wueddie

during analysis of "IO Error: Connection reset", many articles mentioned that it could be caused by java security code (accessing /dev/random) used in JDBC connection. However it is not the root cause in my case. In my environment, Java already use /dev/urandom. 1. $JAVA_HOME/jre/lib/security/java.security securerandom.source=file:/dev/./urandom 2. check with strace. only -Djava.security.egd=file:/dev/../dev/urandom will trigger system call (read on /dev/urandom) all other other path format like below are OK. -Djava.security.egd=file:/dev/./urandom -Djava.security.egd=file:///dev/urandom 3. Keep checking the retropy size, I have never seen it is exhaused. while [ 1 ]; do cat /proc/sys/kernel/random/entropy_avail sleep 1 done usually the avail is in the range from 1000 to 3000. so far, there is no clue about the root cause of "IO Error: Connection reset".

I encountered many issue during installation of Oracle Grid Infrastructure(GI) and Database; with the help of ariticle and documents found through Google search engine, I finally made it. for records, here is the details issues encountered and solutions applied. Major issues were encountered during GI installation. Pre-installation tasks. Issue 1: swapspace is not big enough; (1.3.1 Verify System Requirements) grep MemTotal /proc/meminfo 264G grep SwapTotal /proc/meminfo 2G during OS installation, I take default option and swap space is only 2G. Oracle recommend to have more than 16G swap space in case of more that 32G RAM. dd if=/dev/zero of=/home/swapfile bs=1024 count=33554432 33554432+0 records in 33554432+0 records out 34359738368 bytes (34 GB) copied mkswap /home/swapfile mkswap /home/swapfile chmod 0600 /home/swapfile lessons learned: setup swap space properly according to DB requirement when installing OS. Issue 2: cannot find oracleasm-kmp-default from Oracle site. (1.3.6 Prepare Storage for Oracle Automatic Storage Management) install oracleasmlib and oracleasm-support is easy, just download them from Oracle and install them; Originally oracleasm kernel is provided by Oracle, but now I cannot find it from Oracle; finally I realized that oracleasm kernel is now provided by OS vendor; In my case, it should be installed from SUSE disk; a. to get its name oracleasm-kmp-default zypper se oracle b. map dvd and install zypper in oracleasm-kmp-default rpm -qa|grep oracleasm oracleasm-kmp-default-2.0.8_k3.12.49_11-3.20.x86_64 oracleasm-support-2.1.8-1.SLE12.x86_64 oracleasmlib-2.0.12-1.SLE12.x86_64 asm configure -i asm createdisk DATA /dev/<...> asm listdisks --DATA ls /dev/oracleasm/disks Installation tasks: Issue 3: always failed due to user equivalence check after starting installer OUI with user oracle. however if I manully check with runcluvfy, no issue found at all. ./runcluvfy.sh stage -pre crsinst -n , -verbose I worked around it by using another user to replace user oracle. but it triggered next issue. Issue 4: cannot see ASM disks in OUI. no matter how I change the disk dicovery path. the disk list is empty. but I can find disk manully. /usr/sbin/oracleasm-discover 'ORCL:*' Discovered disk: ORCL:DATA Root cause is that the ASM is configured and created with user oracle. and I aming installing GI with different user other than oracle; so I cannot see the Disk created. change owner of disk device file solved the issue. ls /dev/oracleasm/disks chown /dev/oracleasm/disks -R Issue 5: root.sh execution failed. Failed to create keys in the OLR, rc = 127, Message: clscfg.bin: error while loading shared libraries: libcap.so.1: cannot open shared object file: No such file or directory fixed the issue with command below: zypper in libcap1 ohasd failed to start Failed to start the Clusterware. Last 20 lines of the alert log follow: 2016-07-24 23:10:28.502: [client(1119)]CRS-2101:The OLR was formatted using version 3. I found a good document from SUSE, Oracle RAC 11.2.0.4.0 on SUSE Linux Enterprise Server 12 - x86_64, it make it clear that SUSE 12 is supported by Oracle GI 11.2.0.4, it also mentioned Patch 18370031. "During the Oracle Grid Infrastructure installation, you must apply patch 18370031 before configuring the software that is installed. " The patch 18370031 is actually mentioned in "Oracle quick installation guide on Linux", but not mentioned in "Oracle quick installation guide on Linux". I majored followed up with later one and missed Patch 18370031. issue disappeared after I installed the patch 18370031. ./OPatch/opatch napply -oh -local /18370031 Errors in file : ORA-27091: unable to queue I/O ORA-15081: failed to submit an I/O operation to a disk ORA-06512: at line 4 solved by change owner of disk DATA related file ls -l /dev/oracleasm/iid chown on folder /dev/oracleasm/iid and some .* hidden file. Issue during DB installation Issue 6: report error: in invoking target 'agent nmhs' vi $ORACLE_HOME/sysman/lib/ins_emagent.mk Search for the line $(MK_EMAGENT_NMECTL) Change it to: $(MK_EMAGENT_NMECTL) -lnnz11 refer to https://community.oracle.com/thread/1093616?tstart=0

很多年前裝了Ubuntu和Windows的雙系統(tǒng)，最近因為有了專門的電腦來裝Ubuntu，所以把原先電腦上的Ubuntu卸載了，結(jié)果系統(tǒng)不能引導(dǎo)了。因為GRUB的原理是控制權(quán)從MBR到Ubuntu系統(tǒng)盤，然后Ubuntu系統(tǒng)盤再提供對windows的引導(dǎo)?，F(xiàn)在Ubuntu系統(tǒng)被卸載了。這個啟動的鏈條也就斷了。

這個問題本身不難解決，借了一個Windows安裝盤，恢復(fù)一下MBR即可。但是這個需要windows系統(tǒng)的Administrator密碼。而我的系統(tǒng)不是我裝的，我根本不知道這個密碼。

有的帖子提到破解Administrator密碼，試了一下，覺得太麻煩了。因為電腦上有數(shù)據(jù)，也不能重裝。 

最后的解決方案是在原先的Ubuntu分區(qū)上安裝一個新的Windows。這樣變成了windows的雙系統(tǒng)。安裝完重啟之后可以進(jìn)入任何一個系統(tǒng)（新的或者舊的Windows）。安裝的過程中MBR被自動更新了。再下來就改一下老系統(tǒng)的Administrator密碼，刪除掉多于的Windows新系統(tǒng)即可。

用Gmail的時候不小心點了"存檔"按鈕，一封重要的郵件就此消失了好幾天，今天才機緣巧合找到。

在網(wǎng)絡(luò)上查到的解釋是：
        存檔會將郵件從收件箱移至所有郵件，這樣您不必刪除郵件就可以整理收件箱。

難以理解，坦率地說，這個功能對我來說是徒增煩惱?？磥砣魏喂ぞ叨夹枰闳ミm應(yīng)，磨合。

最近買了一個叫做“華容道”的玩具給兒子晚。這個游戲雖然號稱是中國四大古典智力游戲之一。其實不過百年歷史，而且是從國外引進(jìn)的。不過本地化做得非常好，也算是創(chuàng)造性地吸收國外文明。

手工解決這個游戲有點難度，當(dāng)然已經(jīng)有人給出了解法；不過我還是自己用編程的方式解決了一遍。發(fā)現(xiàn)自己在這方面的編程還是比較弱。大部分時間花在了調(diào)試上。

剛開始是用的深度優(yōu)先搜索。大致知道了答案應(yīng)該長什么樣。后來改進(jìn)為廣度優(yōu)先搜索，得到了最優(yōu)的解法。還有一個就是原先只考慮每次最多移動一格。后來發(fā)現(xiàn)傳統(tǒng)的定義是一個塊的所有連續(xù)移動都算作一步。相應(yīng)地修改了實現(xiàn)算法。

最難的是做界面。為了調(diào)試，隨便寫了個Applet。但是給我兒子玩，就覺得拿不出手了。

Just use this blog to share some meta information.


  

git://github.com/ueddieu/mmix.git

After two weeks' struggle, I have successfully installed Gentoo, a popular GNU/Linux Distribution. For Records, the obstacles I encountered are listed below. (but I can not remember the solution exactly)

0. failed to emerge gpm when I install the links package. If I recall correctly, it is resolved by install gpm manually

1. I encounter issue when I install glib 2.22.5. no update-desktop-database. which is in dev-util/desktop-file-utils. When I try to emerge it, there is a circular dependency on glib. no solution and I forget How I resolve the problem.

2. later after I install glib, with ~amd64 keyword I can install gpm-1.20.6, but it conflicts with the manually inatalled gpm. I remove the conflicted file and emerge successfully.

3. Failed to emerge tiff. edit packages.keywords to add the following. / ~amd64 I am able to use latest tiff in beta-version, which is unstable and masked out.

4. later atk-1.28.0 failed to emerge. edit /etc/make.conf with the following. FEATURES="-stricter". then emerge successfully with only some complain. with out this seting. the warining from GCC will cause that emerge fail.

5. when I run emerge --update system actually gcc will be upgraded from 4.3.4 to 4.4.3. but it failed because of compilation warning, again. add "-stricter" into Features variable in /etc/make.conf work around it.

6. The installation takes a long time, the KDE itself take more than 10 hours. There is still a lot of improvement space! Anyway, it is nice to be able to use it daily.

在C:\Documents and Settings\<user_name>\Application Data\Subversion\servers文件中加入
all=*.*

[all]
http-proxy-host = ***.**.com
http-proxy-port = 8080

這里的all映射到所有的Server。

網(wǎng)絡(luò)環(huán)境的復(fù)雜給我們的工作帶來了一些影響。就拿Proxy的設(shè)置來說，本來理想的情況是在全局做一個設(shè)置就可以了，但是事實上我們要為每個程序做設(shè)置，而且語法還不一樣。

今天從word文檔中拷貝腳本到命令行執(zhí)行。沒有想到的是Word自動加入了空格，導(dǎo)致執(zhí)行失敗。具體如下。
call ttGridCreate('$TT_GRID');
被word變成了
call ttGridCreate(' $TT_GRID');
這個空格可不容易被發(fā)現(xiàn)，尤其你不是腳本作者的時候。提高警惕！

今天做了一個簡單的性能測試。比較訪問Java對象屬性的各種方法的性能差異。
1. 直接訪問對象的屬性。
2. 用方法訪問對象的屬性。
3. 用Map來存儲和訪問。
4. 反射－Field 訪問。
5. 反射－Method訪問。

重復(fù)100次，結(jié)果如下(單位為納秒）。
 * 100 field access, 14,806<br/>
 * 100 method access, 20,393<br/>
 * 100 map access, 66,489<br/>
 * 100 reflection field access, 620,190<br/>
 * 100 reflection method access, 1,832,356<br/>
重復(fù)100000次，結(jié)果如下(單位為納秒）。
 *100000 field access, 2,938,362
 *100000 method access, 3,039,772
 *100000 map access, 10,784,052
 *100000 reflection field access, 144,489,034
 *100000 reflection method access, 37,525,719 <br/>
 
由結(jié)果可見：
1。getter/setter 的性能已經(jīng)接近直接屬性訪問（大約慢50％），沒有必要擔(dān)心getter/setter的性能而采用直接屬性訪問。
2。用Map代替POJO的代價大約是比getter/setter慢三倍。
3。反射訪問比getter/setter慢50到150倍。慎用。追求動態(tài)性的時候也要注意不菲的性能代價。
4。注意重復(fù)次數(shù)增加到100000次，方法訪問和屬性訪問的差距縮??；更有意思的是，反射的Method訪問比Field訪問快四倍。這主要是JIT的作用。

該測試結(jié)果和原先的猜想基本符合。但是性能評估很容易得到片面的結(jié)論，如果有錯誤的地方，請大家不吝指正。謝謝。

0. I am reading the source code of Tomcat 6.0.26. To pay off the effort,
I documents some notes for record. Thanks for the articles about Tomcat
source code, especially the book <<How Tomcat works>>.

1. They are two concepts about server, one is called Server, which
is for managing the Tomcat (start and stop); another is called Connector,
which is the server to serve the application request. they are on the different
ports. The server.xml clearly show the difference.

<Server port="8005" shutdown="SHUTDOWN">
  <Service name="Catalina">
    <Connector port="8080" protocol="HTTP/1.1"
               connectionTimeout="20000"
               redirectPort="8443" />
    <Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />

although the server is the top level element, logically it should not be.
Actually in code, Bootstrap starts the service first, which
in turn start the Server and server's services.

2. My focus in on Connector part. I care how the request is services by the
Tomcat. Here are some key classes.

Connector --> ProtocolHandler (HttpProtocol
                        and AjpProtocol)                       --> JIoEndPoint
                                                                           --> Handler(Http11ConnectionHandler
                                                                           and AjpConnectionHandler)
                                                  
                                                  
3. Connector is most obervious class, but the entry point is not here.
The sequence is like this.

Connector.Acceptor.run()
--> JioEndPoint.processSocke(Socket socket)
    -->SockeProcess.run()
        -->Http11ConnectorHandler.process(Socket socket)
            -->Http11Processor.process(Socket socket)
                -->CoyoteAdapter.service(Request req, Response res)       

The core logic is in method Http11Processor.process(Socket socket)                                                  

CoyoteAdapter.service(Request req, Response res) bridges between Connector module and Container module.

Any comments are welcome. I may continue the source code reading and dig deeper into it if time permit.

It is handy to be able to navigate the source code with Ctrl + ] in Cscope, but I always forget how to navigate back and waste effort many times. So for record, Ctrl+t can navigate back in Cscope.

One more time, Ctrl+] and Ctrl+t can navigate forth and back in Cscope.

How to read the source code in <<TCP/IP Illustrated Volume 2>>

1. Get the source code, original link provided in the book is not available now.
You may need to google it.

2. install cscope and vi.

3. refer to http://cscope.sourceforge.net/large_projects.html for the following steps.

It will include all the source code of the whole OS, not only the kernel.
find src -name '*.[ch]' > cscope.files

we actually only care kernel source.
find src/sys -name '*.[ch]' > cscope.files

4.  wc cscope.files
 1613  1613 45585 cscope.files

5. vim
:help cscope
then you can read the help details.

6. if you run vim in the folder where cscope.out resides. then it will be loaded
automaically.

7. Try a few commands.
:cs find g mbuf
:cs find f vm.h

They works. A good start.

P.S. this book is quite old, if you know it well and can recommend some better alternative for learning TCP/IP, please post a comments, Thanks in advance.

我兒子在彈鋼琴，他阿姨說，“哥哥就喜歡彈難的。”
我外甥女說：“哥哥要彈男的。妹妹要彈女的。”

在高中的時候，知道了用篩法可以得到素數(shù)。當(dāng)時我還有一個錯誤的關(guān)于尋找素數(shù)的猜測。
以為用兩個素數(shù)相乘，其附近存在素數(shù)的幾率很高。比如， 7×11 ＝ 77， 其附近有79，正好是素數(shù)。
當(dāng)時已經(jīng)發(fā)現(xiàn)11×11＝121。7×17＝119；但是錯誤的理解為只有其中一個是平方或次冪時才成立。
后來有了計算機，編程驗證了一下，發(fā)現(xiàn)有很多的反例。對當(dāng)初的錯誤猜測羞赧不已。


這個猜測雖然錯的離譜，但是和現(xiàn)在的素數(shù)理論，尤其是孿生素數(shù)還是很有關(guān)系的?，F(xiàn)在已經(jīng)知道，
素數(shù)有無窮多個，但是素數(shù)在自然數(shù)中所占的比例逐漸趨近于零。

因此孿生素數(shù)在自然數(shù)中的比例也是趨近于零的。現(xiàn)在還沒有證明孿生素數(shù)是否有無窮多個。

這個猜測的樸素之處在于，任何兩個素數(shù)之乘積A，要么A是3n+2，要么A是3n+1；如果是3n+2，則只有A+2
才有可能是素數(shù)；如果是3n+1，則只有A-2才有可能是素數(shù)。但是，事實上，這個猜測成立的比例非常的低。

寫了一個程序驗證了一下。16位的整數(shù)中，大概只有 10％ 能使假設(shè)成立。

由于是在Proxy的網(wǎng)絡(luò)環(huán)境，MSYSGIT 的 git clone 總是失敗。需要配置如下環(huán)境變量。
export http_proxy="http://<proxy domain name>:<port>"
之后http協(xié)議git clone沒有任何問題。但是用git 協(xié)議仍舊有問題。

之后發(fā)現(xiàn)git push 和 git pull 經(jīng)常不能work。多次嘗試后發(fā)現(xiàn)用更全的命令行參數(shù)可以解決問題。
過程如下。

git pull --fail
git pull origin --fail
git pull  git@github.com:ueddieu/mmix.git --it works.

It seems the Command line short cuts are lack of some user information, such as user name "git".
(which is kind of strange at the first glance.)

git push --fail
git push origin --fail
git push git@github.com:ueddieu/mmix.git master --it works.

Anyway, now I can check in code smoothly. :)

There are a few cases in which the un-visible blank character will cause
problem, but it is hard to detect since they are not visible.

One famous case is the '\t' character used by Make file, it is used to mark
the start of a command. If it is replace by blank space character, it does
not work, but you can not see the difference if you only look at the make file.
This kind of problem may get the newbies crazy.

Last week, I have encounter a similar issue, which is also caused by unnecessary
 blank space.
 
As you may know, '\' is used as line-continuation when you have a very long line, e.g.
when you configure the class path for Java in a property file, you may have something like this.

classpath=/lib/A.jar;/lib/B.jar;\
/lib/C.jar;/lib/D.jar;\
/lib/E.jar;/lib/log4j.jar;\
/lib/F.jar;/lib/httpclient.jar;

But if you add extra blank space after the '\', then you can not get the complete
content of classpath. Because only when '\' is followed by a '\n' on Unix or '\r''\n'
on Windows, it will work as line-continuation ; otherwise, e.g '\' is followed by
' ''\n', the line is complete after the '\n', the content after that will be the start of
a new line.

Fortunately, it is easy to check this kind of extra blank space by using vi in Unix.
use command '$' to go to the end of line, if there is no extra blank space after '\',
the current position should be '\', if there are any blank space after '\', the current position
is after the '\'.
 

媽媽和兒子
媽媽，你最近不吃魚，變笨了吧――2009.6.2

兒子又要求錄音了。我們按著臺詞在dialog。
“……”兒子
“……，I like mangoes”媽媽
“媽媽，我昨天剛教會你，又忘了？是I like watermelon。”
“哦，媽媽現(xiàn)在記性不好了！”
“是你這幾天不吃魚了吧，變笨了吧。明天多吃點！”

媽媽你穿這衣服蠻可愛――2009-6-9晚
兒子挑選的故事講完了。“好，ok，我們睡覺吧！”我說。
“唉，媽媽，還沒錄音呢，我去拿mp3。”自從第一次提議給他錄音，兒子每天都要求我能做到。
……
“……，媽媽，你蠻可愛的！……”錄音正起勁，兒子突然插了一句題外話。
“什么？”我沒聽清。
“你穿這衣服蠻可愛的！”兒子賊賊的笑著又重復(fù)了一遍。“因為你的衣服象小斑馬呀！”我終于明白。

我安裝openldap時主要是參考了http://hexstar.javaeye.com/blog/271912

我遇到的一個新問題是執(zhí)行l(wèi)dapsearch報錯如下：
can not find libdb-4.7.so.
我的解決辦法是，建立符號鏈接/usr/lib/libdb-4.7.so， 后者指向/usr/local/BerkeleyDB/lib/libdb-4.7.so
之后沒有遇到其他問題。

凡事都有其內(nèi)在原因，即使是表面上毫無道理的行為，也有其內(nèi)在的原因。今天再次認(rèn)識到這個

道理，還是因為今天早上和我兒子的一段插曲。

 

今天早上，我兒子不肯起床，在床上哭鬧，不讓他媽媽去上班，要他媽媽陪他睡覺。

他媽媽要趕班車，沒時間陪她，留我在家里。我陪他睡了一會，聊了十分鐘，才知道他是有原因

的。

 

昨天晚上，我和她媽媽都很累，我就說今天我們早點睡覺，和兒子一起睡好了?？墒且驗閯倓?/span>

回上海，有很多事情要做，最后還是忙到十點半才睡。兒子就說爸爸說謊了。當(dāng)然他可能還有

其它的原因，比如想我們每天和他一起睡覺。

The analysis of MOR(MXOR) instruction implementation in MMIXWare
 
 -- A stupid way to understand the source code.
 
 the implementation of MOR(MXOR) is in file: mmix-arith.w
 436 octa bool_mult(y,z,xor)
 437   octa y,z; /* the operands */
 438   bool xor; /* do we do xor instead of or? */
 439 {
 440   octa o,x;
 441   register tetra a,b,c;
 442   register int k;
 443   for (k=0,o=y,x=zero_octa;o.h||o.l;k++,o=shift_right(o,8,1))
 444     if (o.l&0xff) {
 445       a=((z.h>>k)&0x01010101)*0xff;
 446       b=((z.l>>k)&0x01010101)*0xff;
 447       c=(o.l&0xff)*0x01010101;
 448       if (xor) x.h^=a&c, x.l^=b&c;
 449       else x.h|=a&c, x.l|=b&c;
 450     }
 451   return x;
 452 }
 
 It takes me several hours to understand the details.
 
 If we treat each octabyte as a matrix, each row corresponds to a byte, then
 y MOR z = z (matrix_mulitiply) y
 
 For a=((z.h>>k)&0x01010101)*0xff;
 (z.h>>k)&0x01010101 will get the four last bit in (z.h>>k). depends on the bit in last row,
 ((z.h>>k)&0x01010101)*0xff will expand the bit (either 0 or 1) into the whole row.
 e.g.
             ff
 *     0x01010101   
 ---------------
 =           ff
           ff
         ff
       ff
 ----------------
 =    ffffffff      
(depending on the last bit in each row of z, the result could be #ff00ff00. #ff0000ff, etc.)

similarily, b=((z.l>>k)&0x01010101)*0xff; will expand the last bit in each byte into the
whole byte.

over all, after these two step, the z becomes the replication of it's last row, since k vary
from 0 to 7, it will loop on all the rows actually.


 For c=(o.l&0xff)*0x01010101, it will get the last byte in o.l and populate it to other three byte.
 since it will not only or/xor h but also l. it is not necessary populate it to o.h.
 
 one example,
 let (z.h>>k)&0x01010101 = 0x01000101, then a= 0xff00ffff;
 let (z.l>>k)&0x01010101 = 0x01010001, then b= 0xffff00ff;
 let (o.l&0xff)=0xuv, then c= 0xuvuvuvuv;
  then a&c=0xuv00uvuv;
         b&c=0xuvuv00uv;
       
 consider the elements [i,j] in result x.  in this round, what value was accumalated in by operation
 or(xor).
 it is the jth bit in last byte of o.l & ith bit in last column of z.(do not consider looping now.)
 in this round, the 64 combination of i and j, contirbute the value to the 64 bits in z.
 
 Noticed that o loop on y from last byte to first byte. There are 8 loop/rounds, in another round.
 say kth round.
 the elements[i,j] will accumuate the jth bit in last (k + 1)th row & the jth bit in last (k+1)th
 column.
 that means the jth column in y multiply the ith row in z. it conform to the definiton for
 z matrix_multiply y.
 
 
         
       
       
 

游戲和數(shù)學(xué)有密切的聯(lián)系。最近在玩九連環(huán)，感受更深。

之所以開始玩九連環(huán)，是因為在高德納的書中提到了格雷碼和九連環(huán)的關(guān)系。為了理解生成格雷碼的算法，特意買了九連環(huán)來玩。畢竟書上的
描述沒有實際玩起來那么容易理解。

通過這個游戲，我不僅會解九連環(huán)了，而且掌握的生成格雷碼的一種算法。

A detailed reading process of a piece of beautiful and trick bitwise operation code.
 
The following code is from MMIXWare, it is used to implement the Wyde difference between two octabyte.
 
     in file: "mmix-arith.w"
     423 tetra wyde_diff(y,z)
     424   tetra y,z;
     425 {
     426   register tetra a=((y>>16)-(z>>16))&0x10000;
     427   register tetra b=((y&0xffff)-(z&0xffff))&0x10000;
     428   return y-(z^((y^z)&(b-a-(b>>16))));
     429 }
 
It is hard to understand it without any thinking or verification, here is the process I used
to check the correctness of this algorithm.

let y = 0xuuuuvvvv;
     z = 0xccccdddd; (please note the [c]s may be different hex number.)
    
then y>>16 = 0x0000uuuu;
     z>>16 = 0x0000cccc;
     
then ((y>>16)-(z>>16)) = 0x1111gggg if #uuuu < #cccc or
     ((y>>16)-(z>>16)) = 0x0000gggg if #uuuu >= #cccc   

so variable a = 0x00010000 if #uuuu < #cccc or
   variable a = 0x00000000 if #uuuu >= #cccc
  
similarly, we can get
   variable b = 0x00010000 if #vvvv < #dddd or
   variable b = 0x00000000 if #vvvv >= #dddd

for (b-a-(b>>16)))), there are four different result depending on the relation between a and b.
when #uuuu >= #cccc and #vvvv >= #dddd, (b-a-(b>>16)))) = 0x00000000;
when #uuuu >= #cccc and #vvvv < #dddd, (b-a-(b>>16)))) = 0x00001111;
when #uuuu < #cccc and #vvvv >= #dddd, (b-a-(b>>16)))) = 0x11110000;
when #uuuu < #cccc and #vvvv < #dddd, (b-a-(b>>16)))) = 0x11111111;
You can see that >= map to #0000 and < map to #1111

for y-(z^((y^z)&(b-a-(b>>16)))), when (b-a-(b>>16)))) is 0x00000000, z^((y^z)&(b-a-(b>>16))) is
z^((y^z)& 0) = z^0=z, so y-(z^((y^z)&(b-a-(b>>16))))=y-z.
similarily, when (b-a-(b>>16)))) is 0x11111111, z^((y^z)&(b-a-(b>>16))) is
z^((y^z)& 1) = z^(y^z)=y, so y-(z^((y^z)&(b-a-(b>>16))))=0.

when (b-a-(b>>16)))) is 0x11110000 or 0x11110000, we can treat the y and z as two separate wydes.
each wyde in the result is correct.

You may think it is a little stupid to verify such kind of details. but for my point of view,
without such detailed analysis, I can not understand the algorithm in the code. with the hard
work like this, I successfully understand it. The pleasure deserve the effort.
 
I am wondering how can the author discover such a genius algorithm.

昨天晚上，瑞瑞睡到十二點的時候，爬出被子，躺在外面。結(jié)果咳嗽得很厲害，吐了好幾口。吃了枇杷膏很快就睡著了。

沒過一會，他就開始做惡夢了。下面是他的夢話：
“爸爸的手不見了”
“爸爸掉下去了”
“我親愛的爸爸不見了，我可怎么辦阿”

估計是最近洪恩的GOGO學(xué)英語的DVD看多了。

After reading the <<MMIX: A RISC Computer for the New Millennium>>, I am ispired to Create a MMIX simulator in Java.
Donald Knuth already created a high quality MMIX simulater in C, why I still bother to creating a new one in Java.
First, I want to learn more about how the computer works. I think re-implement a simulator for MMIX can
help me gain a better understanding.
Second, I want to exercise my Java skills.

After about one month's work, I realize that I can not finish it by myself. I am looking for the help.
If you are interested in MMIX and know Java, Please give me a hand.

Currently I have finished most of the instructions, but some important and complex one are not completed
yet.
I have developed a few JUnit TestCase for some instructions, but it's way far from covering all the instructions (there are 256 instructions total).
Few of the sample MMIX program in Donald Knuth's MMIXware package, such as cp.mmo, hello.mmo can be
simulated successfully, but there are much more to support.

To help on this project, first you need the access to the current source code. It's hosted on Google
code. Please follow the steps below to access the source code.

Use this command to anonymously check out the latest project source code:
# Non-members may check out a read-only working copy anonymously over HTTP.
svn checkout http://mmix.googlecode.com/svn/trunk/ mmix-read-only

If you are willint to help, please comment on this blog with your email address.

There are many questions coming into my mind when I read the Linux kernel book and source code. As time goes by, I become more knowledgeable than before and can address those questions by myself, here is the first question addressed by myself.

 

Q: why kernel have to map the high memory in kernel space, why not just allocate the high memory and only map it in user process.

A: Because kernel also need to access the high memory before it returned the allocated memory to user process. For example, kernel must zero the page or initialized the page for security reason. Please refer to linux device driver page 9.

Q: why not let the clib zero the page or initialize it, it saves the kernel's effort and simplifies the kernel.

A: besides Requesting memory through clib, user program can also request memory through direct System call, in this situation, the security is not guaranteed, the information in memory will be leaked.

9/26/2008 8:57AM
Today I want to research the different ways to substitute text in the file. For records, I written them down.

1. use Ultra Edit, it is super easy for a Windows user if you have Ultra Edit installed.
use Ctrl + R to get Replacement Wizard and follow you intuition.

2. use VI in Unix.
:s/xx/yy/
will replace xx with yy.

3. use filter, such as sed, and awk in Unix.
sed -e 's/xx/yy/g' file.in > file.out
replace xx with yy in all the lines. It seem sed will not change the original input file, so I redirect the out put to file.out

1 WYSIWYM vs WYSIWYG

WYSIWYM stands for What You See is What You Mean; WYSIWYG stands for What You See is What You Get;

Microsoft -- Word is always considered as a example of WYSIWYG. Today I have a look at the tool named LyX, which is an example of WYSIWYM. From an end user's point of view, there are more similarity than difference between them.

They both display the the resulted layout on the fly; they both provide button to typeset the document.

The difference I can see between then is -- LyX use text file, while Word use binary file. But I don't think it matters.

In my humble opinion, the real difference between Word and LyX/LaTeX is as the following. In Word, you typeset in the lower level, you can control all the details but it also need more effort. In LyX/LaTex, you typeset in higher level, you only need to figure out the logic structure of the document. The resulted layout is not decided by you, you actually just share the layout developed by the expert. I think it is the key advantage of WYSIWYM.

Yesterday, we found that the application can not send mail successfully; the performance of the module using email feature is also very bad. I suspect it caused by that the mail server host name can not be resolved in the application server.

I executed the following command
host <mail server host name>
It shows a strange IP. It means it can not properly resolve the mail server host name

Then I execute the command below.
man host
The output tells me to resort to /etc/resolv.conf
open it with
vi /etc/resolv.conf

The context is as following:
nameserver <name server 1>
nameserver <name server 2>
update the config with correct DNS server IP.

Everything is OK.

P.S. It seems that the ping and host commands are different. For some host name, I can ping it but I can not host it.

The reality is far from the idealism - the inelegance in Operating System

I am interested in Operating System, after I know more and more concepts, know more and more details, I realize that the reality is far from the idealism. The root cause is the history and to some extent, it is the back compatibility. we can not afford to make a brand new thing from scratch, we need to include many old things in any things.

Let me give some example about how the history make the current Operation System become complicated and inelegant.
1. DMA
DMA stands for Direct Memory Access, which is a way to improve the parallelism in computer system. Basically, with DMA, peripheral device can access main Memory simultaneously when CPU is running. but for historical reason, in X86 platform, some DMA device only have 24 bit address line. which limit the memory scope to 16M. since X86 platform is also lack of IO-MMU to remap the address, the memory can be used in DMA is [0,16M). It definitely complicated the memory management.

2. High Memory
Since  Linux kernel has only 1G linear address space, it can not address all the 4G physical memory in 32 bit machine. This is actual a design issue in Linux for historical reason. it does not predict that some day, the physical memory will become so large. Later in order to support more than 1 G physical memory, CONFIG_HIGHMEM compile option was added. There are also other way to fix this problem, such as 4G kernel space v.s. 4G user space.

3. PAE
PAE stands for Physical Memory Extension, PAE make it possible to support up to 64G physical memory. but to me, it is just a temporary solution, does not deserve the effort. I even do not want to have a look on the corresponding document. It does not make too much sense. I prefer to directly move to 64 bit platform. 64 bit platform has its own problems though.

the above is just some inelegant in hardware. majorly cause by historical reason. I am wondering how can we keep up the quick development under the burden of history. maybe at some point, we finally need to throw away the history and move on with a brand new start.

Virtual Memory Area

Virtual Memory Area is also called Memory Region in some book.

In the process address spaces, there are many memory areas, contiguous addresses wil be divided into different memory area if the access right of them are different. For example, in one Java Process, there are 359 memory areas.

so the kernel need to find a effective way to insert into, remove from, search from the list of memory areas. The semantics of find_area  API is the as the following.

return null if
    1. The list itself is empty.
    2. The list is not empty, and the address is big than the last memory area.

return found area if
    1. the address is in the region of one area.
    2. the address is not in the region of any area. but is not bigger than the last area.       
        it means it is in the hole between areas. right area besides the hole is returned.
       
 
 
 

The kernel are trying to use as little resource as possible. Here is an example, Originally, in kernerl 2.4, the size of Kernel Stack is 8K. Now, in kernel 2.6, it could be 4K, if you enable it in compilaiton time.

Why will kernel spend effort to support such a feature when most of PC have more than 1 Gigabyte memonry. I think it has something to do with the C10K probleum; C10K means Concurrent 10 Thousand Processes(Threads). considering a system with more thant 10 thousand processes, such as a WEB server, the save of 4K in every kernel stack will become 4K * 10 K = 40 M tatal save of memory, which is a big deal!

How is it possible to achieve that? originally the kernel mode stack is also used in Exception and Interrupt handling, but Exception and Interrupt handling is not specific to any process. so in 2.6, Interrupt and Exception will have their own Stack for each CPU. Kernel stack is only used by process in the kernel mode. so the acutal kernel stack did not become small.
2.4     8K Stack shared between process kernel mode, Exception, Interrupt.
v.s    
2.6     4K Stack specific for process kernel mode Stack
        4K Stack specific for Exception Stack
        4K Stack specific for Interrupt Stack
Besides this, in 8K stack of 2.4, task_struct is at the bottom of stack, which may cost about 1K, in 4K stack of 2.6, only thread_info is at the bottom of stack, the task_struct is put into a per-CPU data structre, thread_info is only about 50 bytes.

Here is just the high level summary of my understanding on Linux Kernel Memory Management. I think it can help achieve a better understanding of the book <<understanding linux kernel>>.

It is said, the memory management is most complex sub-system in linux kernel, at the same time, there aren't too much System Calls for it. Becuase most the the complex mechanism happens trasparently to the user process, such as COW(Copy On Write), On Demand Paging. For user process, to successfully refer to a linear memory address, the following factors are necessary:
    vm_area_struct (Virtual Memory Area, Memory Region) are set up correctly.
    Phsical memory are allocated.
    Page Global Directory, Page Table, and the corresponding entry are correclty set up according to Virtual Memory Area and Phisical Meory.
   
This three factors can be further simplified as
    Virtual Memory
    Phisical Memory
    Mappting between Virtual Momory and Phisical Memory.

From user process's perspective, only Virtual Memory is visible, when user process applys for memory, he got virtual memory; phisical memory may not be allocated yet. All these three factors are managed by the kernel, they can be thought of as three resource managed by the kernel. kernel not only need to manage the Virtual Memoty in user address space, but also need to manage Virtual Memory in kernel address space.

When user process try to use his virtual memory, but the phisical memory is not allocated yet. Page Exception happens, kernel take charge of it and allocate the phisical memory and set up the mapping. user process reexecute the instruction and everything go forward smoothly. It's called On Demand Paging.

Besides that there are many more concepts, such as Memory mapping, non-linear memory mapping. I will continue this article when I dig into the details.

 ps -H -A
can show the relationship between all the processes in a tree format. it is helpful when you want to research the internals of UNIX.
 
 init
   keventd
   ksoftirqd/0
   bdflush
   kswapd   
 
we can see from the above that all the process are the children of init (directly or indirectly). especially the kernel thread are also the children of init process.
process 0 is special, it is not displayed.

From the following:
  sshd
    sshd
      sshd
        bash
          vim
            cscope
    sshd
      sshd
        bash
          ps
 we can see that how ssh works. actually I have created two ssh session to the server.
 

根據(jù)以下Xusage.txt中的說明：
-Xms<size>        set initial Java heap size
-Xmx<size>        set maximum Java heap size

Java -Xms512M 應(yīng)該為Java分配至少512M的內(nèi)存，但是在Linux中用TOP查看，其RSS和SIZE的值遠(yuǎn)小于512M。我的理解是Java向操作系統(tǒng)申請內(nèi)存時，用的是mmap2或者old_mmap系統(tǒng)調(diào)用，這兩個系統(tǒng)調(diào)用其實都沒有真正分配物理內(nèi)存，而僅僅是分配了虛擬內(nèi)存。所以預(yù)先分配的這些內(nèi)存要到實際使用時才能落實到位。

There are not too much grammar. here is just the incomplete summary for the future reference.
meta-character
.                                any character
|                                or
()                                grouping
[]                                character class
[^]                                negative character class

Greedy Quantifier
?                                optional
*                                any amount
+                                at least one
lazy quantifier
??                               
*?
+?
possessing quantifier
?+
*+
++

position related
^                                start ot the line
\A                               
$                                end of the line
\Z
\<                                start of the word
\>                                end of the word
\b                                start or end of the word

non-capturing group                (?:Expression)
non-capturing atomic group        (?>Expression)
positive lookahead                (?=Expression)
negative lookahead                (?!Expression)
positive lookbehind                (?<=Expression)
negative lookbehind             (?<!Expression)

\Q start quoting
\E end quoting

mode modifier
(?modifier)Expression(?-modifier)
valid modifier
i            case insensitive match mode
x             free spacing
s            dot matches all match mode
m            enhanced line-anchor match mode
(?modifier:Expression)

comments:
(?#Comments)

kernel memory mapping summay

Today, finally I become clear about the relationship between
fixed mapping
permanent kernel mapping
temporary kernel mapping
noncontiguous memory area mapping
(I feel that most of the name is not appropriate, to some text, it will mislead the reader.)

4G linear virtual address space is divided into two major part.
kernel space mapping     [3G, 4G)
user space mapping        [0, 3G)

kernel space mapping is divided into more pieces
linear mapping [3G, 3G + 896M)
non linear mapping [3G + 896M + 8M, 4G)
1. Fixed Mapping (wrong name, should be compile time mapping, the virtual address is decided in compile time. )
2. Temporary mapping
3. Permanent mapping
4. noncontiguous memory area mapping (Vmalloc area)

The following is the diagram for the reference.

FIXADDR_TOP            (=0xfffff000)
 
                    fixed_addresses (temporary kernel mapping is part of it)
                     #define __FIXADDR_SIZE  (__end_of_permanent_fixed_addresses << PAGE_SHIFT)
                
FIXADDR_START        (FIXADDR_TOP - __FIXADDR_SIZE)
                        
                    temp fixed addresses (used in boot time)
                     #define __FIXADDR_BOOT_SIZE     (__end_of_fixed_addresses << PAGE_SHIFT)
                
FIXADDR_BOOT_START    (FIXADDR_TOP - __FIXADDR_BOOT_SIZE)
                        
                    Persistent kmap area (4M)
 
PKMAP_BASE            ( (FIXADDR_BOOT_START - PAGE_SIZE*(LAST_PKMAP + 1)) & PMD_MASK )
 
                     2*PAGE_SIZE
 
VMALLOC_END            (PKMAP_BASE-2*PAGE_SIZE) or (FIXADDR_START-2*PAGE_SIZE)

                     noncontiguous memory area mapping (Vmalloc area)
                    
VMALLOC_START        (((unsigned long) high_memory + 2*VMALLOC_OFFSET-1) & ~(VMALLOC_OFFSET-1)) 

high_memory            MIN (896M, phisical memory size)
 
below the excerp of the source code.

#ifdef CONFIG_X86_PAE
#define LAST_PKMAP 512
#else
#define LAST_PKMAP 1024
#endif
                                                                         
#define VMALLOC_OFFSET  (8*1024*1024)
#define VMALLOC_START   (((unsigned long) high_memory + \                         
                        2*VMALLOC_OFFSET-1) & ~(VMALLOC_OFFSET-1))   
                              
#ifdef CONFIG_HIGHMEM                                             
# define VMALLOC_END    (PKMAP_BASE-2*PAGE_SIZE)
#else                                                                    
# define VMALLOC_END    (FIXADDR_START-2*PAGE_SIZE)              
#endif

enum fixed_addresses {
        FIX_HOLE,
        FIX_VDSO,
        FIX_DBGP_BASE,
        FIX_EARLYCON_MEM_BASE,
#ifdef CONFIG_X86_LOCAL_APIC
        FIX_APIC_BASE,  /* local (CPU) APIC) -- required for SMP or not */
#endif
#ifdef CONFIG_X86_IO_APIC
        FIX_IO_APIC_BASE_0,
        FIX_IO_APIC_BASE_END = FIX_IO_APIC_BASE_0 + MAX_IO_APICS-1,
#endif
#ifdef CONFIG_X86_VISWS_APIC
        FIX_CO_CPU,     /* Cobalt timer */
        FIX_CO_APIC,    /* Cobalt APIC Redirection Table */
        FIX_LI_PCIA,    /* Lithium PCI Bridge A */
        FIX_LI_PCIB,    /* Lithium PCI Bridge B */
#endif
#ifdef CONFIG_X86_F00F_BUG
        FIX_F00F_IDT,   /* Virtual mapping for IDT */
#endif
#ifdef CONFIG_X86_CYCLONE_TIMER
        FIX_CYCLONE_TIMER, /*cyclone timer register*/
#endif
#ifdef CONFIG_HIGHMEM
        FIX_KMAP_BEGIN, /* reserved pte's for temporary kernel mappings */
        FIX_KMAP_END = FIX_KMAP_BEGIN+(KM_TYPE_NR*NR_CPUS)-1,
#endif
#ifdef CONFIG_ACPI
        FIX_ACPI_BEGIN,
        FIX_ACPI_END = FIX_ACPI_BEGIN + FIX_ACPI_PAGES - 1,
#endif
#ifdef CONFIG_PCI_MMCONFIG
        FIX_PCIE_MCFG,
#endif
#ifdef CONFIG_PARAVIRT
        FIX_PARAVIRT_BOOTMAP,
#endif
        __end_of_permanent_fixed_addresses,
        /* temporary boot-time mappings, used before ioremap() is functional */
#define NR_FIX_BTMAPS   16
        FIX_BTMAP_END = __end_of_permanent_fixed_addresses,
        FIX_BTMAP_BEGIN = FIX_BTMAP_END + NR_FIX_BTMAPS - 1,
        FIX_WP_TEST,
        __end_of_fixed_addresses
}

scale up     -     vertically scale
scale out     -     horizontally scale

scale out
1. Use share nothing clustering architectures
    The session failover functionality cannot avoid errors completely when failures happen, as my article mentioned, but it will damage the performance and scalability.
   
2. Use scalable session replication mechanisms   
    The most scalable one is paired node replication, the least scalable solution is using database as session persistence storage.

3. Use collocated deployment instead of distributed one.

4. Shared resources and services
    Database servers, JNDI trees, LDAP Servers, and external file systems can be shared by the nodes in the cluster.

5. Memcached
    Memcached's magic lies in its two-stage hash approach. It behaves as though it were a giant hash table, looking up key = value pairs. Give it a key, and set or get some arbitrary data. When doing a memcached lookup, first the client hashes the key against the whole list of servers. Once it has chosen a server, the client then sends its request, and the server does an internal hash key lookup for the actual item data.
   
6. Terracotta   
    Terracotta extends the Java Memory Model of a single JVM to include a cluster of virtual machines such that threads on one virtual machine can interact with threads on another virtual machine as if they were all on the same virtual machine with an unlimited amount of heap.
   
7. Using unorthodox approach to achieve high scalability   

今天遇到了一個奇怪的Hibernate問題。（我用得hibernate是2.1版。比較舊，不知道這個問題在hibernate 3 中是否存在。）
下面這個是捕捉到的異常堆棧。
java.lang.ClassCastException: java.lang.Boolean  
at net.sf.hibernate.type.StringType.set(StringType.java:26)
at net.sf.hibernate.type.NullableType.nullSafeSet(NullableType.java:48)  
at net.sf.hibernate.type.NullableType.nullSafeSet(NullableType.java:35)  
at net.sf.hibernate.persister.EntityPersister.dehydrate(EntityPersister.java:393)
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:466)  
at net.sf.hibernate.persister.EntityPersister.insert(EntityPersister.java:442)  
at net.sf.hibernate.impl.ScheduledInsertion.execute(ScheduledInsertion.java:29)    
at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2382)  
at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2335) 
at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2204)
奇怪之處在于程序在本機Tomcat上運行情況良好，一旦部署到Linux服務(wù)器上就掛了。

仔細(xì)分析之后，發(fā)現(xiàn)要存儲的對象既定義了get方法又定義了is方法。內(nèi)容示例如下
public class FakePO {
    String goodMan;
    public String getGoodMan() {
        return goodMan;
    }
    public void setGoodMan(String goodMan) {
        this.goodMan = goodMan;
    }
    public boolean isGoodMan(){
        return "Y".equalsIgnoreCase(goodMan);
    }
}
懷疑可能是這個衍生的輔助方法isGoodMan()導(dǎo)致的問題。通過追蹤Hibernate 2的源代碼，發(fā)現(xiàn)hibernate 2是按如下方式通過反射API訪問PO的。

private static Method getterMethod(Class theClass, String propertyName) {      
        Method[] methods = theClass.getDeclaredMethods();
        for (int i=0; i<methods.length; i++) {
            // only carry on if the method has no parameters
            if ( methods[i].getParameterTypes().length==0 ) {
                String methodName = methods[i].getName();
               
                // try "get"
                if( methodName.startsWith("get") ) {
                    String testStdMethod = Introspector.decapitalize( methodName.substring(3) );
                    String testOldMethod = methodName.substring(3);
                    if( testStdMethod.equals(propertyName) || testOldMethod.equals(propertyName) ) return methods[i];
                   
                }
               
                // if not "get" then try "is"
                /*boolean isBoolean = methods[i].getReturnType().equals(Boolean.class) ||
                    methods[i].getReturnType().equals(boolean.class);*/
                if( methodName.startsWith("is") ) {
                    String testStdMethod = Introspector.decapitalize( methodName.substring(2) );
                    String testOldMethod = methodName.substring(2);
                    if( testStdMethod.equals(propertyName) || testOldMethod.equals(propertyName) ) return methods[i];
                }
            }
        }
        return null;
    }
仔細(xì)讀以上代碼可以發(fā)現(xiàn)，Hibernate就是簡單的遍歷類的public方法，看是否和屬性名稱匹配，并不檢查方法的返回值是否和屬性的類型匹配。所以在我們的例子中，既可能返回get方法，也可能返回is方法，取決于public方法列表的順序，而這個順序恰恰是沒有任何保證的。這也解釋了為什么這個問題只能在特定平臺上發(fā)生。

最近在看write系統(tǒng)調(diào)用的實現(xiàn)，雖然還有一下細(xì)節(jié)不是很清楚，但是大致的實現(xiàn)機理還是有一定的理解了?？偨Y(jié)如下：
這里假設(shè)最普通的情況，不考慮Direct IO 的情況。從全家的高度看，要往一個文件中寫入內(nèi)容，需要一下幾步。
1. sys_write 將用戶進(jìn)程要寫的內(nèi)容寫入到內(nèi)核的文件頁面緩沖中。sys_write 本身到此就結(jié)束了。
2. pdflush 內(nèi)核線程（定期或者由內(nèi)核閾值觸發(fā)）刷新臟的頁面緩沖，其實只是提交IO請求給底層的驅(qū)動。
3. IO請求并不是同步執(zhí)行的，而是由底層的驅(qū)動調(diào)度執(zhí)行，發(fā)出DMA操作指令。
4. 物理IO完成之后會中斷并通知內(nèi)核，內(nèi)核負(fù)責(zé)更新IO的狀態(tài)。
先要去陪兒子睡覺了。有空會繼續(xù)細(xì)化各個部分的實現(xiàn)。

sys_write 的調(diào)用過程。（我的linux內(nèi)核版本為2.6.24，文件系統(tǒng)為ext3）
asmlinkage ssize_t sys_write(unsigned int fd, const char __user * buf, size_t count)

vfs_write(file, buf, count, &pos);

file->f_op->write(file, buf, count, pos);
這里的file->fop 是在open一個文件是初始化的函數(shù)指針，ext3文件系統(tǒng)對應(yīng)的函數(shù)為do_sync_write。
下面是其實現(xiàn)的要點。
 for (;;) {
300                 ret = filp->f_op->aio_write(&kiocb, &iov, 1, kiocb.ki_pos);
301                 if (ret != -EIOCBRETRY)
302                         break;
303                 wait_on_retry_sync_kiocb(&kiocb);
304         }
305
306         if (-EIOCBQUEUED == ret)
307                 ret = wait_on_sync_kiocb(&kiocb);
 filp->f_op->aio_write(&kiocb, &iov, 1, kiocb.ki_pos); 是實現(xiàn)的核心，其函數(shù)指針指向ext3_file_write。
307行的作用在于等待IO的完成。這里的IO完成指的是進(jìn)入IO的隊列而已，不是物理IO的完成。

generic_file_aio_write(iocb, iov, nr_segs, pos);

__generic_file_aio_write_nolock(iocb, iov, nr_segs,  &iocb->ki_pos);

generic_segment_checks(iov, &nr_segs, &ocount, VERIFY_READ);

generic_file_buffered_write(iocb, iov, nr_segs, pos,ppos,count,written);

generic_file_direct_IO(WRITE, iocb, iov, pos, *nr_segs);

以下的調(diào)用序列還很長，一時還消化不了。僅供自己參考。

最近開始在Unix下讀一些源代碼.下面是一點體會.
1. 工欲善其事,必先利其器
我開始的時候是用find xargs 和 egrep 配合來搜索關(guān)鍵字, 看代碼的效率很低.后來裝了ctags,方便多了.最初沒有裝ctags, 是因為覺得可能裝起來費勁, 其實還是很容易裝的,也就是那么幾步, google一下就搞定了.

2. 要及時實踐.
雖然開始是讀代碼的方式比較笨,不過這種干勁非常有用,只有動手實踐了,才有可能取得進(jìn)步.否則的話,我可能還是停留在閱讀書本上代碼的階段.

3. Unix下的工具看起來不如Windows的工具異用.其實不然,可能是門檻搞一些.多數(shù)人象我一樣因此不敢去碰它.入門以后,會發(fā)現(xiàn)其實Unix下的工具真是短小精悍. 就拿VIM + Ctags 閱讀源代碼來說,覺得性價比高.符合80/20原則.

如題。
同名的Bean配置兩次。根據(jù)配置文件的載入順序，后定義的Bean生效。
看來還得自己小心，不能過于依賴Spring自己的檢查機制（只能在單文件中檢查重復(fù)的bean配置）。

J2EE項目中基本都是遵循分層架構(gòu)的，自然包結(jié)構(gòu)也是基于分層的。DAO層有DAO package。service 層有service package。在這些包下面再根據(jù)模塊劃分子包。

我覺得另一種可行的方案是根據(jù)模塊劃分包，如果包比較復(fù)雜，比如有超過十個的類，再根據(jù)層來劃分子包。一般的模塊比較簡單，無需劃分子包。

從高內(nèi)聚，低偶合的原則來說，這樣劃分具有更高的內(nèi)聚性。如果按層劃分。其實同層的類并入多大的關(guān)系。考慮一下DAO層。這些DAO之間有多少聯(lián)系？

新劃分方法的好處是如果需要修改某個模塊，修改的地方相對集中。因為都位于一個包內(nèi)。

現(xiàn)在分層架構(gòu)已經(jīng)非常普遍，沒有必要在包的劃分上體現(xiàn)分層架構(gòu)。在類名上體現(xiàn)分層架構(gòu)即可。就是說分層架構(gòu)無需通過包結(jié)構(gòu)來體現(xiàn)。

新的劃分方案可能有一個問題。各個模塊之間可能有實現(xiàn)上的冗余。如果采用這個方案，需要在這點上采取預(yù)防措施。

當(dāng)然這還是想法，沒有在項目中實踐。希望大家能指出這個方法可能帶來的問題。

最近看了一些項目代碼，了解了它得架構(gòu)和設(shè)計?；旧虾芘宸?。因為這些代碼是幾年以前寫的。但是很多書中提到的模式，原則都得到了運用。但是也有一些地方有不同看法，我覺得很多地方用得并不恰當(dāng)。
1. 濫用繼承。比如在類結(jié)構(gòu)中已經(jīng)用了模板模式，照理說子類按照需要覆蓋模板中的實現(xiàn)即可?？墒遣恢鲇诤畏N目的。有的子類卻是抽象的，需要從該抽象子類再次擴展，導(dǎo)致繼承樹不必要的深。
2. 濫用接口。經(jīng)?？吹浇涌谥卸x了一堆的方法，而且該接口只有一種實現(xiàn)。這種接口純粹是擺設(shè)，這樣的接口根本不能指望它有穩(wěn)定性。實際情況是接口將隨著實現(xiàn)的改變而改變。你說要這樣的接口干嗎？
3. 喜歡抽象出框架，但是這些框架對于當(dāng)前的應(yīng)用來說真實不必要的復(fù)雜。事實上沒有增加重用，反而降低了代碼的可讀性。
4. 濫用工廠模式。大家不是覺得模式很難實際運用嗎。真想用模式嗎？那還不簡單。給每個對象都定義一個工廠類不就的了嗎？說心里話，我真看不出那些工廠模式到底實現(xiàn)什么設(shè)計上的好處。
5. 抽象的能力不夠。在一個分頁的實現(xiàn)中。把查尋字符串抽象到了一個類中。正確的方法應(yīng)該是把查詢結(jié)果抽象出來。
項目在進(jìn)化的過程中很容易變得越來越難維護(hù)，畢竟很多不同的思想和不同人的代碼揉和到了一起。出現(xiàn)各種問題也是正常的。
希望在別的項目中能引以為戒。

今天成功的完成了一個游戲--孔明鎖

簡單的說,道具就是六根長方形木條, 其中五根都是有榫頭的, 另一根則是直木.
這六根木條最終搭成三個方向互相穿透的立體形狀.

這個玩具我上個周末在黃山買的.試過四次, 一直沒有成功.
今天吃過晚飯, 無意之中和老婆合作完成了.

基本上,我的分析能力還是可以的. 能夠抓住要點.
但是動手能力不如我老婆, 眼看就要成功的時候,還是老婆眼疾手快, 完成最好一塊.

其實剛玩的時候, 我就總結(jié)出一些基本要領(lǐng).盡管不是成功的法則.但是能避免不必要的失敗

1. 要以唯一的一根直木為思考的出發(fā)點.

2. 有兩個方向各有兩個木條和該直木相交. 其中一個方向需要居中的榫頭(剛開始只是總結(jié)出這點).
另一個方向需要偏左或者偏右的榫頭(今天才總結(jié)出這個要點).

3. 分析已經(jīng)有的木條榫頭的形狀, 合理的組合不到20種.

4. 通過手工試驗. 再摸索出一些規(guī)律, 養(yǎng)成空間的感覺. 半個小時應(yīng)該可以完成.


我很少寫B(tài)log.為什么今天例外了.
因為我需要一些東西來證明自己的智商還在. 我說的智商是與具體知識不太相關(guān)的一種能力.
都說成功離不開自信. 自信不僅僅是一種信念. 應(yīng)該通過對自己能力的檢驗來建立自信.

每次項目結(jié)束，都會發(fā)現(xiàn)有一堆的Bug。如何分析這些Bug，避免重蹈覆轍。

有兩種分析方法， 根據(jù)Developer在修復(fù)Bug時選擇的CommonCause，選擇比重最大的CommonCause，

然后從各個方面分析RootcCause。總結(jié)出可以改進(jìn)的地方。

我很難理解這種方法，主要是覺得每次都是泛泛而談，對減少BUG沒有真正的幫助。

(我確信這種分析方法沒有太大的意義,因為缺乏對底層原因的了解. 而且Developer在選擇common Cause的時候完全可能沒有合適的而任選一個.經(jīng)?？吹降囊粋€例子是缺乏UT. 這個就不一定是真正的原因,事實往往是做了UT卻沒有發(fā)現(xiàn)出Bug.這種分析方法是典型的不深入實際的浮夸作風(fēng), 依賴統(tǒng)計的數(shù)據(jù)而沒有看到統(tǒng)計數(shù)據(jù)事實上可能存在問題. 這樣的工作肯定效率不高. )

下面是我的一些思考。
分析的基礎(chǔ)應(yīng)該是Bug，而不是commonCause。直接從CommonCause開始分析，至少可能遺漏一下重要有價值的發(fā)現(xiàn)。
有些Bug是有可能避免的。而有些bug可以說沒有什么好的對策。我們應(yīng)該集中分析有可能避免的Bug。
至于如何分析具體Bug是否能避免，首先應(yīng)該是造成該Bug的Developer自己分析，讓大家知道Bug是如何形成的，然后由大家集體決定。（這樣做的風(fēng)險是大家能否接受。）
其次根據(jù)Bug引入的時間，和最終測試出的時間，總結(jié)有沒有可以改進(jìn)的大方。
能夠由developer改進(jìn)而消除的Bug。是最有希望避免再次發(fā)生的。
比如，有些bug是打字錯誤造成界面上顯示的內(nèi)容有瑕疵，一個可行的改進(jìn)是每次都從需求文檔拷貝。
注意必須要有措施能保證該經(jīng)驗?zāi)鼙凰蠨eveloper知道。
另一個例子是，我有一次，是的，我有一次再修正bug是沒有清除徹底。在總結(jié)的時候我掌握了全局查找、替換的技巧。有效地避免了類似的錯誤再次發(fā)生。

并非所有錯誤都能由devloper來消除，有一些只能由Tester來消除。比如，一般來說，Tester總是比Developer對界面敏感，更能發(fā)現(xiàn)界面bug。
我覺得隨著單元測試的進(jìn)步，現(xiàn)在對Developer的測試水平的要求也提高了。這也許不盡合理。developer對實現(xiàn)花了很多精力，他不可能在測試上達(dá)到同樣的水準(zhǔn)。

Why we need Interface? The most important benefit come from the fact: The code depend on the interface no need to care about the implementaion class. and if the implementation class is changed later, the client code no need to update.
This is the feature of Ploymophism of OOP, such as Java. 

In some projects, the struts framework was adopted, so all the field need to be persisted is in ActionForm. In order to avoid that the Service layer /DAO layer will depends on the struts. One way is to define a interface which have getter and
setter to access all the fields need to be persisted. The design is like this:

XXXActionForm --------> XXXInterface <--------------ServiceLayer/DAO Layer
                                          most of them are 
                                          getter and setter
 
I can understand this concern, it seems follow the paterns in Enterprise Application Architecture Pattern. but I can not agree this kinds of design. I believe this is misuse of interface.

First, in this kinds of design, if we add some fields, we need update the actionForm, them also need to update interface.
It is boring, and in this case, the interface can not provide any abstraction so the interface need to evolve as the implementation changed.

Second, there is only one  kind of implementaion in the system, so the interface can not provide the benifit from making use of polymorphism.

In a word, we can get nothing design benefit from Interface in this case, And Have burden to keep the implementaion and interface synchronized.