= 2011-02-27 =

== Virtual Memory , HEAP ==

 * 長久以來一直不是很懂 top 指令中，VIRT 、RES 與 SHR 的意義。今天還是來了解一下 manpage
{{{
  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
}}}
{{{
o: VIRT  --  Virtual Image (kb)
   The total amount of virtual memory used by the task.
   It includes all code, data and shared libraries plus
   pages that have been swapped out.

   VIRT = SWAP + RES.

q: RES  --  Resident size (kb)
   The non-swapped physical memory a task has used.

t: SHR  --  Shared Mem size (kb)
   The amount of shared memory used by a task. It
   simply reflects memory that could be potentially
   shared with other processes.

p: SWAP  --  Swapped size (kb)
   The swapped out portion of a task's total virtual
   memory image.
}}}
 * 所以 RES 是目前正在使用中的「實體記憶體（physical memory）」，SHR 是不同程序之間共享的記憶體大小，VIRT 則是「虛擬記憶體（virtual memory）」，包括 CODE, DATA, 共享函式庫與 SWAP。
 * 從「[http://stackoverflow.com/questions/561245/virtual-memory-usage-from-java-under-linux-too-much-memory-used Virtual Memory Usage from Java under Linux, too much memory used]」這篇討論，學到一個指令：pmap，隸屬於 [http://packages.debian.org/procps procps 套件]中。用法是 pmap -x $PID (不同身份時，得透過 sudo 取得記憶體狀態）
{{{
procps: /usr/bin/pmap
}}}
{{{
~$ sudo pmap -x $PID
}}}
{{{
jazz@drbl:~$ sudo pmap 24972
24972:   /usr/bin/php5-cgi
0000000000400000   5144K r-x--  /usr/bin/php5-cgi
0000000000b06000    356K rw---  /usr/bin/php5-cgi
0000000000b5f000     32K rw---    [ anon ]
00000000010da000   2212K rw---    [ anon ]
00007f418a02b000     40K r-x--  /lib/libnss_files-2.7.so
00007f418a035000   2048K -----  /lib/libnss_files-2.7.so
00007f418a235000      8K rw---  /lib/libnss_files-2.7.so
00007f418a237000     28K r-x--  /usr/lib/php5/20060613/pdo_mysql.so
00007f418a23e000   2044K -----  /usr/lib/php5/20060613/pdo_mysql.so
00007f418a43d000      4K rw---  /usr/lib/php5/20060613/pdo_mysql.so
... 略 ...
}}}
 * 根據「[http://serverfault.com/questions/141988/avoid-linux-out-of-memory-application-teardown Avoid linux out-of-memory application teardown]」這篇討論，如果出現虛擬記憶體使用過多，核心開始執行 out-of-memory(OOM) killer 的話，可以使用以下方法關閉：
{{{
   1. Disable the OOM Killer (Put vm.oom-kill = 0 in /etc/sysctl.conf)
   2. Disable memory overcommit (Put vm.overcommit_memory = 2 in /etc/sysctl.conf)
      Note that this is a trinary value: 0 = "estimate if we have enough RAM", 1 = "Always say yes", 2 = "say no if we don't have the memory")
}}}
 * 關於 /proc/sys/vm 底下的檔案 - 說明詳見 vm.txt
{{{
linux-doc: /usr/share/doc/linux-doc/sysctl/vm.txt.gz
}}}
 * 可以透過 sysctl 設定 swap - [http://superuser.com/questions/115565/how-do-i-free-virtual-memory-in-ubuntu How do I free virtual memory in Ubuntu?]
{{{
$ cat /proc/sys/vm/swappiness
60
$ sudo sysctl vm.swappiness=0
vm.swappiness = 0
}}}