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「​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 ​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「​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 - ​How do I free virtual memory in Ubuntu?

  $ cat /proc/sys/vm/swappiness
  60
  $ sudo sysctl vm.swappiness=0
  vm.swappiness = 0
  

• ​20.如何取得記憶體的統計資料(memory statistics)？ 介紹如何寫 Java 記憶體

  public class MemoryStats{
      public static void main(String args[]){
          long totalMem = Runtime.getRuntime().totalMemory();
          long freeMem = Runtime.getRuntime().freeMemory();
          System.out.println("Total memory"+ totalMem);
          System.out.println("Free memory" + freeMem);
      }
  }