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Linux高速緩存使用率調查

Linux的高速緩存pagecache對性能的影響至關重要,但是實際系統中我們的利用率如何呢,特別是具體到每個設備的利用情況。

  Linux的高速緩存pagecache對性能的影響至關重要,但是實際系統中我們的利用率如何呢,特別是具體到每個設備的利用情況。

  從下圖我們可以很清楚的看到:

  我們知道IO請求由vfs發起,經過pagecache緩存,擋不住的就落實到io設備去,那麼統計這個利用率就很簡單。 我們只要知道擋不住的IO的比例就好了。

  我寫了個systemtap腳本來解決這個問題:

  $ uname -r

  2.6.18-164.el5

  $ sudo rpm -i kernel-debuginfo-common-2.6.18-164.el5.x86_64.rpm

  $ sudo rpm -i kernel-debuginfo-2.6.18-164.el5.x86_64.rpm

  $ cat >pcmr.stp

  global io_stat, blk_stat

  global RD_CNT=2, WR_CNT=3

  probe vfs.read.return {

  if ($return>0 && devname!="N/A") {

  io_stat[devname, 0] += $return

  io_stat[devname, RD_CNT] ++;

  }

  }

  probe ioblock.request {

  blk_stat[devname, rw] += size

  blk_stat[devname, rw+RD_CNT] ++;

  }

  probe vfs.write.return {

  if ($return>0 && devname!="N/A") {

  io_stat[devname, 1] += $return

  io_stat[devname, WR_CNT] ++;

  }

  }

  global _io,_blk

  probe timer.ms(5000) {

  foreach ([devname, action] in io_stat)

  {

  _io[action] += io_stat[devname, action];

  _blk[action] += blk_stat[devname, action]

  }

  if (_io[0] + _io[1]) {

  printf("\\n%-25s, %s%db/%d, %s%db/%d, %s%db/%d, %s%db/%d, %s%d%%\\n\\n",

  ctime(gettimeofday_s()),

  "IO Read:", _io[0], _io[RD_CNT],

  "IO Write:", _io[1],_io[WR_CNT],

  "Blk Read:", _blk[0], _blk[RD_CNT],

  "Blk Write:", _blk[1],_blk[WR_CNT],

  "PCMR:", (_blk[0]+_blk[1]) * 100 / (_io[0] + _io[1]))

  delete _io

  delete _blk

  /* print header */

  printf("%8s %2s %19s %19s %5s\\n",

  "DEVICE","T", "IO", "BLK", "PCMR")

  }

  /* print top ten I/O */

  foreach ([devname, action] in io_stat- limit 10)

  if(action < RD_CNT)

  printf("%8s %2s %11db/%6d %11db/%6d %4d%%\\n",

  devname, action?"W":"R",

  io_stat[devname, action],io_stat[devname, action+RD_CNT],

  blk_stat[devname, action],blk_stat[devname, action+RD_CNT],

  (blk_stat[devname, action] * 100) / io_stat[devname, action]

  )

  /* clear data */

  delete io_stat

  delete blk_stat

  }

  probe begin

  {

  println("::");

  }

  CTRL+D

  我們來演示下:

  $ sudo stap pcmr.stp

  ::

  ...

  Wed Jun 1 09:16:16 2011 , IO Read:551320328b/124024, IO Write:0b/0, Blk Read:103219200b/17536, Blk Write:0b/0, PCMR:18%

  DEVICE T IO BLK PCMR

  fioa R 493284566b/119460 103219200b/ 17536 20%

  sda2 R 41100816b/ 624 0b/ 0 0%

  sda5 R 16934946b/ 3940 0b/ 0 0%

  Wed Jun 1 09:16:21 2011 , IO Read:554808170b/123568, IO Write:2295b/9, Blk Read:101883904b/17296, Blk Write:135168b/33, PCMR:18%

  DEVICE T IO BLK PCMR

  fioa R 491003095b/118908 101543936b/ 17259 20%

  sda3 R 42119596b/ 136 0b/ 0 0%

  sda5 R 17730097b/ 4061 163840b/ 18 0%

  sda1 R 2399374b/ 33 0b/ 0 0%

  sda2 R 1543938b/ 424 176128b/ 19 11%

  sda6 R 12070b/ 6 0b/ 0 0%

  sda3 W 2295b/ 9 135168b/ 33 5889%

  ...

  參數我們來解釋下:

  IO Read: vfs層面看到的IO讀/次數

  IO Write: vfs層面看到的IO寫/次數

  Blk Read: 塊層面看到的IO讀/次數

  Blk Write: 塊層面看到的IO寫/次數

  PCMR: pagecache miss rate =100 * blk / io, 由於文件讀寫是以page為單位,所以可能>100%

  第一行是匯總統計,每隔5秒打印一次。

  總結: PCMR越小越好,說明高速緩存利用率高。

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