Linux高速缓存使用率调查
Linux的高速缓存pagecache对性能的影响至关重要,但是实际系统中我们的利用率如何呢,特别是具体到每个设备的利用情况。
从下图我们可以很清楚的看到:
我们知道IO请求由vfs发起,经过pagecache缓存,挡不住的就落实到io设备去,那么统计这个利用率就很简单。 我们只要知道挡不住的IO的比例就好了。
我写了个systemtap脚本来解决这个问题:
$ 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 |
global RD_CNT=2, WR_CNT=3 |
if ($ return >0 && devname!= "N/A" ) { |
io_stat[devname, 0] += $ return |
io_stat[devname, RD_CNT] ++; |
blk_stat[devname, rw] += size |
blk_stat[devname, rw+RD_CNT] ++; |
if ($ return >0 && devname!= "N/A" ) { |
io_stat[devname, 1] += $ return |
io_stat[devname, WR_CNT] ++; |
foreach ([devname, action] in io_stat) |
_io[action] += io_stat[devname, action]; |
_blk[action] += blk_stat[devname, action] |
printf ( "\n%-25s, %s%db/%d, %s%db/%d, %s%db/%d, %s%db/%d, %s%d%%\n\n" , |
"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])) |
printf ( "%8s %2s %19s %19s %5s\n" , |
"DEVICE" , "T" , "IO" , "BLK" , "PCMR" ) |
foreach ([devname, action] in io_stat- limit 10) |
printf ( "%8s %2s %11db/%6d %11db/%6d %4d%%\n" , |
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] |
我们来演示下:
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% |
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% |
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% |
参数我们来解释下: IO Read: vfs层面看到的IO读/次数 IO Write: vfs层面看到的IO写/次数 Blk Read: 块层面看到的IO读/次数 Blk Write: 块层面看到的IO写/次数 PCMR: pagecache miss rate =100 * blk / io, 由于文件读写是以page为单位,所以可能>100% 第一行是汇总统计,每隔5秒打印一次。 总结: PCMR越小越好,说明高速缓存利用率高。
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