Page Cache Trace

~$ free -g
              total        used        free      shared  buff/cache   available
Mem:             15           7           1           1           5           5
Swap:             0           0           0

~$ cat /proc/meminfo | grep Cached
Cached:          3869192 kB
SwapCached:            0 kB

mmap, tmpfs, file, shm 会占用Page Cache. 文件删除时, Page Cache自动回收. 内存紧张时, 同步回收(direct reclaim). 也可通过写入 /proc/sys/vm/drop_caches 手动回收.

mmap占用的Page Cache, 在munmap之前, 无法回收. tmpfs 文件占有的Page Cache, 文件删除之前, 无法回收.

Page Cache是系统级别的. cgroup内, 有自己的cache. 那当不同cgroup进程访问同一个文件, 是单个cgroup统计cache, 还是两边都统计?

pmap, smem用来分析内存使用情况:

~$ smem
  PID User     Command                         Swap      USS      PSS      RSS
 7401 yanyg    /home/yanyg/bin/emacsc -t R        0       96      114     1424
 3112 yanyg    /home/yanyg/bin/emacsc -t          0       96      118     1544
19712 yanyg    cat                                0       88      121     1756
19711 yanyg    cat                                0       88      126     1732
21062 yanyg    sh start.sh                        0      200      203     1436

1 page cache tools

1.1 vmtouch

Github: https://github.com/hoytech/vmtouch

Code: https://github.com/hoytech/vmtouch/blob/master/vmtouch.c

基于 mmap 和 mincore 实现.

mincore() returns a vector that indicates whether pages of the calling process's virtual memory are resident in core (RAM), and so will not cause a disk access (page fault) if referenced. The kernel returns residency information about the pages starting at the address addr, and continuing for length bytes.

example:

~$ git clone https://github.com/hoytech/vmtouch.git
~$ cd vmtouch
~$ make && sudo make install
~$ vmtouch test.img
           Files: 1
     Directories: 0
  Resident Pages: 0/320  0/1M  0%
         Elapsed: 0.000293 seconds
~$ cat test.img >/dev/null
~$ vmtouch test.img
           Files: 1
     Directories: 0
  Resident Pages: 320/320  1M/1M  100%
         Elapsed: 0.0001 seconds
~$ vmtouch /var/ 2>/dev/null
           Files: 15756
     Directories: 686
  Resident Pages: 1162109/1162110  4G/4G  100%
         Elapsed: 0.16369 seconds

Page Cache Trace

目录

1 page cache tools

1.1 vmtouch