Table of Contents
I have a openwrt board with 128M ram, for safely upgrade, the free mem should
kept at least 18MB.
So, when I check the free mem is lower than 18MB, I will do the clean cache task:
system("sync"); sleep(1); system("echo 1 > /proc/sys/vm/drop_caches"); system("echo 2 > /proc/sys/vm/drop_caches"); system("echo 3 > /proc/sys/vm/drop_caches"); sleep(1);
but it produce very little effect.
The test result as below:
root@xx:~# free; sync; echo 3 > /proc/sys/vm/drop_caches; free total used free shared buffers Mem: 123776 107948 15828 0 1700 -/+ buffers: 106248 17528 Swap: 0 0 0 total used free shared buffers Mem: 123776 105540 18236 0 204 -/+ buffers: 105336 18440 Swap: 0 0 0
There are still 45588K mem cached, as below:
Mem: 108076K used, 15700K free, 36264K shrd, 1780K buff, 45588K cached
the big files in /tmp (tmpfs) will not be removed by using
sync; echo 3 > /proc/sys/vm/drop_caches
But the size of big files in /tmp will added to the Cached memory value.
3.2 test steps
- check cached mem
root@xx:/tmp# top -bn1 | head -n1o Mem: 106172K used, 17604K free, 26880K shrd, 5644K buff, 39000K cached
- put big file to /tmp
root@xx:/tmp# tftp -gr aa.bin 192.168.1.2
- check cached mem again
root@xx:/tmp# top -bn1 | head -n1 Mem: 120196K used, 3580K free, 40772K shrd, 5644K buff, 52968K cached
Then we know the files in /tmp will increase the cached memory value.
4 base knowledge
When I do command below
root@xx:/tmp# mount | grep /tmp tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime)
I found that tmpfs mounted in /tmp direcoty.
check details from
Tmpfs is a file system which keeps all files in virtual memory.
Everything in tmpfs is temporary in the sense that no files will be created on
your hard drive. If you unmount a tmpfs instance, everything stored therein is
tmpfs puts everything into the kernel internal caches and grows and shrinks to
accommodate the files it contains and is able to swap unneeded pages out to swap
size: The limit of allocated bytes for this tmpfs instance. The default is half
of your physical RAM without swap. If you oversize your tmpfs instances the
machine will deadlock since the OOM handler will not be able to free that memory.
4.2 `cached' in the top command
when I do command below:
root@xx:/tmp# top -bn1 | head -n 1 Mem: 117232K used, 6544K free, 40040K shrd, 4448K buff, 50924K cached
I got the 50924K memory info before `cached'.
Cached memory is memory that Linux uses for disk caching.