Openwrt drop cache not work

Dec 13 2017

1. reference
2. background
3. analysis
- 3.1. result
- 3.2. test steps
4. base knowledge
- 4.1. tmpfs
- 4.2. `cached' in the top command

1 reference

2 background

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

3 analysis

3.1 result

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

4.1 tmpfs

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
https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt

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
lost.

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
space.

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.

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