Ytdyklly

As part of doing some cold cache timings, I'm trying to free the OS cache. The kernel documentation (retrieved January 2019) says:

drop_caches



Writing to this will cause the kernel to drop clean caches, as well as

reclaimable slab objects like dentries and inodes.  Once dropped, their

memory becomes free.



To free pagecache:

    echo 1 > /proc/sys/vm/drop_caches

To free reclaimable slab objects (includes dentries and inodes):

    echo 2 > /proc/sys/vm/drop_caches

To free slab objects and pagecache:

    echo 3 > /proc/sys/vm/drop_caches



This is a non-destructive operation and will not free any dirty objects.

To increase the number of objects freed by this operation, the user may run

`sync' prior to writing to /proc/sys/vm/drop_caches.  This will minimize the

number of dirty objects on the system and create more candidates to be

dropped.



This file is not a means to control the growth of the various kernel caches

(inodes, dentries, pagecache, etc...)  These objects are automatically

reclaimed by the kernel when memory is needed elsewhere on the system.



Use of this file can cause performance problems.  Since it discards cached

objects, it may cost a significant amount of I/O and CPU to recreate the

dropped objects, especially if they were under heavy use.  Because of this,

use outside of a testing or debugging environment is not recommended.



You may see informational messages in your kernel log when this file is

used:



    cat (1234): drop_caches: 3



These are informational only.  They do not mean that anything is wrong

with your system.  To disable them, echo 4 (bit 3) into drop_caches.

I'm a bit sketchy about the details. Running

echo 3 > /proc/sys/vm/drop_caches

frees pagecache, dentries and inodes. Ok.

So, if I want the system to start caching normally again, do I need to reset it to 0 first? My system has the value currently set to 0, which I assume is the default. Or will it reset on its own? I see at least two possibilities here, and I'm not sure which one is true:

1. echo 3 > /proc/sys/vm/drop_caches frees pagecache, dentries and inodes. The system then immediately starts caching again. I'm not sure what I would expect the value in /proc/sys/vm/drop_caches to do if this is the case. Go back to 0 almost immediately?




2. If /proc/sys/vm/drop_caches is set to 3, the system does not do any memory caching till it is reset to 0.

Which case is true?

It isn't sticky - you just write to the file to make it drop the caches and then it immediately starts caching again.

Basically when you write to that file you aren't really changing a setting, you are issuing a command to the kernel. The kernel acts on that command (by dropping the caches) then carries on as before.