Re: Kernel 2.6.8.1: swap storm of death - nr_requests > 1024 on swap partition

Karl Vogel
2004-08-30T18:01:19Z

Marcelo Tosatti wrote:

> What is the problem Karl is seeing again? There seem to be several, lets
> separate them
>
> - OOM killer triggering (if there's swap space available and
> "enough" anonymous memory to be swapped out this should not happen).
> One of his complaint on the initial report (about the OOM killer).

Correct. On my 512Mb RAM system with 1Gb swap partition, running a
calloc(1Gb) causes the process to get OOM killed when using CFQ.
The problem is not CFQ as such.. the problem is when nr_requests is too
large (8192 being the default for CFQ).

The same will happen with the default nr_request of 128 which AS uses,
if you use a low memory system. e.g. I booted with mem=128M and then a
calloc(128Mb) can trigger the OOM.

> - Swap cache not freed after test app exists. Should not be a
> problem because such memory will be freed as soon as theres
> pressure, I think.

After the OOM killer killed the calloc() task, the SwapCache still
contains a large chunk of the original allocation. This get's cleared if
there is alot of I/O (example: dd if=/dev/hdX of=/dev/null).

However, without the I/O's it doesn't seem to get freed.. this also
causes a second run of calloc(1Gb) to fail as the SwapCache still
accounts for used memory.

> How can you reproduce that?

It should be reproducable as follows:
- boot with mem=512M
- have a 1Gb swap partition / swapfile (the size doesn't really matter)
- use CFQ or set nr_requests to 8192 on the drive _hosting the swap_
- run 'expunge 1024' (might work the 1st time, if so, run it again)

--- expunge.c program source ---
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[])
{
char *p= calloc(1, atol(argv[1])*1024L*1024L);
if (!p) {
perror("calloc");
exit(1);
}
return 0;
}
--- expunge.c program source ---

Another thing that you can try:
- boot with mem=128M
- have enough swap
- execute: while true; do expunge 128; done

This will trigger an OOM even with AS (nr_requests = 128)

After the OOM, SwapCache still holds part of the allocation.