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List:       linux-mm
Subject:    Re: Kernel 2.6.8.1: swap storm of death - nr_requests > 1024 on swap
From:       Karl Vogel <karl.vogel () pandora ! be>
Date:       2004-08-30 18:01:19
Message-ID: 41336B6F.6050806 () pandora ! be
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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.

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