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Date: Fri, 20 Apr 2001 01:43:47 -0300 (BRT)
From: Marcelo Tosatti <marcelo@conectiva.com.br>
To: Linus Torvalds <torvalds@transmeta.com>
Cc: lkml <linux-kernel@vger.kernel.org>, Rik van Riel <riel@conectiva.com.br>
Subject: Re: Allocated swap space versus used swap space
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On Fri, 20 Apr 2001, Marcelo Tosatti wrote:

> 
> Hi, 

Argh. Silly. 

Well... 

Right now we can get a task killed by the OOM killer even if there is a
lot of _unused_ (but allocated) swap space. The reason for that is the
pre allocation of swap.

Practical example (128MB swap, 960MB ram): 

# fillmem 960 

...

   procs                      memory    swap          io     system cpu
 r  b  w   swpd   free   buff  cache  si  so    bi    bo   in    cs  us sy
 0  0  0      0 873548   1760   9244   0   0    25    11   32    24   0 1 98
 0  0  0      0 873540   1760   9244   0   0     0     0  103     8   0 0 100
 1  0  0      0 469040   1760   9248   0   0     2     0  221   562   2 17 80
 1  0  1  91980   2068     80  98480   0 362     0   456  277   724   2 26 72
 0  1  2 128516   1560     80 106564  30 24724    44 24724  443  8090   0 9  91
 0  1  0  64840 820804     84  66396 112 22436   148 22438  413  9197   1 13  87
 0  0  0  64840 820416    104  66768   0   0   198     0  119    22   0 0 100
 0  0  0  64840 820416    104  66768   0   0     0     0  102     9   0 0 100

...

Out of Memory: Killed process 763 (fillmem).

Around 60MB was written to swap, so there was unused swap space available,
which means the task should _not_ get killed.

We cannot simply check for "(swapper_space.nrpages > 0)" or count the
number of dirty swap cache pages to avoid the OOM killer from triggering,
obviously.

We can split the swap map in "used" / "allocated" bits (or something which
gives us that information), but that would be painful to do, I guess. 

Comments ?

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