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SubjectRe: out_of_memory() heuristic broken for different mem configurations

On Tue, 6 Nov 2001, Marcelo Tosatti wrote:
>
> Looking at out_of_memory() I've found out that we will only kill a task if
> we happen to call out_of_memory() ten times in one second.

It should be "10 times in at _least_ one second, and at most 5 seconds
apart", but yes, I can imagine that it doesn't work very well if you have
tons of memory.

The problem became more pronounced when we started freeing the swap cache:
it seems that allows the machine to shuffle memory around so efficiently
that I've seen it go for half a minute with vmstat claiming zero IO, and
yet very few out-of-memory messages - I don't know why shrink_cache()
_claims_ success under those circumstances, but I've seen it myself.
Shrink_cache _should_ only return success when it has actually dropped a
page that needs re-loading, but..

> (end of swap)
>
> 23 1 1 4032960 2892 232 7648 16 24246 22 24252 235 997 0 72 28
> 22 1 1 4032960 2872 232 7648 0 0 0 0 106 13 0 99 0
> 23 0 2 4032960 2764 232 7648 0 0 0 0 116 11 0 100 0
> 21 0 1 4032960 2856 232 7648 0 0 0 0 122 45 0 100 0
> 21 0 1 4032960 2848 232 7648 0 0 0 0 117 21 0 100 0
> 21 0 1 4032960 2588 232 7648 38 0 38 0 118 41 0 100 0
> 21 0 1 4032960 2584 232 7648 0 0 0 0 123 10 0 100 0

Note how you also go for seconds with no IO and no shrinking of the
caches, while shrink_cache() is apparently happy (and no, it does not take
several seconds to traverse even a 16GB inactive queue, there's something
else going on)

With the more aggressive max_mapped, the oom failure count could be
dropped to something smaller, as a false positive from shrink_caches
should be fairly rare. I don't think it needs to be tunable on memory
size, I just didn't even try any other values on my machines (I noticed
that the old values were too high once max_mapped was upped and the swap
cache reclaiming was re-done, but I didn't try if five seconds and ten
failures was any better than 10 seconds and five failures, for example)

Linus

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