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SubjectQuestion regarding unexpected OOM kill on 3.8.0, swap stays untouched
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Hi,

I've seen quite a few OOM kills, which all followed a common pattern of swap depletion first. But today I get a reproducible one without major swap use.

Here is vmstat output while kill occurred:

# vmstat 3
procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
r b swpd free buff cache si so bi bo in cs us sy id wa
2 0 8924 10352 18376 162232 0 0 1668 24 627 8948 69 26 0 5
1 0 8924 11624 18848 161680 0 0 2476 1352 713 10533 68 26 0 7
0 1 8924 4028 15948 55080 0 0 33487 24 725 5429 44 30 0 26 < last before kill
1 0 8924 200392 616 5932 11 1 19433 465 331 752 2 11 72 15 < first after kill
0 0 8924 200332 616 6004 0 0 0 0 66 11 0 0 100 0
0 0 8924 200332 624 6004 0 0 0 4 65 11 0 0 98 2

Since swap should not be used so fast, that change in usage goes unnoticed, OOM seems to have happened while swap still available. According to e.g. [1], OOM kill should not happen then.

dmesg output:

[21745.390625] select 1 (init), adj 0, size 108, to kill
[21745.390632] select 1215 (bash), adj 0, size 198, to kill
[21745.390633] select 20178 (bash), adj 0, size 205, to kill
[21745.390634] select 25644 (fakeroot), adj 0, size 370, to kill
[21745.390636] select 25659 (rules), adj 0, size 550, to kill
[21745.390637] select 602 (make), adj 0, size 658, to kill
[21745.390638] select 2711 (bash), adj 0, size 799, to kill
[21745.390639] select 2712 (ld), adj 0, size 48287, to kill
[21745.390639] send sigkill to 2712 (ld), adj 0, size 48287 << the size is always quite similar, ranging 48000-52000, no matter how much swap is available.

Is there a known bug in the 3.8 series or am I missing something relevant? Could it be, that kernel is running short on special pages, but why should simple compiling of linux-kernel deplete those? Could it be, that swapping is to slow to cope with speed of new pages and this also triggers OOM and the cited kernel.org documentation is wrong? Apart from that, the machine is quite strange, it has 256MB RAM and 2.5GB swap and is usually not used for that kind of task. Since virtual swapdisk should be fast, I did not bother to change the VM-parameters just for a single compile run.

Roman

[1] https://www.kernel.org/doc/gorman/html/understand/understand016.html


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