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SubjectRe: 2.4.16 & OOM killer screw up (fwd)
Marcelo Tosatti wrote:
> Andrea,
> Could you please start looking at any 2.4 VM issues which show up ?

Just fwiw, I did some testing on this yesterday.

Buffers and cache data are sitting on the active list, and shrink_caches()
is *not* getting them off the active list, and onto the inactive list
where they can be freed.

So we end up with enormous amounts of anon memory on the inactive
list, so this code:

/* try to keep the active list 2/3 of the size of the cache */
ratio = (unsigned long) nr_pages * nr_active_pages / ((nr_inactive_pages + 1) * 2);

just calls refill_inactive(0) all the time. Nothing gets moved
onto the inactive list - it remains full of unfreeable anon
allocations. And with no swap, there's nowhere to go.

I think a little fix is to add

if (ratio < nr_pages)
ratio = nr_pages;

so we at least move *something* onto the inactive list.

Also refill_inactive needs to be changed so that it counts
the number of pages which it actually moved, rather than
the number of pages which it inspected.

In my swapless testing, I burnt HUGE amounts of CPU in flush_tlb_others().
So we're madly trying to swap pages out and finding that there's no swap
space. I beleive that when we find there's no swap left we should move
the page onto the active list so we don't keep rescanning it pointlessly.

A fix may be to just remove the use-once stuff. It is one of the
sources of this problem, because it's overpopulating the inactive list.

In my testing last night, I tried to allocate 650 megs on a 768 meg
swapless box. Got oom-killed when there was almost 100 megs of freeable
memory: half buffercache, half filecache. Presumably, all of it was
stuck on the active list with no way to get off.

We also need to do something about shrink_[di]cache_memory(),
which seem to be called in the wrong place.

There's also the report concerning modify_ldt() failure in a
similar situation. I'm not sure why this one occurred. It
vmallocs 64k of memory and that seems to fail.

I did some similar testing a week or so ago, also tested
the -aa patches. They seemed to maybe help a tiny bit,
but not significantly.

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