Re: [patch] mm: skip rebalance of hopeless zones

[Ying Han <yinghan@google.com>]

On Thu, Dec 9, 2010 at 6:44 AM, Mel Gorman <mel@csn.ul.ie> wrote:
> On Wed, Dec 08, 2010 at 05:23:24PM -0800, Andrew Morton wrote:
>> On Wed, 8 Dec 2010 16:36:21 -0800 Simon Kirby <sim@hostway.ca> wrote:
>>
>> > On Wed, Dec 08, 2010 at 04:16:59PM +0100, Johannes Weiner wrote:
>> >
>> > > Kswapd tries to rebalance zones persistently until their high
>> > > watermarks are restored.
>> > >
>> > > If the amount of unreclaimable pages in a zone makes this impossible
>> > > for reclaim, though, kswapd will end up in a busy loop without a
>> > > chance of reaching its goal.
>> > >
>> > > This behaviour was observed on a virtual machine with a tiny
>> > > Normal-zone that filled up with unreclaimable slab objects.
>> > >
>> > > This patch makes kswapd skip rebalancing on such 'hopeless' zones and
>> > > leaves them to direct reclaim.
>> >
>> > Hi!
>> >
>> > We are experiencing a similar issue, though with a 757 MB Normal zone,
>> > where kswapd tries to rebalance Normal after an order-3 allocation while
>> > page cache allocations (order-0) keep splitting it back up again.  It can
>> > run the whole day like this (SSD storage) without sleeping.
>>
>> People at google have told me they've seen the same thing.  A fork is
>> taking 15 minutes when someone else is doing a dd, because the fork
>> enters direct-reclaim trying for an order-one page.  It successfully
>> frees some order-one pages but before it gets back to allocate one, dd
>> has gone and stolen them, or split them apart.
>>
>
> Is there a known test case for this or should I look at doing a
> streaming-IO test with a basic workload constantly forking in the
> background to measure the fork latency?

We were seeing some system daemons(sshd) being OOM killed while
running in the same
memory container as dd test. I assume we can generate the test case
while running dd on
10G of file in 1G container, at the same time running
unixbench(fork/exec loop)?

--Ying

>
>> This problem would have got worse when slub came along doing its stupid
>> unnecessary high-order allocations.
>>
>> Billions of years ago a direct-reclaimer had a one-deep cache in the
>> task_struct into which it freed the page to prevent it from getting
>> stolen.
>>
>> Later, we took that out because pages were being freed into the
>> per-cpu-pages magazine, which is effectively task-local anyway.  But
>> per-cpu-pages are only for order-0 pages.  See slub stupidity, above.
>>
>> I expect that this is happening so repeatably because the
>> direct-reclaimer is dong a sleep somewhere after freeing the pages it
>> needs - if it wasn't doing that then surely the window wouldn't be wide
>> enough for it to happen so often.  But I didn't look.
>>
>> Suitable fixes might be
>>
>> a) don't go to sleep after the successful direct-reclaim.
>>
>
> I submitted a patch for this a long time ago but at the time we didn't
> have a test case that made a difference to it. Might be worth
> revisiting. I can't find the related patch any more but it was fairly
> trivial.

If you have the patch, maybe we can give a try on our case.

--Ying
>
> --
> Mel Gorman
> Part-time Phd Student                          Linux Technology Center
> University of Limerick                         IBM Dublin Software Lab
>
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