Re: [PATCH v2] vmscan: limit concurrent reclaimers in shrink_zone

[Minchan Kim]

Hi, Rik.

On Sat, Dec 12, 2009 at 6:46 AM, Rik van Riel <riel@xxxxxxxxxx> wrote:
> Under very heavy multi-process workloads, like AIM7, the VM can
> get into trouble in a variety of ways. ÂThe trouble start when
> there are hundreds, or even thousands of processes active in the
> page reclaim code.
>
> Not only can the system suffer enormous slowdowns because of
> lock contention (and conditional reschedules) between thousands
> of processes in the page reclaim code, but each process will try
> to free up to SWAP_CLUSTER_MAX pages, even when the system already
> has lots of memory free.
>
> It should be possible to avoid both of those issues at once, by
> simply limiting how many processes are active in the page reclaim
> code simultaneously.
>
> If too many processes are active doing page reclaim in one zone,
> simply go to sleep in shrink_zone().
>
> On wakeup, check whether enough memory has been freed already
> before jumping into the page reclaim code ourselves. ÂWe want
> to use the same threshold here that is used in the page allocator
> for deciding whether or not to call the page reclaim code in the
> first place, otherwise some unlucky processes could end up freeing
> memory for the rest of the system.

I am worried about one.

Now, we can put too many processes reclaim_wait with NR_UNINTERRUBTIBLE state.
If OOM happens, OOM will kill many innocent processes since
uninterruptible task
can't handle kill signal until the processes free from reclaim_wait list.

I think reclaim_wait list staying time might be long if VM pressure is heavy.
Is this a exaggeration?

If it is serious problem, how about this?

We add new PF_RECLAIM_BLOCK flag and don't pick the process
in select_bad_process.

-- 
Kind regards,
Minchan Kim
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