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

[Larry Woodman <lwoodman@redhat.com>]

On Mon, 2009-12-14 at 08:14 -0500, Christoph Hellwig wrote:
> On Thu, Dec 10, 2009 at 06:56:26PM -0500, Rik van Riel wrote:
> > Under very heavy multi-process workloads, like AIM7, the VM can
> > get into trouble in a variety of ways.  The 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.  In Larry's case, this resulted in over
> > 6000 processes fighting over locks in the page reclaim code, even
> > though the system already had 1.5GB of free memory.
> >
> > 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.
> > 
> 
> This sounds like a very good argument against using direct reclaim at
> all.  It reminds a bit of the issue we had in XFS with lots of processes
> pushing the AIL and causing massive slowdowns due to lock contention
> and cacheline bonucing.  Moving all the AIL pushing into a dedicated
> thread solved that nicely.  In the VM we already have that dedicated
> per-node kswapd thread, so moving off as much as possible work to
> should be equivalent.

Some of the new systems have 16 CPUs per-node.

> 
> Of course any of this kind of tuning really requires a lot of testing
> and benchrmarking to verify those assumptions.
> 

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