Re: [PATCH/RFC 1/14] Reclaim Scalability: Convert anon_vma lock to read/write lock

[mel@skynet.ie (Mel Gorman)]

On (18/09/07 11:41), KAMEZAWA Hiroyuki didst pronounce:
> On Mon, 17 Sep 2007 12:02:35 +0100
> mel@skynet.ie (Mel Gorman) wrote:
> 
> > On (14/09/07 16:54), Lee Schermerhorn didst pronounce:
> > > [PATCH/RFC] 01/14 Reclaim Scalability:  Convert anon_vma list lock a read/write lock
> > > 
> > > Against 2.6.23-rc4-mm1
> > > 
> > > Make the anon_vma list lock a read/write lock.  Heaviest use of this
> > > lock is in the page_referenced()/try_to_unmap() calls from vmscan
> > > [shrink_page_list()].  These functions can use a read lock to allow
> > > some parallelism for different cpus trying to reclaim pages mapped
> > > via the same set of vmas.
> <snip>
> > In light of what Peter and Linus said about rw-locks being more expensive
> > than spinlocks, we'll need to measure this with some benchmark. The plus
> > side is that this patch can be handled in isolation because it's either a
> > scalability fix or it isn't. It's worth investigating because you say it
> > fixed a real problem where under load the job was able to complete with
> > this patch and live-locked without it.
> >
> > When you decide on a test-case, I can test just this patch and see what
> > results I find.
> > 
> 
> One of the case I can imagine is..
> ==
> 1. Use NUMA.
> 2. create *large* anon_vma and use it with MPOL_INTERLEAVE
> 3. When memory is exhausted (on several nodes), all kswapd on nodes will
>    see one anon_vma->lock.
> ==
> Maybe the worst case.

It certainly sounds like a bad case. Would be very difficult to measure
as part of a test though as latencies in kswapd are not very obvious.

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

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