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Date: Mon, 11 Nov 2013 12:47:28 -0800
From: Davidlohr Bueso <davidlohr@...com>
To: Ingo Molnar <mingo@...nel.org>
Cc: Michel Lespinasse <walken@...gle.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Hugh Dickins <hughd@...gle.com>, Mel Gorman <mgorman@...e.de>,
Rik van Riel <riel@...hat.com>,
Guan Xuetao <gxt@...c.pku.edu.cn>,
"Chandramouleeswaran, Aswin" <aswin@...com>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH] mm: cache largest vma
On Mon, 2013-11-11 at 13:04 +0100, Ingo Molnar wrote:
> * Michel Lespinasse <walken@...gle.com> wrote:
>
> > On Sun, Nov 10, 2013 at 8:12 PM, Davidlohr Bueso <davidlohr@...com> wrote:
> > > 2) Oracle Data mining (4K pages)
> > > +------------------------+----------+------------------+---------+
> > > | mmap_cache type | hit-rate | cycles (billion) | stddev |
> > > +------------------------+----------+------------------+---------+
> > > | no mmap_cache | - | 63.35 | 0.20207 |
> > > | current mmap_cache | 65.66% | 19.55 | 0.35019 |
> > > | mmap_cache+largest VMA | 71.53% | 15.84 | 0.26764 |
> > > | 4 element hash table | 70.75% | 15.90 | 0.25586 |
> > > | per-thread mmap_cache | 86.42% | 11.57 | 0.29462 |
> > > +------------------------+----------+------------------+---------+
> > >
> > > This workload sure makes the point of how much we can benefit of
> > > caching the vma, otherwise find_vma() can cost more than 220% extra
> > > cycles. We clearly win here by having a per-thread cache instead of
> > > per address space. I also tried the same workload with 2Mb hugepages
> > > and the results are much more closer to the kernel build, but with the
> > > per-thread vma still winning over the rest of the alternatives.
> > >
> > > All in all I think that we should probably have a per-thread vma
> > > cache. Please let me know if there is some other workload you'd like
> > > me to try out. If folks agree then I can cleanup the patch and send it
> > > out.
> >
> > Per thread cache sounds interesting - with per-mm caches there is a real
> > risk that some modern threaded apps pay the cost of cache updates
> > without seeing much of the benefit. However, how do you cheaply handle
> > invalidations for the per thread cache ?
>
> The cheapest way to handle that would be to have a generation counter for
> the mm and to couple cache validity to a specific value of that.
> 'Invalidation' is then the free side effect of bumping the generation
> counter when a vma is removed/moved.
I was basing the invalidations on the freeing of vm_area_cachep, so I
mark current->mmap_cache = NULL whenever we call
kmem_cache_free(vm_area_cachep, ...). But I can see this being a problem
if more than one task's mmap_cache points to the same vma, as we end up
invalidating only one. I'd really like to use a similar logic and base
everything around the existence of the vma instead of adding a counting
infrastructure. Sure we'd end up doing more reads when we do the lookup
in find_vma() but the cost of maintaining it comes free. I just ran into
a similar idea from 2 years ago:
http://lkml.indiana.edu/hypermail/linux/kernel/1112.1/01352.html
While there are several things that aren't needed, it does do the
is_kmem_cache() to verify that the vma is still a valid slab.
Thanks,
Davidlohr
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