Date: Fri, 16 Apr 2010 16:12:26 +1000
From: Nick Piggin <npiggin@suse.de>
To: Steven Whitehouse <swhiteho@redhat.com>
Cc: Minchan Kim <minchan.kim@gmail.com>, linux-mm@kvack.org,
       linux-kernel@vger.kernel.org
Subject: Re: vmalloc performance
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On Thu, Apr 15, 2010 at 09:33:08AM +0100, Steven Whitehouse wrote:
> Hi,
> 
> On Thu, 2010-04-15 at 01:35 +0900, Minchan Kim wrote:
> > On Thu, 2010-04-15 at 00:13 +0900, Minchan Kim wrote:
> > > On Wed, Apr 14, 2010 at 9:49 PM, Steven Whitehouse <swhiteho@redhat.com> wrote:
> > > >> When this module is run on my x86_64, 8 core, 12 Gb machine, then on an
> > > >> otherwise idle system I get the following results:
> > > >>
> > > >> vmalloc took 148798983 us
> > > >> vmalloc took 151664529 us
> > > >> vmalloc took 152416398 us
> > > >> vmalloc took 151837733 us
> > > >>
> > > >> After applying the two line patch (see the same bz) which disabled the
> > > >> delayed removal of the structures, which appears to be intended to
> > > >> improve performance in the smp case by reducing TLB flushes across cpus,
> > > >> I get the following results:
> > > >>
> > > >> vmalloc took 15363634 us
> > > >> vmalloc took 15358026 us
> > > >> vmalloc took 15240955 us
> > > >> vmalloc took 15402302 us
> > 
> > 
> > > >>
> > > >> So thats a speed up of around 10x, which isn't too bad. The question is
> > > >> whether it is possible to come to a compromise where it is possible to
> > > >> retain the benefits of the delayed TLB flushing code, but reduce the
> > > >> overhead for other users. My two line patch basically disables the delay
> > > >> by forcing a removal on each and every vfree.
> > > >>
> > > >> What is the correct way to fix this I wonder?
> > > >>
> > > >> Steve.
> > > >>
> > 
> > In my case(2 core, mem 2G system), 50300661 vs 11569357. 
> > It improves 4 times. 
> > 
> Looking at the code, it seems that the limit, against which my patch
> removes a test, scales according to the number of cpu cores. So with
> more cores, I'd expect the difference to be greater. I have a feeling
> that the original reporter had a greater number than the 8 of my test
> machine.
> 
> > It would result from larger number of lazy_max_pages.
> > It would prevent many vmap_area freed.
> > So alloc_vmap_area takes long time to find new vmap_area. (ie, lookup
> > rbtree)
> > 
> > How about calling purge_vmap_area_lazy at the middle of loop in
> > alloc_vmap_area if rbtree lookup were long?
> > 
> That may be a good solution - I'm happy to test any patches but my worry
> is that any change here might result in a regression in whatever
> workload the lazy purge code was originally designed to improve. Is
> there any way to test that I wonder?

Ah this is interesting. What we could do is have a "free area cache"
like the user virtual memory allocator has, which basically avoids
restarting the search from scratch.

Or we could perhaps go one better and do a more sophisticated free space
allocator.

Bigger systems will indeed get hurt by increasing flushes so I'd prefer
to avoid that. But that's not a good justification for a slowdown for
small systems. What good is having cake if you can't also eat it? :)


> > BTW, Steve. Is is real issue or some test?
> > I doubt such vmalloc bomb workload is real. 
> 
> Well the answer is both yes and no :-) So this is how I came across the
> issue. I received a report that GFS2 performance had regressed in recent
> kernels in relation to a test which basically fires lots of requests at
> it via NFS. The reporter of this problem gave me two bits of
> information: firstly that by eliminating all readdir calls from the
> test, the regression is never seen and secondly that oprofile showed
> that two functions related to vmalloc (rb_next, find_vmap_area,
> alloc_vmap_area in that order) were taking between them about 60% of the
> total cpu time.

Thanks for tracking this down. I didn't realize GFS2 used vmalloc
extensively. How large are typical vmalloc requests here, can you
tell me? There is a per-cpu virtual memory allocator that is more
scalable than the global one, and would help avoid these problems
too.

XFS is using it at the moment, but we are looking for some more
users of the API so as to get more testing coverage. I was
considering moving vmalloc over to use it (vm_map_ram).

It's still probably a good idea to improve the global allocator
regression first, but that might get you even more performance.

Thanks,
Nick

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