Re: vmalloc performance

[Minchan Kim <minchan.kim@gmail.com>]

On Fri, Apr 16, 2010 at 3:12 PM, Nick Piggin <npiggin@suse.de> wrote:
> 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.


AFAIR, vmalloc's performance regression is first. I am not sure
whoever suffers from it and
didn't report. Anyway, with fist report, complicated allocator
implement is rather overkill, I think.

So I votes free_area_cache.

Early ending of lookup from last cache point makes overflow fast and
it results in flush.
I think it's good in that it doesn't depends on system resource environment.
And it could improve search time than one from scratch unless it's
very unfortunate.

>
> 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? :)

Indeed. :)

-- 
Kind regards,
Minchan Kim

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