Re: [PATCH v3 13/14] mm, hugetlb: retry if failed to allocate and there is concurrent user

[Joonsoo Kim <iamjoonsoo.kim@lge.com>]

On Fri, Jan 03, 2014 at 11:55:45AM -0800, Davidlohr Bueso wrote:
> Hi Joonsoo,
> 
> Sorry about the delay...
> 
> On Mon, 2013-12-23 at 11:11 +0900, Joonsoo Kim wrote:
> > On Mon, Dec 23, 2013 at 09:44:38AM +0900, Joonsoo Kim wrote:
> > > On Fri, Dec 20, 2013 at 10:48:17PM -0800, Davidlohr Bueso wrote:
> > > > On Fri, 2013-12-20 at 14:01 +0000, Mel Gorman wrote:
> > > > > On Thu, Dec 19, 2013 at 05:02:02PM -0800, Andrew Morton wrote:
> > > > > > On Wed, 18 Dec 2013 15:53:59 +0900 Joonsoo Kim <iamjoonsoo.kim@lge.com> wrote:
> > > > > > 
> > > > > > > If parallel fault occur, we can fail to allocate a hugepage,
> > > > > > > because many threads dequeue a hugepage to handle a fault of same address.
> > > > > > > This makes reserved pool shortage just for a little while and this cause
> > > > > > > faulting thread who can get hugepages to get a SIGBUS signal.
> > > > > > > 
> > > > > > > To solve this problem, we already have a nice solution, that is,
> > > > > > > a hugetlb_instantiation_mutex. This blocks other threads to dive into
> > > > > > > a fault handler. This solve the problem clearly, but it introduce
> > > > > > > performance degradation, because it serialize all fault handling.
> > > > > > > 
> > > > > > > Now, I try to remove a hugetlb_instantiation_mutex to get rid of
> > > > > > > performance degradation.
> > > > > > 
> > > > > > So the whole point of the patch is to improve performance, but the
> > > > > > changelog doesn't include any performance measurements!
> > > > > > 
> > > > > 
> > > > > I don't really deal with hugetlbfs any more and I have not examined this
> > > > > series but I remember why I never really cared about this mutex. It wrecks
> > > > > fault scalability but AFAIK fault scalability almost never mattered for
> > > > > workloads using hugetlbfs.  The most common user of hugetlbfs by far is
> > > > > sysv shared memory. The memory is faulted early in the lifetime of the
> > > > > workload and after that it does not matter. At worst, it hurts application
> > > > > startup time but that is still poor motivation for putting a lot of work
> > > > > into removing the mutex.
> > > > 
> > > > Yep, important hugepage workloads initially pound heavily on this lock,
> > > > then it naturally decreases.
> > > > 
> > > > > Microbenchmarks will be able to trigger problems in this area but it'd
> > > > > be important to check if any workload that matters is actually hitting
> > > > > that problem.
> > > > 
> > > > I was thinking of writing one to actually get some numbers for this
> > > > patchset -- I don't know of any benchmark that might stress this lock. 
> > > > 
> > > > However I first measured the amount of cycles it costs to start an
> > > > Oracle DB and things went south with these changes. A simple 'startup
> > > > immediate' calls hugetlb_fault() ~5000 times. For a vanilla kernel, this
> > > > costs ~7.5 billion cycles and with this patchset it goes up to ~27.1
> > > > billion. While there is naturally a fair amount of variation, these
> > > > changes do seem to do more harm than good, at least in real world
> > > > scenarios.
> > > 
> > > Hello,
> > > 
> > > I think that number of cycles is not proper to measure this patchset,
> > > because cycles would be wasted by fault handling failure. Instead, it
> > > targeted improved elapsed time. 
> 
> Fair enough, however the fact of the matter is this approach does en up
> hurting performance. Regarding total startup time, I didn't see hardly
> any differences, with both vanilla and this patchset it takes close to
> 33.5 seconds.
> 
> > Could you tell me how long it
> > > takes to fault all of it's hugepages?
> > > 
> > > Anyway, this order of magnitude still seems a problem. :/
> > > 
> > > I guess that cycles are wasted by zeroing hugepage in fault-path like as
> > > Andrew pointed out.
> > > 
> > > I will send another patches to fix this problem.
> > 
> > Hello, Davidlohr.
> > 
> > Here goes the fix on top of this series.
> 
> ... and with this patch we go from 27 down to 11 billion cycles, so this
> approach still costs more than what we currently have. A perf stat shows
> that an entire 1Gb huge page aware DB startup costs around ~30 billion
> cycles on a vanilla kernel, so the impact of hugetlb_fault() is
> definitely non trivial and IMO worth considering.

Really thanks for your help. :)

> 
> Now, I took my old patchset (https://lkml.org/lkml/2013/7/26/299) for a
> ride and things do look quite better, which is basically what Andrew was
> suggesting previously anyway. With the hash table approach the startup
> time did go down to ~25.1 seconds, which is a nice -24.7% time
> reduction, with hugetlb_fault() consuming roughly 5.3 billion cycles.
> This hash table was on a 80 core system, so since we do the power of two
> round up we end up with 256 entries -- I think we can do better if we
> enlarger further, maybe something like statically 1024, or probably
> better, 8-ish * nr cpus.
> 
> Thoughts? Is there any reason why we cannot go with this instead? Yes,
> we still keep the mutex, but the approach is (1) proven better for
> performance on real world workloads and (2) far less invasive. 

I have no more idea to improve my patches now, so I agree with your approach.
When I reviewed your approach last time, I found one race condition. In that
time, I didn't think of a solution about it. If you resend it, I will review
and re-think about it.

Thanks.

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