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

[Davidlohr Bueso]

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@xxxxxxx> 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.

Thanks,
Davidlohr

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