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

[Davidlohr Bueso]

On Thu, 2013-12-19 at 17:02 -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!
> 
> Please, run some quantitative tests and include a nice summary of the
> results in the changelog.

I was actually spending this afternoon testing these patches with Oracle
(I haven't seen any issues so far) and unless Joonsoo already did so, I
want to run these by the libhugetlb test cases - I got side tracked by
futexes though :/

Please do consider that performance wise I haven't seen much in
particular. The thing is, I started dealing with this mutex once I
noticed it as the #1 hot lock in Oracle DB starts, but then once the
faults are done, it really goes away. So I wouldn't say that the mutex
is a bottleneck except for the first few minutes.

> 
> This is terribly important, because if the performance benefit is
> infinitesimally small or negative, the patch goes into the bit bucket ;)

Well, this mutex is infinitesimally ugly and needs to die (as long as
performance isn't hurt).

Thanks,
Davidlohr

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