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

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

Hello, Davidlohr.

On Thu, Dec 19, 2013 at 06:31:21PM -0800, Davidlohr Bueso wrote:
> 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@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!
> > 
> > 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 :/

Really thanks for your time to test these patches.
I already did libhugetlbfs test cases and passed it.

> 
> 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.

What I want to be sure is for the first few minutes you mentioned.
If possible, let me know the result like as following link.
https://lkml.org/lkml/2013/7/12/428

Thanks in advance. :)

> > 
> > 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).

Yes, I agreed.

Thanks.

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