Re: + mm-thp-avoid-unnecessary-swapin-in-khugepaged.patch added to -mm tree

From: Michal Hocko <mhocko@kernel.org>
To: Minchan Kim <minchan@kernel.org>
Cc: akpm@linux-foundation.org, ebru.akagunduz@gmail.com,
	aarcange@redhat.com, aneesh.kumar@linux.vnet.ibm.com,
	boaz@plexistor.com, gorcunov@openvz.org, hannes@cmpxchg.org,
	hughd@google.com, iamjoonsoo.kim@lge.com,
	kirill.shutemov@linux.intel.com, mgorman@suse.de,
	n-horiguchi@ah.jp.nec.com, riel@redhat.com, rientjes@google.com,
	vbabka@suse.cz, mm-commits@vger.kernel.org, linux-mm@kvack.org
Subject: Re: + mm-thp-avoid-unnecessary-swapin-in-khugepaged.patch added to -mm tree
Date: Fri, 20 May 2016 09:34:32 +0200	[thread overview]
Message-ID: <20160520073432.GE19172@dhcp22.suse.cz> (raw)
In-Reply-To: <20160520072624.GD6808@bbox>

On Fri 20-05-16 16:26:24, Minchan Kim wrote:
> On Fri, May 20, 2016 at 08:39:17AM +0200, Michal Hocko wrote:
> > On Fri 20-05-16 09:21:55, Minchan Kim wrote:
> > [...]
> > > I think other important thing we should consider is how the THP page is likely
> > > to be hot without split in a short time like KSM is doing double checking to
> > > merge stable page. Of course, it wouldn't be easyI to implement but I think
> > > algorithm is based on such *hotness* basically and then consider the number
> > > of max_swap_ptes. IOW, I think we should approach more conservative rather
> > > than optimistic because a page I/O overhead by wrong choice could be bigger
> > > than benefit of a few TLB hit.
> > > If we approach that way, maybe we don't need to detect memory pressure.
> > > 
> > > For that way, how about raising bar for swapin allowance?
> > > I mean now we allows swapin
> > > 
> > > from
> > >         64 pages below swap ptes and 1 page young in 512 ptes 
> > > to
> > >         64 pages below swap ptes and 256 page young in 512 ptes 
> > 
> > I agree that the current 1 page threshold for collapsing is way too
> > optimistic. Same as the defaults we had for the page fault THP faulting
> > which has caused many issues. So I would be all for changing it. I do
> 
> I don't know we should change all but if we change it for THP faulting,
> I believe THP swapin should be more conservative value rather than THP
> faulting because cost of THP swapin collapsing would be heavier.
> 
> > not have good benchmarks to back any "good" number unfortunately. So
> > such a change would be quite arbitrary based on feeling... If you have
> > some workload where collapsing THP pages causes some real issues that
> > would be great justification though.
> 
> Hope to have but our products never have turned on THP. :(
> I just wanted to say current problem and suggestion so a THP guy
> can have an interest on that.
> If it's not worth to do, simple igonore.

I guess this is worth changing I am just not sure about the
justification
 
> > That being said khugepaged_max_ptes_none = HPAGE_PMD_NR/2 sounds like a
> 
> max_ptes_none?

Not sure I understand what you mean here.
-- 
Michal Hocko
SUSE Labs

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