linux-kernel - Re: [RFC 0/6] the big khugepaged redesign

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Message-ID: <54F88BD4.3090006@suse.cz>
Date:	Thu, 05 Mar 2015 18:01:08 +0100
From:	Vlastimil Babka <vbabka@...e.cz>
To:	Andres Freund <andres@...razel.de>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Davidlohr Bueso <dave@...olabs.net>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	Michal Hocko <mhocko@...e.cz>,
	Ebru Akagunduz <ebru.akagunduz@...il.com>,
	Alex Thorlton <athorlton@....com>,
	David Rientjes <rientjes@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Robert Haas <robertmhaas@...il.com>,
	Josh Berkus <josh@...iodbs.com>
Subject: Re: [RFC 0/6] the big khugepaged redesign

On 03/05/2015 05:52 PM, Andres Freund wrote:
> Hi,
> 
> On 2015-03-05 17:30:16 +0100, Vlastimil Babka wrote:
>> That however means the workload is based on hugetlbfs and shouldn't trigger THP
>> page fault activity, which is the aim of this patchset. Some more googling made
>> me recall that last LSF/MM, postgresql people mentioned THP issues and pointed
>> at compaction. See http://lwn.net/Articles/591723/ That's exactly where this
>> patchset should help, but I obviously won't be able to measure this before LSF/MM...
>> 
>> I'm CCing the psql guys from last year LSF/MM - do you have any insight about
>> psql performance with THPs enabled/disabled on recent kernels, where e.g.
>> compaction is no longer synchronous for THP page faults?
> 
> What exactly counts as "recent" in this context? Most of the bigger
> installations where we found THP to be absolutely prohibitive (slowdowns
> on the order of a magnitude, huge latency spikes) unfortunately run
> quite old kernels...  I guess 3.11 does *not* count :/? That'd be a

Yeah that's too old :/ 3.17 has patches to make compaction less aggressive on
THP page faults, and 3.18 prevents khugepaged from holding mmap_sem during
compaction, which could be also relevant.

> bigger machine where I could relatively quickly reenable THP to check
> whether it's still bad. I might be able to trigger it to be rebooted
> onto a newer kernel, will ask.

Thanks, that would be great, if you could do that.
I also noticed that you now support hugetlbfs. That could be also interesting
data point, if the hugetlbfs usage helped because THP code wouldn't trigger.

Vlastimil

> Greetings,
> 
> Andres Freund
> 

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