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

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Message-ID: <20150224114503.GH5542@redhat.com>
Date:	Tue, 24 Feb 2015 12:45:03 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Vlastimil Babka <vbabka@...e.cz>
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>,
	"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>
Subject: Re: [RFC 0/6] the big khugepaged redesign

On Tue, Feb 24, 2015 at 12:24:12PM +0100, Andrea Arcangeli wrote:
> I would advise not to make changes for app that are already the
> biggest users ever of hugetlbfs (like Oracle). Those already are
> optimized by other means. THP target are apps that have several

Before somebody risks to misunderstand perhaps I should clarify
further: what I meant is that if the khugepaged boost helps Oracle or
other heavy users of hugetlbfs, but it _hurts_ everything else as I'd
guess, I'd advise against it. Because if an app can deal with
hugetlbfs it's much simpler to optimize by other means and it's not
the primary target of THP so the priority for THP default behavior
should be biased towards those apps that can't easily fit into
hugetlbfs and numa hard pins static placement models.

Of course it'd be perfectly fine to make THP changes that helps even
the biggest hugetlbfs users out there, as long as these changes don't
hurt all other normal use cases (where THP is always guaranteed to
provide a significant performance boost if enabled). Chances are the
benchmarks are also comparing "hugetlbfs+THP" vs "hugetlbfs" without
THP, and not "nothing" vs "THP".

Clearly I'd like to optimize for all apps including the biggest
hugetlbfs users, and this is why I'd like to optimize redis as well,
considering it's simple enough to do it with just one madvise to
change the behavior of COW faults and it'd be guaranteed not to hurt
any other common usage. If we were to instead change the default
behavior of COW faults we'd need first to collect data for a variety
of apps and personally I doubt such a change would be a good universal
tradeoff, while it's a fine change for a behavioral change through
madvise.
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