linux-kernel - Re: [PATCH v2 01/13] mm: support madvise(MADV

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Date:	Wed, 4 Nov 2015 16:29:00 -0500
From:	Daniel Micay <danielmicay@...il.com>
To:	Shaohua Li <shli@...nel.org>, Minchan Kim <minchan@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-api@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Rik van Riel <riel@...hat.com>, Mel Gorman <mgorman@...e.de>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jason Evans <je@...com>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	Michal Hocko <mhocko@...e.cz>, yalin.wang2010@...il.com,
	bmaurer@...com
Subject: Re: [PATCH v2 01/13] mm: support madvise(MADV_FREE)

> That's comparable to Android's pinning / unpinning API for ashmem and I
> think it makes sense if it's faster. It's different than the MADV_FREE
> API though, because the new allocations that are handed out won't have
> the usual lazy commit which MADV_FREE provides. Pages in an allocation
> that's handed out can still be dropped until they are actually written
> to. It's considered active by jemalloc either way, but only a subset of
> the active pages are actually committed. There's probably a use case for
> both of these systems.

Also, consider that MADV_FREE would allow jemalloc to be extremely
aggressive with purging when it actually has to do it. It can start with
the largest span of memory and it can mark more than strictly necessary
to drop below the ratio as there's no cost to using the memory again
(not even a system call).

Since the main cost is using the system call at all, there's going to be
pressure to mark the largest possible spans in one go. It will mean
concentration on memory compaction will improve performance. I think
that's the right direction for the kernel to be guiding userspace. It
will play better with THP than the allocator trying to be very precise
with purging based on aging.

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