linux-kernel - Re: [PATCH 2/3] mm: make the threshold of enabling THP configurable

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Message-ID: <4E015672.2020407@redhat.com>
Date:	Wed, 22 Jun 2011 10:41:54 +0800
From:	Cong Wang <amwang@...hat.com>
To:	Mel Gorman <mgorman@...e.de>
CC:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mel@....ul.ie>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-mm@...ck.org
Subject: Re: [PATCH 2/3] mm: make the threshold of enabling THP configurable

于 2011年06月21日 17:36, Mel Gorman 写道:
 >
> Fragmentation avoidance benefits from tuning min_free_kbytes to a higher
> value and minimising fragmentation-related problems is crucial if THP is
> to allocate its necessary pages.
>
> THP tunes min_free_kbytes automatically and this value is in part
> related to the number of zones. At 512M on a single node machine, the
> recommended min_free_kbytes is close to 10% of memory which is barely
> tolerable as it is. At 256M, it's 17%, at 128M, it's 34% so tuning the
> value lower has diminishing returns as the performance impact of giving
> up such a high percentage of free memory is not going to be offset by
> reduced TLB misses. Tuning it to a higher value might make some sense
> if the higher min_free_kbytes was a problem but it would be much more
> rational to tune it as a sysctl than making it a compile-time decision.
>

What this patch changed is the check of total memory pages in hugepage_init(),
which I don't think is suitable as a sysctl.

If you mean min_free_kbytes could be tuned as a sysctl, that should be done
in other patch, right? :)

Thanks.
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