linux-kernel - Re: [PATCH v6 0/7] vfs: Non-blockling buffered fs read (page cache only)

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Message-Id: <E1Xwo5O-00GRl4-Cc@intern.SerNet.DE>
Date:	Fri, 5 Dec 2014 09:17:54 +0100
From:	Volker Lendecke <Volker.Lendecke@...Net.DE>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Milosz Tanski <milosz@...in.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@...radead.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	"linux-aio@...ck.org" <linux-aio@...ck.org>,
	Mel Gorman <mgorman@...e.de>, Tejun Heo <tj@...nel.org>,
	Jeff Moyer <jmoyer@...hat.com>, Theodore Ts'o <tytso@....edu>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux API <linux-api@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	linux-arch@...r.kernel.org
Subject: Re: [PATCH v6 0/7] vfs: Non-blockling buffered fs read (page cache
 only)

On Thu, Dec 04, 2014 at 03:11:02PM -0800, Andrew Morton wrote:
> I can see all that, but it's handwaving.  Yes, preadv2() will perform
> better in some circumstances than fincore+pread.  But how much better? 
> Enough to justify this approach, or not?
> 
> Alas, the only way to really settle that is to implement fincore() and
> to subject it to a decent amount of realistic quantitative testing.
> 
> Ho hum.
> 
> Could you please hunt down some libuv developers, see if we can solicit
> some quality input from them?  As I said, we really don't want to merge
> this then find that people don't use it for some reason, or that it
> needs changes.

All I can say from a Samba perspective is that none of the ARM based
Storage boxes I have seen so far do AIO because of the base footprint
for every read. For sequential reads kernel-level readahead could kick
in properly and we should be able to give them the best of both worlds:
No context switches in the default case but also good parallel behaviour
for other workloads.  The most important benchmark for those guys is to
read a DVD image, whether it makes sense or not.

Volker

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