linux-kernel - Re: [PATCH 8/9] readahead: basic support for backwards prefetching

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Date:	Wed, 30 Nov 2011 08:24:25 +0800
From:	Wu Fengguang <fengguang.wu@...el.com>
To:	Pádraig Brady <P@...igBrady.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 8/9] readahead: basic support for backwards prefetching

On Wed, Nov 30, 2011 at 12:37:55AM +0800, Pádraig Brady wrote:
> On 11/29/2011 03:35 PM, Jan Kara wrote:
> >   Someone already mentioned this earlier and I don't think I've seen a
> > response: Do you have a realistic usecase for this? I don't think I've ever
> > seen an application reading file backwards...
> 
> tac, tail -n$large, ...

Indeed!
             tac-4425  [000] 73358.419777: readahead: readahead-random(dev=0:16, ino=1548445, req=750+1, ra=750+1-0, async=0) = 1
             tac-4425  [004] 73358.442030: readahead: readahead-backwards(dev=0:16, ino=1548445, req=748+2, ra=746+5-0, async=0) = 4
             tac-4425  [004] 73358.443312: readahead: readahead-backwards(dev=0:16, ino=1548445, req=744+2, ra=726+25-0, async=0) = 20

            tail-4369  [000] 72633.696307: readahead: readahead-random(dev=0:16, ino=1548450, req=750+1, ra=750+1-0, async=0) = 1
            tail-4369  [004] 72634.042106: readahead: readahead-backwards(dev=0:16, ino=1548450, req=748+2, ra=746+5-0, async=0) = 4
            tail-4369  [004] 72634.043231: readahead: readahead-backwards(dev=0:16, ino=1548450, req=744+2, ra=726+25-0, async=0) = 20
            tail-4369  [004] 72634.176216: readahead: readahead-backwards(dev=0:16, ino=1548450, req=724+2, ra=626+125-0, async=0) = 100

However I see the readahead requests always be snapped to EOF.

So it's obvious the "snap to EOF" logic need some limiting based on
max readahead size.

Thanks,
Fengguang
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