From: david@lang.hm
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Olivier Galibert <galibert@pobox.com>,
Wu Fengguang <fengguang.wu@intel.com>,
Andrew Morton <akpm@linux-foundation.org>,
Jens Axboe <jens.axboe@oracle.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Linux Memory Management List <linux-mm@kvack.org>,
linux-fsdevel@vger.kernel.org,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 10/11] readahead: dont do start-of-file readahead after lseek()
Date: Tue, 2 Feb 2010 11:59:03 -0800 (PST) [thread overview]
Message-ID: <alpine.DEB.2.00.1002021157280.3707@asgard.lang.hm> (raw)
In-Reply-To: <alpine.LFD.2.00.1002021111240.3664@localhost.localdomain>
On Tue, 2 Feb 2010, Linus Torvalds wrote:
> Rememebr: read-ahead is about filling the empty IO spaces _between_ reads,
> and turning many smaller reads into one bigger one. If you only have a
> single big read, read-ahead cannot help.
>
> Also, keep in mind that read-ahead is not always a win. It can be a huge
> loss too. Which is why we have _heuristics_. They fundamentally cannot
> catch every case, but what they aim for is to do a good job on average.
as a note from the field, I just had an application that needed to be
changed because it did excessive read-ahead. it turned a 2 min reporting
run into a 20 min reporting run because for this report the access was
really random and the app forced large read-ahead.
David Lang
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next prev parent reply other threads:[~2010-02-02 20:00 UTC|newest]
Thread overview: 35+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-02-02 15:28 [PATCH 00/11] [RFC] 512K readahead size with thrashing safe readahead Wu Fengguang
2010-02-02 15:28 ` [PATCH 01/11] readahead: limit readahead size for small devices Wu Fengguang
2010-02-02 19:38 ` Jens Axboe
2010-02-03 6:13 ` Wu Fengguang
2010-02-03 8:23 ` Jens Axboe
2010-02-04 8:24 ` Clemens Ladisch
2010-02-04 13:00 ` Wu Fengguang
2010-02-02 15:28 ` [PATCH 02/11] readahead: bump up the default readahead size Wu Fengguang
2010-02-02 15:28 ` [PATCH 03/11] readahead: introduce {MAX|MIN}_READAHEAD_PAGES macros for ease of use Wu Fengguang
2010-02-02 15:28 ` [PATCH 04/11] readahead: replace ra->mmap_miss with ra->ra_flags Wu Fengguang
2010-02-02 15:28 ` [PATCH 05/11] readahead: retain inactive lru pages to be accessed soon Wu Fengguang
2010-02-02 15:28 ` [PATCH 06/11] readahead: thrashing safe context readahead Wu Fengguang
2010-02-02 15:28 ` [PATCH 07/11] readahead: record readahead patterns Wu Fengguang
2010-02-02 15:28 ` [PATCH 08/11] readahead: add tracing event Wu Fengguang
2010-02-12 16:19 ` Steven Rostedt
2010-02-14 3:56 ` Wu Fengguang
2010-02-02 15:28 ` [PATCH 09/11] readahead: add /debug/readahead/stats Wu Fengguang
2010-02-02 15:28 ` [PATCH 10/11] readahead: dont do start-of-file readahead after lseek() Wu Fengguang
2010-02-02 17:39 ` Linus Torvalds
2010-02-02 18:13 ` Olivier Galibert
2010-02-02 18:40 ` Linus Torvalds
2010-02-02 18:48 ` Olivier Galibert
2010-02-02 19:14 ` Linus Torvalds
2010-02-02 19:59 ` david [this message]
2010-02-02 20:22 ` Linus Torvalds
2010-02-02 15:28 ` [PATCH 11/11] radixtree: speed up next/prev hole search Wu Fengguang
2010-02-02 22:38 ` [PATCH 00/11] [RFC] 512K readahead size with thrashing safe readahead Vivek Goyal
2010-02-02 23:17 ` Vivek Goyal
2010-02-03 6:27 ` Wu Fengguang
2010-02-03 15:24 ` Vivek Goyal
2010-02-03 15:58 ` Vivek Goyal
2010-02-04 13:21 ` Wu Fengguang
2010-02-04 15:52 ` Vivek Goyal
2010-02-04 13:44 ` Wu Fengguang
2010-02-07 4:10 [PATCH 00/11] " Wu Fengguang
2010-02-07 4:10 ` [PATCH 10/11] readahead: dont do start-of-file readahead after lseek() Wu Fengguang
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