archive mirror
 help / color / mirror / Atom feed
From: Chuck Lever <>
To: Marcelo Tosatti <>
Cc: Linux Kernel Mailing List <>
Subject: [PATCH] 2.4 read ahead never reads the last page in a file
Date: Wed, 3 Dec 2003 18:10:31 -0500 (EST)	[thread overview]
Message-ID: <> (raw)

hi marcelo-

i posted this a while back on fsdevel for comments, but never heard
anything.  i'd like this patch to be included in 2.4.24, as it improves
NFS client read performance by a significant margin, and doesn't appear to
have any negative impact (see fsdevel archives for benchmarks).  this is
against 2.4.23.

generic_file_readahead never reads the last page of a file.  this means
the last page is always read synchronously by do_generic_file_read.
normally this is not an issue, as most local disk reads are fast.
however, this means that the NFS client is never given the opportunity to
coalesce the last page of a file, which must be read in a separate
synchronous read operation.

this is especially honerous if the network round trip time is long, and/or
the workload consists of reading many uncached small files.

i also explored changing the way that generic_file_readahead computes the
index of the last page of the file.  the fix below appears to be the best

diff -X /home/cel/src/linux/dont-diff -Naurp 00-stock/mm/filemap.c 01-readahead1/mm/filemap.c
--- 00-stock/mm/filemap.c	2003-11-28 13:26:21.000000000 -0500
+++ 01-readahead1/mm/filemap.c	2003-12-03 16:46:11.000000000 -0500
@@ -1299,11 +1299,14 @@ static void generic_file_readahead(int r
 	ahead = 0;
 	while (ahead < max_ahead) {
-		ahead ++;
-		if ((raend + ahead) >= end_index)
+		unsigned long ra_index = raend + ahead + 1;
+		if (ra_index > end_index)
-		if (page_cache_read(filp, raend + ahead) < 0)
+		if (page_cache_read(filp, ra_index) < 0)
+		ahead++;
  * If we tried to read ahead some pages,

	- Chuck Lever
corporate:	<cel at netapp dot com>
personal:	<chucklever at bigfoot dot com>

             reply	other threads:[~2003-12-03 23:12 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2003-12-03 23:10 Chuck Lever [this message]
2003-12-05  1:32 ` [PATCH] 2.4 read ahead never reads the last page in a file Andrea Arcangeli
2003-12-05 15:51   ` Marcelo Tosatti
2003-12-05  1:54 ` Mike Fedyk
2003-12-06  8:20 ` Willy Tarreau

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).