lkml.org 
[lkml]   [2010]   [Feb]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patches in this message
/
Date
From
Subject[PATCH] writeback: Fix broken sync writeback
Hi,

There's currently a writeback bug in the 2.6.32 and 2.6.33-rc kernels
that prevent proper writeback when sync(1) is being run. Instead of
flushing everything older than the sync run, it will do chunks of normal
MAX_WRITEBACK_PAGES writeback and restart over and over. This results in
very suboptimal writeback for many files, see the initial report from
Jan Engelhardt:

http://lkml.org/lkml/2010/1/22/382

This fixes it by using the passed in page writeback count, instead of
doing MAX_WRITEBACK_PAGES batches, which gets us much better performance
(Jan reports it's up from ~400KB/sec to 10MB/sec) and makes sync(1)
finish properly even when new pages are being dirted.

Thanks to Jan Kara <jack@suse.cz> for spotting this problem!

Cc: stable@kernel.org
Reported-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

---

I'm sending this out as a patch on the list instead since I'll be going
away for a week skiing very shortly, a mail+patch is easier to discuss
here.

Greg, I'm attaching a 2.6.32 based patch as well, since this one will
not apply to 2.6.32.

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 1a7c42c..55aeea9 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -749,6 +749,8 @@ static long wb_writeback(struct bdi_writeback *wb,
}

for (;;) {
+ long to_write = 0;
+
/*
* Stop writeback when nr_pages has been consumed
*/
@@ -762,12 +764,17 @@ static long wb_writeback(struct bdi_writeback *wb,
if (args->for_background && !over_bground_thresh())
break;

+ if (args->sync_mode == WB_SYNC_ALL)
+ to_write = args->nr_pages;
+ if (!to_write)
+ to_write = MAX_WRITEBACK_PAGES;
+
wbc.more_io = 0;
- wbc.nr_to_write = MAX_WRITEBACK_PAGES;
+ wbc.nr_to_write = to_write;
wbc.pages_skipped = 0;
writeback_inodes_wb(wb, &wbc);
- args->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
- wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+ args->nr_pages -= to_write - wbc.nr_to_write;
+ wrote += to_write - wbc.nr_to_write;

/*
* If we consumed everything, see if we have more
@@ -782,7 +789,7 @@ static long wb_writeback(struct bdi_writeback *wb,
/*
* Did we write something? Try for more
*/
- if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
+ if (wbc.nr_to_write < to_write)
continue;
/*
* Nothing written. Wait for some inode to
--
Jens Axboe

commit 057226ca7447880e4e766a82cf32197e492ba963
Author: Jens Axboe <jens.axboe@oracle.com>
Date: Fri Feb 12 10:14:34 2010 +0100

writeback: Fix broken sync writeback

There's currently a writeback bug in the 2.6.32 and 2.6.33-rc kernels
that prevent proper writeback when sync(1) is being run. Instead of
flushing everything older than the sync run, it will do chunks of normal
MAX_WRITEBACK_PAGES writeback and restart over and over. This results in
very suboptimal writeback for many files, see the initial report from
Jan Engelhardt:

http://lkml.org/lkml/2010/1/22/382

This fixes it by using the passed in page writeback count, instead of
doing MAX_WRITEBACK_PAGES batches, which gets us much better performance
(Jan reports it's up from ~400KB/sec to 10MB/sec) and makes sync(1)
finish properly even when new pages are being dirted.

Thanks to Jan Kara <jack@suse.cz> for spotting this problem!

Cc: stable@kernel.org
Reported-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 9d5360c..8a46c67 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -773,6 +773,8 @@ static long wb_writeback(struct bdi_writeback *wb,
}

for (;;) {
+ long to_write = 0;
+
/*
* Stop writeback when nr_pages has been consumed
*/
@@ -786,13 +788,18 @@ static long wb_writeback(struct bdi_writeback *wb,
if (args->for_background && !over_bground_thresh())
break;

+ if (args->sync_mode == WB_SYNC_ALL)
+ to_write = args->nr_pages;
+ if (!to_write)
+ to_write = MAX_WRITEBACK_PAGES;
+
wbc.more_io = 0;
wbc.encountered_congestion = 0;
- wbc.nr_to_write = MAX_WRITEBACK_PAGES;
+ wbc.nr_to_write = to_write;
wbc.pages_skipped = 0;
writeback_inodes_wb(wb, &wbc);
- args->nr_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
- wrote += MAX_WRITEBACK_PAGES - wbc.nr_to_write;
+ args->nr_pages -= to_write - wbc.nr_to_write;
+ wrote += to_write - wbc.nr_to_write;

/*
* If we consumed everything, see if we have more
@@ -807,7 +814,7 @@ static long wb_writeback(struct bdi_writeback *wb,
/*
* Did we write something? Try for more
*/
- if (wbc.nr_to_write < MAX_WRITEBACK_PAGES)
+ if (wbc.nr_to_write < to_write)
continue;
/*
* Nothing written. Wait for some inode to
\
 
 \ /
  Last update: 2010-02-12 10:19    [W:0.114 / U:0.924 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site