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writeback: speed up writeback of big dirty files 

After making dirty a 100M file, the normal behavior is to start the
writeback for all data after 30s delays.  But sometimes the following
happens instead:

	- after 30s:    ~4M
	- after 5s:     ~4M
	- after 5s:     all remaining 92M

Some analyze shows that the internal io dispatch queues goes like this:

		s_io            s_more_io
		-------------------------
	1)	100M,1K         0
	2)	1K              96M
	3)	0               96M
1) initial state with a 100M file and a 1K file

2) 4M written, nr_to_write <= 0, so write more

3) 1K written, nr_to_write > 0, no more writes(BUG)

nr_to_write > 0 in (3) fools the upper layer to think that data have all
been written out.  The big dirty file is actually still sitting in
s_more_io.  We cannot simply splice s_more_io back to s_io as soon as s_io
becomes empty, and let the loop in generic_sync_sb_inodes() continue: this
may starve newly expired inodes in s_dirty.  It is also not an option to
draw inodes from both s_more_io and s_dirty, an let the loop go on: this
might lead to live locks, and might also starve other superblocks in sync
time(well kupdate may still starve some superblocks, that's another bug).

We have to return when a full scan of s_io completes.  So nr_to_write > 0
does not necessarily mean that "all data are written".  This patch
introduces a flag writeback_control.more_io to indicate that more io should
be done.  With it the big dirty file no longer has to wait for the next
kupdate invokation 5s later.

In sync_sb_inodes() we only set more_io on super_blocks we actually
visited.  This avoids the interaction between two pdflush deamons.

Also in __sync_single_inode() we don't blindly keep requeuing the io if the
filesystem cannot progress.  Failing to do so may lead to 100% iowait.

Tested-by: Mike Snitzer <snitzer@gmail.com>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Michael Rubin <mrubin@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Fengguang Wu 2008-02-04 22:29:36 -08:00 committed by Linus Torvalds
parent a322f8ab66
commit 8bc3be2751
3 changed files with 23 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

18
fs/fs-writeback.c
View File

@ -284,7 +284,17 @@ __sync_single_inode(struct inode *inode, struct writeback_control *wbc)
* soon as the queue becomes uncongested.
*/
inode->i_state |= I_DIRTY_PAGES;
requeue_io(inode);
if (wbc->nr_to_write <= 0) {
/*
* slice used up: queue for next turn
*/
requeue_io(inode);
} else {
/*
* somehow blocked: retry later
*/
redirty_tail(inode);
}
} else {
/*
* Otherwise fully redirty the inode so that
@ -468,8 +478,12 @@ sync_sb_inodes(struct super_block *sb, struct writeback_control *wbc)
iput(inode);
cond_resched();
spin_lock(&inode_lock);
if (wbc->nr_to_write <= 0)
if (wbc->nr_to_write <= 0) {
wbc->more_io = 1;
break;
}
if (!list_empty(&sb->s_more_io))
wbc->more_io = 1;
}
return; /* Leave any unwritten inodes on s_io */
}

1
include/linux/writeback.h
View File

@ -62,6 +62,7 @@ struct writeback_control {
unsigned for_reclaim:1; /* Invoked from the page allocator */
unsigned for_writepages:1; /* This is a writepages() call */
unsigned range_cyclic:1; /* range_start is cyclic */
unsigned more_io:1; /* more io to be dispatched */
};
/*

9
mm/page-writeback.c
View File

@ -567,6 +567,7 @@ static void background_writeout(unsigned long _min_pages)
global_page_state(NR_UNSTABLE_NFS) < background_thresh
&& min_pages <= 0)
break;
wbc.more_io = 0;
wbc.encountered_congestion = 0;
wbc.nr_to_write = MAX_WRITEBACK_PAGES;
wbc.pages_skipped = 0;
@ -574,8 +575,9 @@ static void background_writeout(unsigned long _min_pages)
min_pages -= MAX_WRITEBACK_PAGES - wbc.nr_to_write;
if (wbc.nr_to_write > 0 || wbc.pages_skipped > 0) {
/* Wrote less than expected */
congestion_wait(WRITE, HZ/10);
if (!wbc.encountered_congestion)
if (wbc.encountered_congestion || wbc.more_io)
congestion_wait(WRITE, HZ/10);
else
break;
}
}
@ -640,11 +642,12 @@ static void wb_kupdate(unsigned long arg)
global_page_state(NR_UNSTABLE_NFS) +
(inodes_stat.nr_inodes - inodes_stat.nr_unused);
while (nr_to_write > 0) {
wbc.more_io = 0;
wbc.encountered_congestion = 0;
wbc.nr_to_write = MAX_WRITEBACK_PAGES;
writeback_inodes(&wbc);
if (wbc.nr_to_write > 0) {
if (wbc.encountered_congestion)
if (wbc.encountered_congestion || wbc.more_io)
congestion_wait(WRITE, HZ/10);
else
break; /* All the old data is written */