---- Original Message ----
> From: Fengguang Wu <wfg@xxxxxxxxxxxxxxxx>
> To: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Mike Snitzer <snitzer@xxxxxxxxx>; Martin Knoblauch <spamtrap@xxxxxxxxxxxx>; Peter Zijlstra <peterz@xxxxxxxxxxxxx>; jplatte@xxxxxxxxx; Ingo Molnar <mingo@xxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; "linux-ext4@xxxxxxxxxxxxxxx" <linux-ext4@xxxxxxxxxxxxxxx>
> Sent: Thursday, January 17, 2008 6:28:18 AM
> Subject: [PATCH] writeback: speed up writeback of big dirty files
>
> On Jan 16, 2008 9:15 AM, Martin Knoblauch
>
>
wrote:
> > Fengguang's latest writeback patch applies cleanly, builds, boots
> on
>
2.6.24-rc8.
>
> Linus, if possible, I'd suggest this patch be merged for 2.6.24.
>
> It's a safer version of the reverted patch. It was tested on
> ext2/ext3/jfs/xfs/reiserfs and won't 100% iowait even without the
> other bug fixing patches.
>
> Fengguang
> ---
>
> 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 invocation
> 5s
>
later.
>
> In sync_sb_inodes() we only set more_io on super_blocks we
> actually
>
visited.
> This aviods 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
> Signed-off-by: Fengguang Wu
> ---
> fs/fs-writeback.c | 18 ++++++++++++++++--
> include/linux/writeback.h | 1 +
> mm/page-writeback.c | 9 ++++++---
> 3 files changed, 23 insertions(+), 5 deletions(-)
>
> --- linux.orig/fs/fs-writeback.c
> +++ linux/fs/fs-writeback.c
> @@ -284,7 +284,17 @@ __sync_single_inode(struct inode *inode,
> * 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
> @@ -479,8 +489,12 @@ sync_sb_inodes(struct super_block *sb, s
> 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 */
> }
> --- linux.orig/include/linux/writeback.h
> +++ linux/include/linux/writeback.h
> @@ -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 */
> };
>
> /*
> --- linux.orig/mm/page-writeback.c
> +++ linux/mm/page-writeback.c
> @@ -558,6 +558,7 @@ static void background_writeout(unsigned
> 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;
> @@ -565,8 +566,9 @@ static void background_writeout(unsigned
> 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;
> }
> }
> @@ -631,11 +633,12 @@ 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 */
>
>
>
Hi Fenguang,
sorry for not coming back earlier. I compiled -rc8 with your patch. It boots and works with my test cases. More I cannot say. The performance decrease I see compared to -rc5 has been discussed elsewhere in this thread and is not related to your work.
Cheers
Martin
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