Re: [PATCH] writeback: speed up writeback of big dirty files

[Martin Knoblauch <spamtrap@knobisoft.de>]

---- Original Message ----
> From: Fengguang Wu <wfg@mail.ustc.edu.cn>
> To: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Mike Snitzer <snitzer@gmail.com>; Martin Knoblauch <spamtrap@knobisoft.de>; Peter Zijlstra <peterz@infradead.org>; jplatte@naasa.net; Ingo Molnar <mingo@elte.hu>; linux-kernel@vger.kernel.org; "linux-ext4@vger.kernel.org" <linux-ext4@vger.kernel.org>
> 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