Re: [PATCH v3] writeback: Do not sync data dirtied after sync start

[Dave Chinner <david@fromorbit.com>]

On Fri, Sep 27, 2013 at 11:44:40AM +0200, Jan Kara wrote:
> When there are processes heavily creating small files while sync(2) is
> running, it can easily happen that quite some new files are created
> between WB_SYNC_NONE and WB_SYNC_ALL pass of sync(2). That can happen
> especially if there are several busy filesystems (remember that sync
> traverses filesystems sequentially and waits in WB_SYNC_ALL phase on one
> fs before starting it on another fs). Because WB_SYNC_ALL pass is slow
> (e.g. causes a transaction commit and cache flush for each inode in
> ext3), resulting sync(2) times are rather large.
> 
> The following script reproduces the problem:
> 
> function run_writers
> {
>   for (( i = 0; i < 10; i++ )); do
>     mkdir $1/dir$i
>     for (( j = 0; j < 40000; j++ )); do
>       dd if=/dev/zero of=$1/dir$i/$j bs=4k count=4 &>/dev/null
>     done &
>   done
> }
> 
> for dir in "$@"; do
>   run_writers $dir
> done
> 
> sleep 40
> time sync
> ======
> 
> Fix the problem by disregarding inodes dirtied after sync(2) was called
> in the WB_SYNC_ALL pass. To allow for this, sync_inodes_sb() now takes a
> time stamp when sync has started which is used for setting up work for
> flusher threads.
> 
> To give some numbers, when above script is run on two ext4 filesystems on
> simple SATA drive, the average sync time from 10 runs is 267.549 seconds
> with standard deviation 104.799426. With the patched kernel, the average
> sync time from 10 runs is 2.995 seconds with standard deviation 0.096.
> 
> Signed-off-by: Jan Kara <jack@suse.cz>

This version looks good.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
-- 
Dave Chinner
david@fromorbit.com