Re: [PATCH] writeback: fix writeback cache thrashing

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On Tue 01-01-13 08:51:04, Wanpeng Li wrote:
> On Mon, Dec 31, 2012 at 12:30:54PM +0100, Jan Kara wrote:
> >On Sun 30-12-12 14:59:50, Namjae Jeon wrote:
> >> From: Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
> >> 
> >> Consider Process A: huge I/O on sda
> >>         doing heavy write operation - dirty memory becomes more
> >>         than dirty_background_ratio
> >>         on HDD - flusher thread flush-8:0
> >> 
> >> Consider Process B: small I/O on sdb
> >>         doing while [1]; read 1024K + rewrite 1024K + sleep 2sec
> >>         on Flash device - flusher thread flush-8:16
> >> 
> >> As Process A is a heavy dirtier, dirty memory becomes more
> >> than dirty_background_thresh. Due to this, below check becomes
> >> true(checking global_page_state in over_bground_thresh)
> >> for all bdi devices(even for very small dirtied bdi - sdb):
> >> 
> >> In this case, even small cached data on 'sdb' is forced to flush
> >> and writeback cache thrashing happens.
> >> 
> >> When we added debug prints inside above 'if' condition and ran
> >> above Process A(heavy dirtier on bdi with flush-8:0) and
> >> Process B(1024K frequent read/rewrite on bdi with flush-8:16)
> >> we got below prints:
> >> 
> >> [Test setup: ARM dual core CPU, 512 MB RAM]
> >> 
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56064 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56704 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 84720 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 94720 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   384 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   960 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =    64 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92160 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   256 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   768 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =    64 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   256 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   320 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =     0 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92032 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 91968 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   192 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =  1024 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =    64 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   192 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   576 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =     0 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 84352 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   192 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =   512 KB
> >> [over_bground_thresh]: wakeup flush-8:16 : BDI_RECLAIMABLE =     0 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92608 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE = 92544 KB
> >> 
> >> As mentioned in above log, when global dirty memory > global background_thresh
> >> small cached data is also forced to flush by flush-8:16.
> >> 
> >> If removing global background_thresh checking code, we can reduce cache
> >> thrashing of frequently used small data.
> >  It's not completely clear to me:
> >  Why is this a problem? Wearing of the flash? Power consumption? I'd like
> >to understand this before changing the code...
> >
> >> And It will be great if we can reserve a portion of writeback cache using
> >> min_ratio.
> >> 
> >> After applying patch:
> >> $ echo 5 > /sys/block/sdb/bdi/min_ratio
> >> $ cat /sys/block/sdb/bdi/min_ratio
> >> 5
> >> 
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56064 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  56704 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  84160 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  96960 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  94080 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93120 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93120 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  91520 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  89600 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93696 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  93696 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  72960 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  90624 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  90624 KB
> >> [over_bground_thresh]: wakeup flush-8:0 : BDI_RECLAIMABLE =  90688 KB
> >> 
> >> As mentioned in the above logs, once cache is reserved for Process B,
> >> and patch is applied there is less writeback cache thrashing on sdb
> >> by frequent forced writeback by flush-8:16 in over_bground_thresh.
> >> 
> >> After all, small cached data will be flushed by periodic writeback
> >> once every dirty_writeback_interval.
> >  OK, in principle something like this makes sence to me. But if there are
> >more BDIs which are roughly equally used, it could happen none of them are
> >over threshold due to percpu counter & rounding errors. So I'd rather
> >change the conditions to something like:
> >	reclaimable = bdi_stat(bdi, BDI_RECLAIMABLE);
> >	bdi_bground_thresh = bdi_dirty_limit(bdi, background_thresh);
> >
> >  	if (reclaimable > bdi_bground_thresh)
> >		return true;
> >	/*
> >	 * If global background limit is exceeded, kick the writeback on
> >	 * BDI if there's a reasonable amount of data to write (at least
> >	 * 1/2 of BDI's background dirty limit).
> >	 */
> >	if (global_page_state(NR_FILE_DIRTY) +
> >	    global_page_state(NR_UNSTABLE_NFS) > background_thresh &&
> >	    reclaimable * 2 > bdi_bground_thresh)
> >		return true;
> >
> 
> Hi Jan,
> 
> If there are enough BDIs and percpu counter of each bdi roughly equally
> used less than 1/2 of BDI's background dirty limit, still nothing will 
> be flushed even if over global background_thresh.
  Yes, although then the percpu counter error would have to be quite big.
Anyway, we can change the last condition to:
     if (global_page_state(NR_FILE_DIRTY) +
         global_page_state(NR_UNSTABLE_NFS) > background_thresh &&
         reclaimable * 2 + bdi_stat_error(bdi) * 2 > bdi_bground_thresh)

  That should be safe and for machines with resonable number of CPUs it
should save the wakeup as well.

								Honza

> >> Suggested-by: Wanpeng Li <liwanp@xxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Namjae Jeon <namjae.jeon@xxxxxxxxxxx>
> >> Signed-off-by: Vivek Trivedi <t.vivek@xxxxxxxxxxx>
> >> Cc: Fengguang Wu <fengguang.wu@xxxxxxxxx>
> >> Cc: Jan Kara <jack@xxxxxxx>
> >> Cc: Dave Chinner <dchinner@xxxxxxxxxx>
> >> ---
> >>  fs/fs-writeback.c |    4 ----
> >>  1 file changed, 4 deletions(-)
> >> 
> >> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> >> index 310972b..070b773 100644
> >> --- a/fs/fs-writeback.c
> >> +++ b/fs/fs-writeback.c
> >> @@ -756,10 +756,6 @@ static bool over_bground_thresh(struct backing_dev_info *bdi)
> >>  
> >>  	global_dirty_limits(&background_thresh, &dirty_thresh);
> >>  
> >> -	if (global_page_state(NR_FILE_DIRTY) +
> >> -	    global_page_state(NR_UNSTABLE_NFS) > background_thresh)
> >> -		return true;
> >> -
> >>  	if (bdi_stat(bdi, BDI_RECLAIMABLE) >
> >>  				bdi_dirty_limit(bdi, background_thresh))
> >>  		return true;
> >> -- 
> >> 1.7.9.5
> >> 
> >-- 
> >Jan Kara <jack@xxxxxxx>
> >SUSE Labs, CR
> >
> >--
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> 
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR

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