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Date:	Thu, 3 Jan 2013 13:35:39 +0900
From:	Namjae Jeon <linkinjeon@...il.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>, fengguang.wu@...el.com,
	linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Namjae Jeon <namjae.jeon@...sung.com>,
	Vivek Trivedi <t.vivek@...sung.com>,
	Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH] writeback: fix writeback cache thrashing

2013/1/2, Jan Kara <jack@...e.cz>:
> 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@...sung.com>
>> >>
>> >> 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...
Hi Jan.
Yes, it can reduce wearing and fragmentation of flash. And also from
one scenario - we
think it might reduce power consumption also.

>> >
>> >> 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.
I agree and will send v2 patch as your suggestion.

Thanks Jan.
>
> 								Honza
>
>> >> Suggested-by: Wanpeng Li <liwanp@...ux.vnet.ibm.com>
>> >> Signed-off-by: Namjae Jeon <namjae.jeon@...sung.com>
>> >> Signed-off-by: Vivek Trivedi <t.vivek@...sung.com>
>> >> Cc: Fengguang Wu <fengguang.wu@...el.com>
>> >> Cc: Jan Kara <jack@...e.cz>
>> >> Cc: Dave Chinner <dchinner@...hat.com>
>> >> ---
>> >>  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@...e.cz>
>> >SUSE Labs, CR
>> >
>> >--
>> >To unsubscribe, send a message with 'unsubscribe linux-mm' in
>> >the body to majordomo@...ck.org.  For more info on Linux MM,
>> >see: http://www.linux-mm.org/ .
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>>
> --
> Jan Kara <jack@...e.cz>
> SUSE Labs, CR
>
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