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Date: Fri, 04 Jan 2013 18:46:43 -0600 From: Simon Jeons <simon.jeons@...il.com> To: Namjae Jeon <linkinjeon@...il.com> Cc: Jan Kara <jack@...e.cz>, 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 On Fri, 2013-01-04 at 16:41 +0900, Namjae Jeon wrote: > 2013/1/4, Simon Jeons <simon.jeons@...il.com>: > > On Thu, 2013-01-03 at 13:35 +0900, Namjae Jeon wrote: > >> 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. > > > > Hi Namjae, > > > > Why use bdi_stat_error here? What's the meaning of its comment "maximal > > error of a stat counter"? > Hi Simon, > > As you know bdi stats (BDI_RECLAIMABLE, BDI_WRITEBACK …) are kept in > percpu counters. > When these percpu counters are incremented/decremented simultaneously > on multiple CPUs by small amount (individual cpu counter less than > threshold BDI_STAT_BATCH), > it is possible that we get approximate value (not exact value) of > these percpu counters. > In order, to handle these percpu counter error we have used > bdi_stat_error. bdi_stat_error is the maximum error which can happen > in percpu bdi stats accounting. > > bdi_stat(bdi, BDI_RECLAIMABLE); > -> This will give approximate value of BDI_RECLAIMABLE by reading > previous value of percpu count. > > bdi_stat_sum(bdi, BDI_RECLAIMABLE); > ->This will give exact value of BDI_RECLAIMABLE. It will take lock > and add current percpu count of individual CPUs. > It is not recommended to use it frequently as it is expensive. We > can better use “bdi_stat” and work with approx value of bdi stats. > Hi Namjae, thanks for your clarify. But why compare error stat count to bdi_bground_thresh? What's the relationship between them? I also see bdi_stat_error compare to bdi_thresh/bdi_dirty in function balance_dirty_pages. > Thanks. > > > >> > >> 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/ . > >> >> >Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> > >> >> > >> > -- > >> > 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/ . > >> Don't email: <a href=mailto:"dont@...ck.org"> email@...ck.org </a> > > > > > > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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