On Thu, 2013-01-03 at 13:35 +0900, Namjae Jeon wrote: > 2013/1/2, Jan Kara <jack@xxxxxxx>: > > 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... > 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"? > > Thanks Jan. > > > > 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 > >> > > >> >-- > >> >To unsubscribe, send a message with 'unsubscribe linux-mm' in > >> >the body to majordomo@xxxxxxxxx. For more info on Linux MM, > >> >see: http://www.linux-mm.org/ . > >> >Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> > >> > > -- > > Jan Kara <jack@xxxxxxx> > > SUSE Labs, CR > > > > -- > To unsubscribe, send a message with 'unsubscribe linux-mm' in > the body to majordomo@xxxxxxxxx. For more info on Linux MM, > see: http://www.linux-mm.org/ . > Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a> -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>