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. Regards, Wanpeng Li > 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. 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