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Date: Sat, 5 Jan 2013 11:18:17 +0800 From: Fengguang Wu <fengguang.wu@...el.com> To: Namjae Jeon <linkinjeon@...il.com> Cc: linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, liwanp@...ux.vnet.ibm.com, Namjae Jeon <namjae.jeon@...sung.com>, Vivek Trivedi <t.vivek@...sung.com>, Jan Kara <jack@...e.cz>, Dave Chinner <dchinner@...hat.com>, Simon Jeons <simon.jeons@...il.com> Subject: Re: [PATCH] writeback: fix writeback cache thrashing Hi Namjae, On Sun, Dec 30, 2012 at 02:59:50PM +0900, 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 Yeah, that IO pattern is not good. Perhaps it's 6 small IOs in /one/ second? However that's not quite in line with "sleep 2sec" in your workload description. Note that I assume flush-8:0 works on a hard disk, so each flush-8:0 line indicates roughly 1 second interval elapsed. It would be much more clear if the printk timestamps are turned on (CONFIG_PRINTK_TIME=y). > [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. > 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 The below log looks all perfect. However the min_ratio setup is a problem. If possible, I'd like the final patch being able to work reasonably well with min_ratio=0 (the system default), too. > [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. > > 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; > - That global test should be kept in some form (see Jan's proposal). Because the below per-bdi test can be inaccurate in various ways: - bdi_stat() may have errors up to bdi_stat_error() - bdi_dirty_limit() may be arbitrarily shifted by min_ratio etc. - bdi_dirty_limit() may be totally wrong due to the estimation in bdi_writeout_fraction() is in its initial value 0, or is still trying to catch up with sudden workload changes. > if (bdi_stat(bdi, BDI_RECLAIMABLE) > > bdi_dirty_limit(bdi, background_thresh)) > return true; I suspect even removing the global test as in your patch, the above bdi test will still mostly return true for your described workload, due to bdi_dirty_limit() returning a value close to 0, because the writeout fraction of sdb is close to 0. You cleverly avoided this in your test by raising min_ratio to 5. However I'd suggest to test with min_ratio=0 and try solutions that can work well in such default configuration. Thanks, Fengguang -- 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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