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Date: Fri, 4 Jan 2013 16:41:24 +0900 From: Namjae Jeon <linkinjeon@...il.com> To: Simon Jeons <simon.jeons@...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 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. 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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