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Message-ID: <20110929121501.GA19582@localhost>
Date: Thu, 29 Sep 2011 20:15:01 +0800
From: Wu Fengguang <fengguang.wu@...el.com>
To: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc: "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
Dave Chinner <david@...morbit.com>,
Greg Thelen <gthelen@...gle.com>,
Minchan Kim <minchan.kim@...il.com>,
Vivek Goyal <vgoyal@...hat.com>,
Andrea Righi <arighi@...eler.com>,
linux-mm <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 10/18] writeback: dirty position control - bdi reserve
area
On Wed, Sep 28, 2011 at 10:02:05PM +0800, Wu Fengguang wrote:
> Hi Peter,
>
> On Sun, Sep 18, 2011 at 10:47:51PM +0800, Wu Fengguang wrote:
> > > BTW, I also compared the IO-less patchset and the vanilla kernel's
> > > JBOD performance. Basically, the performance is lightly improved
> > > under large memory, and reduced a lot in small memory servers.
> > >
> > > vanillla IO-less
> > > --------------------------------------------------------------------------------
> > [...]
> > > 26508063 17706200 -33.2% JBOD-10HDD-thresh=100M/xfs-100dd-1M-16p-5895M-100M
> > > 23767810 23374918 -1.7% JBOD-10HDD-thresh=100M/xfs-10dd-1M-16p-5895M-100M
> > > 28032891 20659278 -26.3% JBOD-10HDD-thresh=100M/xfs-1dd-1M-16p-5895M-100M
> > > 26049973 22517497 -13.6% JBOD-10HDD-thresh=100M/xfs-2dd-1M-16p-5895M-100M
> > >
> > > There are still some itches in JBOD..
> >
> > OK, in the dirty_bytes=100M case, I find that the bdi threshold _and_
> > writeout bandwidth may drop close to 0 in long periods. This change
> > may avoid one bdi being stuck:
> >
> > /*
> > * bdi reserve area, safeguard against dirty pool underrun and disk idle
> > *
> > * It may push the desired control point of global dirty pages higher
> > * than setpoint. It's not necessary in single-bdi case because a
> > * minimal pool of @freerun dirty pages will already be guaranteed.
> > */
> > - x_intercept = min(write_bw, freerun);
> > + x_intercept = min(write_bw + MIN_WRITEBACK_PAGES, freerun);
>
> After lots of experiments, I end up with this bdi reserve point
>
> + x_intercept = bdi_thresh / 2 + MIN_WRITEBACK_PAGES;
>
> together with this chunk to avoid a bdi stuck in bdi_thresh=0 state:
>
> @@ -590,6 +590,7 @@ static unsigned long bdi_position_ratio(
> */
> if (unlikely(bdi_thresh > thresh))
> bdi_thresh = thresh;
> + bdi_thresh = max(bdi_thresh, (limit - dirty) / 8);
> /*
> * scale global setpoint to bdi's:
> * bdi_setpoint = setpoint * bdi_thresh / thresh
>
> The above changes are good enough to keep reasonable amount of bdi
> dirty pages, so the bdi underrun flag ("[PATCH 11/18] block: add bdi
> flag to indicate risk of io queue underrun") is dropped.
>
> I also tried various bdi freerun patches, however the results are not
> satisfactory. Basically the bdi reserve area approach (this patch)
> yields noticeably more smooth/resilient behavior than the
> freerun/underrun approaches. I noticed that the bdi underrun flag
> could lead to sudden surge of dirty pages (especially if not
> safeguarded by the dirty_exceeded condition) in the very small
> window..
>
> To dig performance increases/drops out of the large number of test
> results, I wrote a convenient script (attached) to compare the
> vmstat:nr_written numbers between 2+ set of test runs. It helped a lot
> for fine tuning the parameters for different cases.
>
> The current JBOD performance numbers are encouraging:
>
> $ ./compare.rb JBOD*/*-vanilla+ JBOD*/*-bgthresh3+
> 3.1.0-rc4-vanilla+ 3.1.0-rc4-bgthresh3+
> ------------------------ ------------------------
> 52934365 +3.2% 54643527 JBOD-10HDD-thresh=100M/ext4-100dd-1M-24p-16384M-100M:10-X
> 45488896 +18.2% 53785605 JBOD-10HDD-thresh=100M/ext4-10dd-1M-24p-16384M-100M:10-X
> 47217534 +12.2% 53001031 JBOD-10HDD-thresh=100M/ext4-1dd-1M-24p-16384M-100M:10-X
> 32286924 +25.4% 40492312 JBOD-10HDD-thresh=100M/xfs-10dd-1M-24p-16384M-100M:10-X
> 38676965 +14.2% 44177606 JBOD-10HDD-thresh=100M/xfs-1dd-1M-24p-16384M-100M:10-X
> 59662173 +11.1% 66269621 JBOD-10HDD-thresh=800M/ext4-10dd-1M-24p-16384M-800M:10-X
> 57510438 +2.3% 58855181 JBOD-10HDD-thresh=800M/ext4-1dd-1M-24p-16384M-800M:10-X
> 63691922 +64.0% 104460352 JBOD-10HDD-thresh=800M/xfs-100dd-1M-24p-16384M-800M:10-X
> 51978567 +16.0% 60298210 JBOD-10HDD-thresh=800M/xfs-10dd-1M-24p-16384M-800M:10-X
> 47641062 +6.4% 50681038 JBOD-10HDD-thresh=800M/xfs-1dd-1M-24p-16384M-800M:10-X
[snip]
I forgot to mention one important change that lead to the increased
JBOD performance: the per-bdi background threshold as in the below
patch.
One thing puzzled me is that in JBOD case, the per-disk writeout
performance is smaller than the corresponding single-disk case even
when they have comparable bdi_thresh. So I wrote the attached tracing
patch and find that in single disk case, bdi_writeback is always kept
high while in JBOD case, it could drop low from time to time and
correspondingly bdi_reclaimable could sometimes rush high.
The fix is to watch bdi_reclaimable and kick background writeback as
soon as it goes high. This resembles the global background threshold
but in per-bdi manner. The trick is, as long as bdi_reclaimable does
not go high, bdi_writeback naturally won't go low because
bdi_reclaimable+bdi_writeback ~= bdi_thresh. With enough writeback
pages, good performance is maintained.
Thanks,
Fengguang
---
--- linux-next.orig/fs/fs-writeback.c 2011-09-25 10:08:43.000000000 +0800
+++ linux-next/fs/fs-writeback.c 2011-09-25 15:36:41.000000000 +0800
@@ -678,14 +678,18 @@ long writeback_inodes_wb(struct bdi_writ
return nr_pages - work.nr_pages;
}
-static inline bool over_bground_thresh(void)
+static bool over_bground_thresh(struct backing_dev_info *bdi)
{
unsigned long background_thresh, dirty_thresh;
global_dirty_limits(&background_thresh, &dirty_thresh);
- return (global_page_state(NR_FILE_DIRTY) +
- global_page_state(NR_UNSTABLE_NFS) > background_thresh);
+ if (global_page_state(NR_FILE_DIRTY) +
+ global_page_state(NR_UNSTABLE_NFS) > background_thresh)
+ return true;
+
+ return bdi_stat(bdi, BDI_RECLAIMABLE) >
+ bdi_dirty_limit(bdi, background_thresh);
}
/*
@@ -747,7 +751,7 @@ static long wb_writeback(struct bdi_writ
* For background writeout, stop when we are below the
* background dirty threshold
*/
- if (work->for_background && !over_bground_thresh())
+ if (work->for_background && !over_bground_thresh(wb->bdi))
break;
if (work->for_kupdate) {
@@ -831,7 +835,7 @@ static unsigned long get_nr_dirty_pages(
static long wb_check_background_flush(struct bdi_writeback *wb)
{
- if (over_bground_thresh()) {
+ if (over_bground_thresh(wb->bdi)) {
struct wb_writeback_work work = {
.nr_pages = LONG_MAX,
View attachment "trace-bdi-dirty-state.patch" of type "text/x-diff" (2123 bytes)
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