Re: [PATCH 10/18] writeback: dirty position control - bdi reservearea

From: Wu Fengguang
Date: Wed Sep 28 2011 - 10:02:15 EST

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

The common single disk cases also see good numbers except for slight
drops in the dirty_bytes=100MB case:

$ ./compare.rb thresh*/*vanilla+ thresh*/*bgthresh3+
3.1.0-rc4-vanilla+ 3.1.0-rc4-bgthresh3+
------------------------ ------------------------
4092719 -2.5% 3988742 thresh=100M/ext4-10dd-4k-8p-4096M-100M:10-X
4956323 -4.0% 4758884 thresh=100M/ext4-1dd-4k-8p-4096M-100M:10-X
4640118 -0.4% 4621240 thresh=100M/ext4-2dd-4k-8p-4096M-100M:10-X
3545136 -3.5% 3420717 thresh=100M/xfs-10dd-4k-8p-4096M-100M:10-X
4399437 -0.9% 4361830 thresh=100M/xfs-1dd-4k-8p-4096M-100M:10-X
4100655 -3.3% 3964043 thresh=100M/xfs-2dd-4k-8p-4096M-100M:10-X
4780624 -0.1% 4776216 thresh=1G/ext4-10dd-4k-8p-4096M-1024M:10-X
4904565 +0.0% 4905293 thresh=1G/ext4-1dd-4k-8p-4096M-1024M:10-X
3578539 +9.1% 3903390 thresh=1G/xfs-10dd-4k-8p-4096M-1024M:10-X
4029890 +0.8% 4063717 thresh=1G/xfs-1dd-4k-8p-4096M-1024M:10-X
2449031 +20.0% 2937926 thresh=1M/ext4-10dd-4k-8p-4096M-1M:10-X
4161896 +7.5% 4472552 thresh=1M/ext4-1dd-4k-8p-4096M-1M:10-X
3437787 +18.8% 4085707 thresh=1M/ext4-2dd-4k-8p-4096M-1M:10-X
1921914 +14.8% 2206897 thresh=1M/xfs-10dd-4k-8p-4096M-1M:10-X
2537481 +65.8% 4207336 thresh=1M/xfs-1dd-4k-8p-4096M-1M:10-X
3329176 +12.3% 3739888 thresh=1M/xfs-2dd-4k-8p-4096M-1M:10-X
4587856 +1.8% 4672501 thresh=400M-300M/ext4-10dd-4k-8p-4096M-400M:300M-X
4883525 +0.0% 4884957 thresh=400M-300M/ext4-1dd-4k-8p-4096M-400M:300M-X
4799105 +2.3% 4907525 thresh=400M-300M/ext4-2dd-4k-8p-4096M-400M:300M-X
3931315 +3.0% 4048277 thresh=400M-300M/xfs-10dd-4k-8p-4096M-400M:300M-X
4238389 +3.9% 4401927 thresh=400M-300M/xfs-1dd-4k-8p-4096M-400M:300M-X
4032798 +2.3% 4123838 thresh=400M-300M/xfs-2dd-4k-8p-4096M-400M:300M-X
2425253 +35.2% 3279302 thresh=8M/ext4-10dd-4k-8p-4096M-8M:10-X
4728506 +2.2% 4834878 thresh=8M/ext4-1dd-4k-8p-4096M-8M:10-X
2782860 +62.1% 4511120 thresh=8M/ext4-2dd-4k-8p-4096M-8M:10-X
1966133 +24.3% 2443874 thresh=8M/xfs-10dd-4k-8p-4096M-8M:10-X
4238402 +1.7% 4308416 thresh=8M/xfs-1dd-4k-8p-4096M-8M:10-X
3299446 +13.3% 3739810 thresh=8M/xfs-2dd-4k-8p-4096M-8M:10-X

Thanks,
Fengguang

Attachment: compare.rb
Description: application/ruby