Date:	Fri, 24 Apr 2009 23:37:06 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Aaron Carroll <aaronc@....unsw.edu.au>,
	Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Reduce latencies for syncronous writes and high I/O priority 
	requests in deadline IO scheduler

On Fri, Apr 24, 2009 at 6:07 PM, Corrado Zoccolo <czoccolo@...il.com> wrote:
> Ok, this is the first patch of the series, and contains code cleanup
> needed before changing read/write to sync/async. No behavioral change
> is introduced by this patch.
>

And this one is the second patch, that changes read/write to sync vs. async.
fsync-tester results,with
     while : ; do time sh -c "dd if=/dev/zero of=bigfile bs=8M
count=256 ; sync; rm bigfile"; done:
running in the background
./fsync-tester
fsync time: 0.6383
fsync time: 0.1835
fsync time: 0.1744
fsync time: 0.1103
fsync time: 0.1535
fsync time: 0.1545
fsync time: 0.1491
fsync time: 0.1524
fsync time: 0.1609
fsync time: 0.1168
fsync time: 0.1458
fsync time: 0.1328
fsync time: 0.1655
fsync time: 0.1731
fsync time: 0.1356
fsync time: 0.1746
fsync time: 0.1166
fsync time: 0.1609
fsync time: 0.1370
fsync time: 0.1379
fsync time: 0.2096
fsync time: 0.1438
fsync time: 0.1652
fsync time: 0.1612
fsync time: 0.1438

compared with original deadline:
fsync time: 0.7963
fsync time: 4.5914
fsync time: 4.2347
fsync time: 1.1670
fsync time: 0.8164
fsync time: 1.9783
fsync time: 4.9726
fsync time: 2.4929
fsync time: 2.5448
fsync time: 3.9627

Usual tio-test, 32 threads 80Mb each:
Tiotest results for 32 concurrent io threads:
,----------------------------------------------------------------------.
| Item                  | Time     | Rate         | Usr CPU  | Sys CPU |
+-----------------------+----------+--------------+----------+---------+
| Write        2560 MBs |  103.6 s |  24.705 MB/s |   9.7 %  | 495.6 % |
| Random Write  125 MBs |   95.9 s |   1.304 MB/s |  -1.9 %  |  23.8 % |
| Read         2560 MBs |  164.3 s |  15.580 MB/s |   3.5 %  |  70.1 % |
| Random Read   125 MBs |  129.7 s |   0.964 MB/s |  -0.0 %  |  16.2 % |
`----------------------------------------------------------------------'
Tiotest latency results:
,-------------------------------------------------------------------------.
| Item         | Average latency | Maximum latency | % >2 sec | % >10 sec |
+--------------+-----------------+-----------------+----------+-----------+
| Write        |        4.040 ms |     8949.999 ms |  0.07614 |   0.00000 |
| Random Write |        1.252 ms |     5439.023 ms |  0.03125 |   0.00000 |
| Read         |        7.920 ms |      792.899 ms |  0.00000 |   0.00000 |
| Random Read  |      123.807 ms |      910.263 ms |  0.00000 |   0.00000 |
|--------------+-----------------+-----------------+----------+-----------|
| Total        |        8.613 ms |     8949.999 ms |  0.03703 |   0.00000 |
`--------------+-----------------+-----------------+----------+-----------'

compared with original deadline:
Tiotest results for 32 concurrent io threads:
,----------------------------------------------------------------------.
| Item                  | Time     | Rate         | Usr CPU  | Sys CPU |
+-----------------------+----------+--------------+----------+---------+
| Write        2560 MBs |  103.0 s |  24.848 MB/s |  10.6 %  | 522.2 % |
| Random Write  125 MBs |   98.8 s |   1.265 MB/s |  -1.6 %  |  16.1 % |
| Read         2560 MBs |  166.2 s |  15.400 MB/s |   4.2 %  |  82.7 % |
| Random Read   125 MBs |  193.3 s |   0.647 MB/s |  -0.8 %  |  14.5 % |
`----------------------------------------------------------------------'
Tiotest latency results:
,-------------------------------------------------------------------------.
| Item         | Average latency | Maximum latency | % >2 sec | % >10 sec |
+--------------+-----------------+-----------------+----------+-----------+
| Write        |        4.122 ms |    17922.920 ms |  0.07980 |   0.00061 |
| Random Write |        0.599 ms |     1245.200 ms |  0.00000 |   0.00000 |
| Read         |        8.032 ms |     1125.759 ms |  0.00000 |   0.00000 |
| Random Read  |      181.968 ms |      972.657 ms |  0.00000 |   0.00000 |
|--------------+-----------------+-----------------+----------+-----------|
| Total        |       10.044 ms |    17922.920 ms |  0.03804 |   0.00029 |
`--------------+-----------------+-----------------+----------+-----------'

We see the improvement on random read (not as pronounced as with full
set of heuristics, but still noticeable) and smaller increase on
random write latency, but this will not harm, since now we have
distinction between sync vs async reads.
I can test which of the other heuristics will provide the other few %
of improvements on random read if you are interested.
Otherwise, I'm more inclined on submitting an other patch to add
io-priorities to this series.

-- 
__________________________________________________________________________

dott. Corrado Zoccolo                          mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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