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Date: Wed, 22 Apr 2009 23:07:32 +0200
From: Corrado Zoccolo <czoccolo@...il.com>
To: jens.axboe@...cle.com, Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Reduce latencies for syncronous writes and high I/O priority requests
in deadline IO scheduler
Hi,
deadline I/O scheduler currently classifies all I/O requests in only 2
classes, reads (always considered high priority) and writes (always
lower).
The attached patch, intended to reduce latencies for syncronous writes
and high I/O priority requests, introduces more levels of priorities:
* real time reads: highest priority and shortest deadline, can starve
other levels
* syncronous operations (either best effort reads or RT/BE writes),
mid priority, starvation for lower level is prevented as usual
* asyncronous operations (async writes and all IDLE class requests),
lowest priority and longest deadline
The patch also introduces some new heuristics:
* for non-rotational devices, reads (within a given priority level)
are issued in FIFO order, to improve the latency perceived by readers
* minimum batch timespan (time quantum): partners with fifo_batch to
improve throughput, by sending more consecutive requests together. A
given number of requests will not always take the same time (due to
amount of seek needed), therefore fifo_batch must be tuned for worst
cases, while in best cases, having longer batches would give a
throughput boost.
* batch start request is chosen fifo_batch/3 requests before the
expired one, to improve fairness for requests with lower start sector,
that otherwise have higher probability to miss a deadline than
mid-sector requests.
I did few performance comparisons:
* HDD, ext3 partition with data=writeback, tiotest with 32 threads,
each writing 80MB of data
** deadline-original
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 |
`--------------+-----------------+-----------------+----------+-----------'
** cfq (2.6.30-rc2)
Tiotest results for 32 concurrent io threads:
,----------------------------------------------------------------------.
| Item | Time | Rate | Usr CPU | Sys CPU |
+-----------------------+----------+--------------+----------+---------+
| Write 2560 MBs | 132.4 s | 19.342 MB/s | 8.5 % | 400.4 % |
| Random Write 125 MBs | 107.8 s | 1.159 MB/s | -1.6 % | 16.8 % |
| Read 2560 MBs | 107.6 s | 23.788 MB/s | 5.4 % | 95.7 % |
| Random Read 125 MBs | 158.4 s | 0.789 MB/s | 0.9 % | 7.7 % |
`----------------------------------------------------------------------'
Tiotest latency results:
,-------------------------------------------------------------------------.
| Item | Average latency | Maximum latency | % >2 sec | % >10 sec |
+--------------+-----------------+-----------------+----------+-----------+
| Write | 5.362 ms | 21081.012 ms | 0.09811 | 0.00244 |
| Random Write | 23.310 ms | 31865.095 ms | 0.13437 | 0.06250 |
| Read | 5.048 ms | 3694.001 ms | 0.15167 | 0.00000 |
| Random Read | 146.523 ms | 2880.409 ms | 0.52187 | 0.00000 |
|--------------+-----------------+-----------------+----------+-----------|
| Total | 8.916 ms | 31865.095 ms | 0.13435 | 0.00262 |
`--------------+-----------------+-----------------+----------+-----------'
** deadline-patched
Tiotest results for 32 concurrent io threads:
,----------------------------------------------------------------------.
| Item | Time | Rate | Usr CPU | Sys CPU |
+-----------------------+----------+--------------+----------+---------+
| Write 2560 MBs | 105.3 s | 24.301 MB/s | 10.5 % | 514.8 % |
| Random Write 125 MBs | 95.9 s | 1.304 MB/s | -1.8 % | 17.3 % |
| Read 2560 MBs | 165.1 s | 15.507 MB/s | 2.7 % | 61.9 % |
| Random Read 125 MBs | 110.6 s | 1.130 MB/s | 0.8 % | 12.2 % |
`----------------------------------------------------------------------'
Tiotest latency results:
,-------------------------------------------------------------------------.
| Item | Average latency | Maximum latency | % >2 sec | % >10 sec |
+--------------+-----------------+-----------------+----------+-----------+
| Write | 4.131 ms | 17456.831 ms | 0.08041 | 0.00275 |
| Random Write | 2.780 ms | 5073.180 ms | 0.07500 | 0.00000 |
| Read | 7.748 ms | 936.499 ms | 0.00000 | 0.00000 |
| Random Read | 104.849 ms | 695.192 ms | 0.00000 | 0.00000 |
|--------------+-----------------+-----------------+----------+-----------|
| Total | 8.168 ms | 17456.831 ms | 0.04008 | 0.00131 |
`--------------+-----------------+-----------------+----------+-----------'
* SD card, nilfs2 partition, tiotest with 16 threads, each writing 80MB of data
** cfq(2.6.30-rc2)
Tiotest results for 16 concurrent io threads:
,----------------------------------------------------------------------.
| Item | Time | Rate | Usr CPU | Sys CPU |
+-----------------------+----------+--------------+----------+---------+
| Write 1280 MBs | 217.8 s | 5.878 MB/s | 3.7 % | 92.2 % |
| Random Write 62 MBs | 18.2 s | 3.432 MB/s | -2.3 % | 28.7 % |
| Read 1280 MBs | 114.7 s | 11.156 MB/s | 7.3 % | 76.6 % |
| Random Read 62 MBs | 3.4 s | 18.615 MB/s | -5.4 % | 274.2 % |
`----------------------------------------------------------------------'
Tiotest latency results:
,-------------------------------------------------------------------------.
| Item | Average latency | Maximum latency | % >2 sec | % >10 sec |
+--------------+-----------------+-----------------+----------+-----------+
| Write | 9.943 ms | 10223.581 ms | 0.14252 | 0.00488 |
| Random Write | 12.287 ms | 5097.196 ms | 0.25625 | 0.00000 |
| Read | 5.352 ms | 1550.162 ms | 0.00000 | 0.00000 |
| Random Read | 3.051 ms | 1507.837 ms | 0.00000 | 0.00000 |
|--------------+-----------------+-----------------+----------+-----------|
| Total | 7.649 ms | 10223.581 ms | 0.07391 | 0.00233 |
`--------------+-----------------+-----------------+----------+-----------'
** deadline-patched:
Tiotest results for 16 concurrent io threads:
,----------------------------------------------------------------------.
| Item | Time | Rate | Usr CPU | Sys CPU |
+-----------------------+----------+--------------+----------+---------+
| Write 1280 MBs | 220.9 s | 5.794 MB/s | 4.0 % | 93.9 % |
| Random Write 62 MBs | 20.5 s | 3.044 MB/s | -2.2 % | 24.9 % |
| Read 1280 MBs | 113.2 s | 11.304 MB/s | 6.8 % | 72.8 % |
| Random Read 62 MBs | 2.9 s | 21.896 MB/s | 5.1 % | 293.8 % |
`----------------------------------------------------------------------'
Tiotest latency results:
,-------------------------------------------------------------------------.
| Item | Average latency | Maximum latency | % >2 sec | % >10 sec |
+--------------+-----------------+-----------------+----------+-----------+
| Write | 10.078 ms | 13303.036 ms | 0.14160 | 0.00031 |
| Random Write | 14.350 ms | 5265.088 ms | 0.40000 | 0.00000 |
| Read | 5.455 ms | 434.495 ms | 0.00000 | 0.00000 |
| Random Read | 2.685 ms | 12.652 ms | 0.00000 | 0.00000 |
|--------------+-----------------+-----------------+----------+-----------|
| Total | 7.801 ms | 13303.036 ms | 0.07682 | 0.00015 |
`--------------+-----------------+-----------------+----------+-----------'
* fsync-tester results, on HDD, empty ext3 partition, mounted with
data=writeback
** deadline-original:
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
** cfq 2.6.30-rc2
fsync time: 0.0288
fsync time: 0.0528
fsync time: 0.0299
fsync time: 0.0397
fsync time: 0.5720
fsync time: 0.0409
fsync time: 0.0876
fsync time: 0.0294
fsync time: 0.0485
** deadline-patched
fsync time: 0.0772
fsync time: 0.0381
fsync time: 0.0604
fsync time: 0.2923
fsync time: 0.2488
fsync time: 0.0924
fsync time: 0.0144
fsync time: 1.4824
fsync time: 0.0789
fsync time: 0.0565
fsync time: 0.0550
fsync time: 0.0421
** deadline-patched, ionice -c1:
fsync time: 0.2569
fsync time: 0.0500
fsync time: 0.0681
fsync time: 0.2863
fsync time: 0.0140
fsync time: 0.0171
fsync time: 0.1198
fsync time: 0.0530
fsync time: 0.0503
fsync time: 0.0462
fsync time: 0.0484
fsync time: 0.0328
fsync time: 0.0562
fsync time: 0.0451
fsync time: 0.0576
fsync time: 0.0444
fsync time: 0.0469
fsync time: 0.0368
fsync time: 0.2865
Corrado
--
__________________________________________________________________________
dott. Corrado Zoccolo mailto:czoccolo@...il.com
PhD - Department of Computer Science - University of Pisa, Italy
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