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Date: Tue, 24 Nov 2009 08:11:33 -0500
From: "Alan D. Brunelle" <Alan.Brunelle@...com>
To: linux-kernel@...r.kernel.org
Cc: Jens Axboe <jens.axboe@...cle.com>
Subject: [PATCH 0/1] Correct sorting problem in cfq_service_tree_add
Found this whilst reviewing the CFQ I/O scheduler code: Currently, this
routine only sorts using the I/O priority class - it does not properly
sort prioritized queues within a specific class. The patch changes the
sort to utilize the full I/O priority (class & priority).
A simple test shows the problem & fixed results: on a 16-way box, for
each of 12 attached disks I started up 17 processes (one process at each
possible class/priority). Each process operated on a separate file in
the file system. I then did two types of tests: (a) direct/synchronous
and (b) direct/asynchronous w/ an 80/20 read/write split.
I then tabulated the overall I/O performed per task: (first column is
priority class (1==RT, 2==BE, 3==IDLE), second column is the I/O
priority (0==highest), then two groupings of read/write data moved
(total KiBs over a span of 120 seconds):
Synchronous:
2.6.32-rc8 2.6.32-rc8+patch
Read Write Read Write
---------------- ----------------
1 0 | 311164 310760 | 424260 424116 |
1 1 | 129712 129792 | 390208 393232 |
1 2 | 72312 71284 | 448 420 |
1 3 | 40364 41052 | 28 20 |
1 4 | 26788 26352 | 28 24 |
1 5 | 16936 16940 | 52 32 |
1 6 | 11196 11140 | 28 20 |
1 7 | 6476 6648 | 20 28 |
2 0 | 24 24 | 40 8 |
2 1 | 24 24 | 12 36 |
2 2 | 20 28 | 20 28 |
2 3 | 28 20 | 24 24 |
2 4 | 28 20 | 28 20 |
2 5 | 28 20 | 20 28 |
2 6 | 24 24 | 20 28 |
2 7 | 24 24 | 36 12 |
3 | 36 12 | 28 20 |
---------------- ----------------
Sum 615184 614164 815300 818096
You can see that due to the "random" nature of the unpatched kernel
lower priority real-time processes get elevated I/O amounts. With the
patched kernel, real-time priorities 0&1 get the vast majority of the
available throughput (as expected). [Basically: priority 0 & 1
flip-flop: when priority 0's I/O finishes, priority 1's gets inserted
then priority 0 comes back with another I/O quick enough (most of the
time) and bumps all the other queues out of the way.]
More I/O is performed with the patched kernel (most likely) because
there is much less thrashing/seeking on the disk.
Asynchronous:
2.6.32-rc8 2.6.32-rc8+patch
Read Write Read Write
---------------- ----------------
1 0 | 1969220 1967036 | 2272660 2266220 |
1 1 | 65880 66216 | 71552 71424 |
1 2 | 30760 30808 | 3532 3508 |
1 3 | 17352 17336 | 2996 3148 |
1 4 | 11496 11288 | 3028 3116 |
1 5 | 7836 8036 | 3008 3136 |
1 6 | 5432 5448 | 2992 3152 |
1 7 | 3692 3860 | 3068 3076 |
2 0 | 3172 2972 | 3052 3092 |
2 1 | 3100 3044 | 3000 3144 |
2 2 | 3140 3004 | 3056 3088 |
2 3 | 3108 3036 | 3084 3060 |
2 4 | 3116 3028 | 2968 3176 |
2 5 | 3068 3076 | 3096 3048 |
2 6 | 2884 3260 | 3084 3060 |
2 7 | 3112 3032 | 3208 2936 |
3 | 3172 2972 | 2968 3176 |
---------------- ----------------
Sum 2139540 2137452 2390352 2384560
With Asynch I/O priority 0 gets the vast (vast!) majority of the
bandwidth because it is issuing more I/Os in one go (128 asynch I/Os at
a time).
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