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Date: Sun, 29 Jan 2012 17:28:06 +0800
From: Wu Fengguang <wfg@...ux.intel.com>
To: Eric Dumazet <eric.dumazet@...il.com>,
Andrew Morton <akpm@...ux-foundation.org>,
LKML <linux-kernel@...r.kernel.org>,
Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>
Subject: Re: Bad SSD performance with recent kernels
On Sun, Jan 29, 2012 at 09:42:59AM +0100, Herbert Poetzl wrote:
> On Sun, Jan 29, 2012 at 01:59:17PM +0800, Wu Fengguang wrote:
> > On Sat, Jan 28, 2012 at 02:33:31PM +0100, Eric Dumazet wrote:
> >> Le samedi 28 janvier 2012 à 20:51 +0800, Wu Fengguang a écrit :
>
> >>> Would you please create a filesystem and large file on sda
> >>> and run the tests on the file? There was some performance bug
> >>> on reading the raw /dev/sda device file..
>
> as promised, I did the tests on a filesystem, created on
> a partition of the disk, and here are the (IMHO quite
> interesting) results:
>
> kernel -- write --- ------------------read -----------------
> --- noop --- --- noop --- - deadline - ---- cfs ---
> [MB/s] %CPU [MB/s] %CPU [MB/s] %CPU [MB/s] %CPU
> ----------------------------------------------------------------
> 2.6.38.8 268.76 49.6 169.20 11.3 169.17 11.3 167.89 11.4
Hmm, read performance drops between 2.6.38 and 2.6.39...
> 2.6.39.4 269.73 50.3 162.03 10.9 161.58 10.9 161.64 11.0
> 3.0.18 269.17 42.0 161.87 9.9 161.36 10.0 161.68 10.1
Between 3.0 and 3.1, the writeback chunk size is raised by commit
1a12d8bd7b2998b ("writeback: scale IO chunk size up to half device
bandwidth") which should be the main reason for the improved write
throughput.
> 3.1.10 271.62 43.1 161.91 9.9 161.68 9.9 161.25 10.1
> 3.2.2 270.95 42.6 162.36 9.9 162.63 9.9 162.65 10.1
>
> so while the 'expected' performance should be somewhere around
> 300MB/s for read and write (raw disk access) we end up with
> good write performance and roughly half the read performance
> with 'dd bs=1M' on ext3
That could be explained by large write chunk size (>=4MB) and small
readahead size (128KB). Long time ago I collected some read
experiments on SSD and find it asks for 4MB readahead size to get
best performance:
SSD 80G Intel x25-M SSDSA2M080 (reported by Li Shaohua)
rasize 1st run 2nd run
----------------------------------
4k 123 MB/s 122 MB/s
16k 153 MB/s 153 MB/s
32k 161 MB/s 162 MB/s
64k 167 MB/s 168 MB/s
128k 197 MB/s 197 MB/s
256k 217 MB/s 217 MB/s
512k 238 MB/s 234 MB/s
1M 251 MB/s 248 MB/s
2M 259 MB/s 257 MB/s
==> 4M 269 MB/s 264 MB/s
8M 266 MB/s 266 MB/s
Note that ==> points to the readahead size that yields plateau throughput.
SSD 22G MARVELL SD88SA02 MP1F (reported by Jens Axboe)
rasize 1st 2nd
--------------------------------
4k 41 MB/s 41 MB/s
16k 85 MB/s 81 MB/s
32k 102 MB/s 109 MB/s
64k 125 MB/s 144 MB/s
128k 183 MB/s 185 MB/s
256k 216 MB/s 216 MB/s
512k 216 MB/s 236 MB/s
1024k 251 MB/s 252 MB/s
2M 258 MB/s 258 MB/s
==> 4M 266 MB/s 266 MB/s
8M 266 MB/s 266 MB/s
Thanks,
Fengguang
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