Re: Bad SSD performance with recent kernels

[Wu Fengguang]

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 Ã 20:51 +0800, Wu Fengguang a Ãcrit :
> 
> >>> 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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