Re: READAHEAD

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Date:	Fri, 31 Oct 2003 13:20:07 +0100
To: Andrew Morton <akpm@osdl.org>

Andrew Morton wrote:
> age <ahuisman@cistron.nl> wrote:
>
>>I have a problem which i don`t understand and i hope that you
>> will and can help me. The problem is that i experience strange disk
>> read performance. I have to set hdparm -m16 -u1 -c1 -d1 -a4096 /dev/hde
>> to get timing buffered disk reads of 56 MB/SEC.
>> When i disable readahead i get 17 MB/SEC
>> When i enable readahead with -a8 i get 17 MB/SEC
>> When i enable readahead with -a16 i get 24,5 MB/SEC
>> When i enable readahead with -a32 i get 30,5 MB/SEC
>> When i enable readahead with -a64 i get 35 MB/SEC
>> When i enable readahead with -a128 i get 39 MB/SEC
>> When i enable readahead with -a256 i get 39 MB/SEC
>> When i enable readahead with -a512 i get 41 MB/SEC
>> When i enable readahead with -a1024 i get 50 MB/SEC
>> When i enable readahead with -a2048 i get 50 MB/SEC
>> When i enable readahead with -a4096 i get 56 MB/SEC
>> With -a8192,-a16384 and -a32768 i get also 56MB/SEC
>>
>> Before, i never had to set readahead so high
>> Please could you tell me, what is going on here ?
>
>
> Lots of people have been reporting this. It's rather weird.
>
> Is the same effect observable when reading a large file, or is it only
> observable via `hdparm -t'?
>

Hi Andrew,

Here are some tests with bonnie++.
The used command is : bonnie++ -d /home/test -s 1024 -n 10 -u root
TCQ and write cache enabled.

The first test with hdparm -a0 -c1 -m16 -u1 -d1 /dev/hde :

Version 1.03 ------Sequential Output------ --Sequential Input- --Random-
                     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
wuuk 1G 3610 98 56878 85 10010 35 3129 90 17355 55 181.2 2
                     ------Sequential Create------ --------Random Create--------
                     -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
               files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
                  10 11117 95 +++++ +++ 18414 98 11888 99 +++++ +++ 19189 98

The second test with hdparm -a16 -c1 -m16 -u1 -d1 /dev/hde :

Version 1.03 ------Sequential Output------ --Sequential Input- --Random-
                     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
wuuk 1G 3611 98 56404 85 12104 36 3260 92 27741 54 193.3 1
                     ------Sequential Create------ --------Random Create--------
                     -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
               files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
                  10 11571 99 +++++ +++ 18477 99 11901 99 +++++ +++ 19250 99

The third test with hdparm -a256 -c1 -m16 -u1 -d1 /dev/hde :

Version 1.03 ------Sequential Output------ --Sequential Input- --Random-
                     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
wuuk 1G 3614 98 57510 85 15597 28 3621 95 43004 45 188.5 1
                     ------Sequential Create------ --------Random Create--------
                     -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
               files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
                  10 11622 99 +++++ +++ 18439 99 11905 99 +++++ +++ 19378 99

The fourth test with hdparm -a4096 -c1 -m16 -u1 -d1 /dev/hde :

Version 1.03 ------Sequential Output------ --Sequential Input- --Random-
                     -Per Chr- --Block-- -Rewrite- -Per Chr- --Block-- --Seeks--
Machine Size K/sec %CP K/sec %CP K/sec %CP K/sec %CP K/sec %CP /sec %CP
wuuk 1G 3610 98 57770 85 18518 32 3636 96 42748 39 186.9 1
                     ------Sequential Create------ --------Random Create--------
                     -Create-- --Read--- -Delete-- -Create-- --Read--- -Delete--
               files /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP /sec %CP
                  10 11653 98 +++++ +++ 18608 99 11908 99 +++++ +++ 19503 99

If you need more information, please tell me.

groetjes,(greetings)

Age Huisman.

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