MD Raid 6: poor algorithm choice?

MD Raid 6: poor algorithm choice?

[Chris Boot]

Hi all,

[4295106.360000] raid6: int32x1    714 MB/s
[4295106.381000] raid6: int32x2    742 MB/s
[4295106.403000] raid6: int32x4    632 MB/s
[4295106.429000] raid6: int32x8    523 MB/s
[4295106.453000] raid6: mmxx1     1476 MB/s
[4295106.474000] raid6: mmxx2     2500 MB/s
[4295106.500000] raid6: sse1x1    1375 MB/s
[4295106.521000] raid6: sse1x2    2339 MB/s
[4295106.524000] raid6: using algorithm sse1x2 (2339 MB/s)
[4295106.531000] md: raid6 personality registered for level 6

I just loaded the raid6 module for fun (might end up using it one day), and I 
was surprised at its choice of algorithm. By the messages above, I would have 
assumed it would choose the mmxx2 algorithm at 2500 MB/s instead of sse1x2 at 
the slightly slower 2339 MB/s. This is probably entirely expected behaviour, but 
why?

Cheers,
Chris

-- 
Chris Boot
bootc@bootc.net
http://www.bootc.net/

Re: MD Raid 6: poor algorithm choice?

[thunder7]

From: Chris Boot <bootc@bootc.net>
Date: Thu, Feb 23, 2006 at 06:00:10PM +0000
> Hi all,
> 
> [4295106.474000] raid6: mmxx2     2500 MB/s
> [4295106.521000] raid6: sse1x2    2339 MB/s
> [4295106.524000] raid6: using algorithm sse1x2 (2339 MB/s)
> [4295106.531000] md: raid6 personality registered for level 6
> 
> I just loaded the raid6 module for fun (might end up using it one day), and 
> I was surprised at its choice of algorithm. By the messages above, I would 
> have assumed it would choose the mmxx2 algorithm at 2500 MB/s instead of 
> sse1x2 at the slightly slower 2339 MB/s. This is probably entirely expected 
> behaviour, but why?
> 
Because it is more cache-efficient (allows the other code to run at a
higher speed). This comes up now and again, perhaps these messages
should be rephrased.

Jurriaan
-- 
Uhm, well, what we are talking about in privy terms, is the very latest
in front-wall fresh air orifices combined with a wide capacity gutter
installation below.
You mean crap out of the window?
	Blackadder II
Debian (Unstable) GNU/Linux 2.6.16-rc3-mm1 5503 bogomips load load 0.01

Re: MD Raid 6: poor algorithm choice?

[Chris Boot]

thunder7@xs4all.nl wrote:
> From: Chris Boot <bootc@bootc.net>
> Date: Thu, Feb 23, 2006 at 06:00:10PM +0000
>   
>> Hi all,
>>
>> [4295106.474000] raid6: mmxx2     2500 MB/s
>> [4295106.521000] raid6: sse1x2    2339 MB/s
>> [4295106.524000] raid6: using algorithm sse1x2 (2339 MB/s)
>> [4295106.531000] md: raid6 personality registered for level 6
>>
>> I just loaded the raid6 module for fun (might end up using it one day), and 
>> I was surprised at its choice of algorithm. By the messages above, I would 
>> have assumed it would choose the mmxx2 algorithm at 2500 MB/s instead of 
>> sse1x2 at the slightly slower 2339 MB/s. This is probably entirely expected 
>> behaviour, but why?
>>
>>     
> Because it is more cache-efficient (allows the other code to run at a
> higher speed). This comes up now and again, perhaps these messages
> should be rephrased.
>   
I knew there would be a good reason for this, I just couldn't figure it 
out from those messages. That's a very good explanation indeed, and as 
you say, perhaps the messages should be updated.

Cheers,
Chris

-- 
Chris Boot
bootc@bootc.net