Re: RAID5 XOR speed vs RAID6 Q speed (was Re: AVX RAID5 xor checksumming)

[Jim Kukunas <james.t.kukunas@linux.intel.com>]

On Sat, Mar 31, 2012 at 12:38:56PM +0100, John Robinson wrote:
> On 29/03/2012 22:44, Jim Kukunas wrote:
> > Based on xor_speed, the AVX implementation appears to be ~32% faster than the
> > SSE implementation on my i7 2600:
> >
> > 	generic_sse: 15088.000 MB/sec
> > 	avx: 19936.000 MB/sec
> 
> I just noticed in my logs the other day (recent el5 kernel on a Core 2):
> 
> raid5: automatically using best checksumming function: generic_sse
>     generic_sse:  7805.000 MB/sec
> raid5: using function: generic_sse (7805.000 MB/sec)
> raid6: int64x1   2635 MB/s
> raid6: int64x2   3208 MB/s
> raid6: int64x4   3020 MB/s
> raid6: int64x8   2519 MB/s
> raid6: sse2x1    5099 MB/s
> raid6: sse2x2    5742 MB/s
> raid6: sse2x4    8237 MB/s
> raid6: using algorithm sse2x4 (8237 MB/s)
> 
> I was just wondering how it's possible to do the RAID6 Q calculation 
> faster than the RAID5 XOR calculation - or am I reading this log excerpt 
> wrongly?

Out of curiosity, are you running with CONFIG_PREEMPT=y?

Thanks.

> 
> It's probably academic, since the machine this is running on only has a 
> maximum of about 4500 MB/s of memory throughput, and a lot of that would 
> be consumed sending data to disc in amongst the calculations being done.
> 
> Cheers,
> 
> John.
> 
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-- 
Jim Kukunas
Intel Open Source Technology Center