On 02/04/2012 23:48, Jim Kukunas wrote:
On Sat, Mar 31, 2012 at 12:38:56PM +0100, John Robinson wrote:
[...]
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: 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?
No. Here's an excerpt from my .config:
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_BKL=y
CONFIG_PREEMPT_NOTIFIERS=y
But this is a Xen dom0 kernel, 2.6.18-308.1.1.el5.centos.plusxen. Now, a
non-Xen kernel (2.6.18-308.1.1.el5) says:
raid5: automatically using best checksumming function: generic_sse
generic_sse: 11892.000 MB/sec
raid5: using function: generic_sse (11892.000 MB/sec)
raid6: int64x1 2644 MB/s
raid6: int64x2 3238 MB/s
raid6: int64x4 3011 MB/s
raid6: int64x8 2503 MB/s
raid6: sse2x1 5375 MB/s
raid6: sse2x2 5851 MB/s
raid6: sse2x4 9136 MB/s
raid6: using algorithm sse2x4 (9136 MB/s)
Looks like it loses a chunk of performance running as a Xen dom0.
Even still, 11892 MB/s for XOR vs 9136 MB/s for XOR+Q - it still seems
remarkable that the XOR can't be done several times faster than the Q.
Cheers,
John.
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