[adding Boaz since he also made an attempt at fixing this] http://marc.info/?l=linux-crypto-vger&m=131829241111450&w=2 ...I had meant to follow up on this, but was buried in 'isci' issues. On Tue, Apr 3, 2012 at 4:56 PM, Jim Kukunas <james.t.kukunas@xxxxxxxxxxxxxxx> wrote: > On Tue, Apr 03, 2012 at 11:23:16AM +0100, John Robinson wrote: >> 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. > > Taking a look at do_xor_speed, I see two issues which might be the cause > of the disparity you reported. > > 0) In the RAID5 xor benchmark, we get the current jiffy, then run do_2() until > the jiffy increments. This means we could potentially be testing for less > than a full jiffy. The RAID6 benchmark handles this by obtaining the current > jiffy, then calling cpu_relax() until the jiffy increments, and then running > the test. This is addressed by my first patch. > > 1) The only way I could reproduce your findings of a higher throughput for > RAID6 than for RAID5 xor checksumming was with CONFIG_PREEMPT=y. It seems > that you encountered this while running as XEN dom0. Currently, we disable > preemption during the RAID6 benchmark, but don't in the RAID5 benchmark. > This is addressed by my second patch. > > I've added linux-crypto to the discussion as both of these patches affect > code in crypto/ > > Thanks. > > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
