From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Robinson Subject: Re: RAID5 XOR speed vs RAID6 Q speed (was Re: AVX RAID5 xor checksumming) Date: Tue, 03 Apr 2012 11:23:16 +0100 Message-ID: <4F7ACF94.5080505@anonymous.org.uk> References: <1333057458-2986-1-git-send-email-james.t.kukunas@linux.intel.com> <4F76ECD0.1060608@anonymous.org.uk> <20120402224818.GA2248@jtkukuna_gentoo_sb> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20120402224818.GA2248@jtkukuna_gentoo_sb> Sender: linux-raid-owner@vger.kernel.org To: linux-raid@vger.kernel.org List-Id: linux-raid.ids 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.