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Message-Id: <20080215.152200.145584182.davem@davemloft.net> Date: Fri, 15 Feb 2008 15:22:00 -0800 (PST) From: David Miller <davem@...emloft.net> To: dada1@...mosbay.com Cc: yanmin_zhang@...ux.intel.com, herbert@...dor.apana.org.au, linux-kernel@...r.kernel.org, netdev@...r.kernel.org Subject: Re: tbench regression in 2.6.25-rc1 From: Eric Dumazet <dada1@...mosbay.com> Date: Fri, 15 Feb 2008 15:21:48 +0100 > On linux-2.6.25-rc1 x86_64 : > > offsetof(struct dst_entry, lastuse)=0xb0 > offsetof(struct dst_entry, __refcnt)=0xb8 > offsetof(struct dst_entry, __use)=0xbc > offsetof(struct dst_entry, next)=0xc0 > > So it should be optimal... I dont know why tbench prefers __refcnt being > on 0xc0, since in this case lastuse will be on a different cache line... > > Each incoming IP packet will need to change lastuse, __refcnt and __use, > so keeping them in the same cache line is a win. > > I suspect then that even this patch could help tbench, since it avoids > writing lastuse... I think your suspicions are right, and even moreso it helps to keep __refcnt out of the same cache line as input/output/ops which are read-almost-entirely :-) I haven't done an exhaustive analysis, but it seems that the write traffic to lastuse and __refcnt are about the same. However if we find that __refcnt gets hit more than lastuse in this workload, it explains the regression. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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