netdev - Re: tbench regression in 2.6.25-rc1

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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.


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