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Re: [PATCH] loop unrolling in net/sched/sch_generic.c

To: tgraf@xxxxxxx
Subject: Re: [PATCH] loop unrolling in net/sched/sch_generic.c
From: "David S. Miller" <davem@xxxxxxxxxxxxx>
Date: Tue, 05 Jul 2005 16:45:03 -0700 (PDT)
Cc: dada1@xxxxxxxxxxxxx, netdev@xxxxxxxxxxx
In-reply-to: <20050705234104.GR16076@xxxxxxxxxxxxxx>
References: <20050705.143548.28788459.davem@xxxxxxxxxxxxx> <42CB14B2.5090601@xxxxxxxxxxxxx> <20050705234104.GR16076@xxxxxxxxxxxxxx>
Sender: netdev-bounce@xxxxxxxxxxx
From: Thomas Graf <tgraf@xxxxxxx>
Date: Wed, 6 Jul 2005 01:41:04 +0200

> I still think we can fix this performance issue without manually
> unrolling the loop or we should at least try to. In the end gcc
> should notice the constant part of the loop and move it out so
> basically the only difference should the additional prio++ and
> possibly a failing branch prediction.

But the branch prediction is where I personally think a lot
of the lossage is coming from.  These can cost upwards of 20
or 30 processor cycles, easily.  That's getting close to the
cost of a L2 cache miss.

I see the difficulties with this change now, why don't we revisit
this some time in the future?

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