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Message-ID: <20090803071539.GA5506@ff.dom.local>
Date: Mon, 3 Aug 2009 07:15:39 +0000
From: Jarek Poplawski <jarkao2@...il.com>
To: Krishna Kumar2 <krkumar2@...ibm.com>
Cc: davem@...emloft.net, herbert@...dor.apana.org.au, kaber@...sh.net,
netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Avoid enqueuing skb for default qdiscs
On Sun, Aug 02, 2009 at 02:21:30PM +0530, Krishna Kumar2 wrote:
> Krishna Kumar2/India/IBM@...IN wrote on 07/28/2009 09:20:55 PM:
>
> > Subject [RFC] [PATCH] Avoid enqueuing skb for default qdiscs
> >
> > From: Krishna Kumar <krkumar2@...ibm.com>
> >
> > dev_queue_xmit enqueue's a skb and calls qdisc_run which
> > dequeue's the skb and xmits it. In most cases (after
> > instrumenting the code), the skb that is enqueue'd is the
> > same one that is dequeue'd (unless the queue gets stopped
> > or multiple cpu's write to the same queue and ends in a
> > race with qdisc_run). For default qdiscs, we can remove
> > this path and simply xmit the skb since this is a work
> > conserving queue.
>
> Any comments on this patch?
Maybe I missed something, but I didn't get this patch, and can't see
it e.g. in the patchwork.
Jarek P.
> Thanks,
>
> - KK
>
> > The patch uses a new flag - TCQ_F_CAN_BYPASS to identify
> > the default fast queue. I plan to use this flag for the
> > previous patch also (rename if required). The controversial
> > part of the patch is incrementing qlen when a skb is
> > requeued, this is to avoid checks like the second line below:
> >
> > + } else if ((q->flags & TCQ_F_CAN_BYPASS) && !qdisc_qlen(q) &&
> > >> THIS LINE: !q->gso_skb &&
> > + !test_and_set_bit(__QDISC_STATE_RUNNING, &q->state)) {
> >
> > Results of a 4 hour testing for multiple netperf sessions
> > (1, 2, 4, 8, 12 sessions on a 4 cpu system-X and 1, 2, 4,
> > 8, 16, 32 sessions on a 16 cpu P6). Aggregate Mb/s across
> > the iterations:
> >
> > -----------------------------------------------------------------
> > | System-X | P6
> > -----------------------------------------------------------------
> > Size | ORG BW NEW BW | ORG BW NEW BW
> > -----|---------------------------|-------------------------------
> > 16K | 154264 156234 | 155350 157569
> > 64K | 154364 154825 | 155790 158845
> > 128K | 154644 154803 | 153418 155572
> > 256K | 153882 152007 | 154784 154596
> > -----------------------------------------------------------------
> >
> > Netperf reported Service demand reduced by 15% on the P6 but
> > no noticeable difference on the system-X box.
> >
> > Please review.
> >
> > Thanks,
> >
> > - KK
>
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