netdev - Re: [RFC] [PATCH] Avoid enqueuing skb for default qdiscs

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Date:	Sun, 2 Aug 2009 14:21:30 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	davem@...emloft.net, herbert@...dor.apana.org.au,
	jarkao2@...il.com, kaber@...sh.net, netdev@...r.kernel.org
Subject: Re: [RFC] [PATCH] Avoid enqueuing skb for default qdiscs

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?

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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