Re: [virtio-net][PATCH] Don't arm tx hrtimer with a constant 500us each transmit

Dor Laor
2007-12-18T12:55:27Z

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Rusty Russell wrote:
> On Wednesday 12 December 2007 23:54:00 Dor Laor wrote:
>
>> commit 763769621d271d92204ed27552d75448587c1ac0
>> Author: Dor Laor <dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
>> Date: Wed Dec 12 14:52:00 2007 +0200
>>
>> [virtio-net][PATCH] Don't arm tx hrtimer with a constant 50us each
>> transmit
>>
>> The current start_xmit sets 500us hrtimer to kick the host.
>> The problem is that if another xmit happens before the timer was
>> fired then
>> the first xmit will have to wait additional 500us.
>> This patch does not re-arm the timer if there is existing one.
>> This will shorten the latency for tx.
>>
>
> Hi Dor!
>
> Yes, I pondered this when I wrote the code. On the one hand, it's a
> low-probability pathological corner case, on the other, your patch reduces
> the number of timer reprograms in the normal case.
>
> So I've applied it, thanks!
> Rusty.
>
>
>
Thanks, it actually improved my tx performance in 20%!
Expecting the host side timer with your new patch.
Cheers,
Dor.
>
>
>
>> Signed-off-by: Dor Laor <dor.laor-atKUWr5tajBWk0Htik3J/w@public.gmane.org>
>>
>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>> index 7b051d5..8bb17d1
>> --- a/drivers/net/virtio_net.c
>> +++ b/drivers/net/virtio_net.c
>> @@ -406,10 +405,10 @@ again:
>> virtio_debug(vdebug, "%s: before calling kick %d\n",
>> __FUNCTION__, __LINE__);
>> vi->svq->vq_ops->kick(vi->svq);
>> vi->out_num = 0;
>> - } else {
>> - vi->stats.hrtimer_starts++;
>> - hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
>> - HRTIMER_MODE_REL);
>> + } else if (!hrtimer_is_queued(&vi->tx_timer)) {
>> + vi->stats.hrtimer_starts++;
>> + hrtimer_start(&vi->tx_timer, ktime_set(0,500000),
>> + HRTIMER_MODE_REL);
>> }
>> return 0;
>> }
>>
>
>
>
>

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