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

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Message-Id: <200712181824.57235.rusty@rustcorp.com.au>
Date:	Tue, 18 Dec 2007 18:24:56 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Avi Kivity <avi@...ranet.com>
Cc:	dor.laor@...ranet.com, kvm-devel <kvm-devel@...ts.sourceforge.net>,
	netdev@...r.kernel.org,
	virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [kvm-devel] [virtio-net][PATCH] Don't arm tx hrtimer with a	constant 500us each transmit

On Tuesday 18 December 2007 16:30:08 Avi Kivity wrote:
> Rusty Russell wrote:
> >     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.
>
> One thing that came up in our discussions is to let the host do the
> timer processing instead of the guest.  When tx exit mitigation is
> enabled, the guest bumps the queue pointer, but carefully refrains from
> kicking the host.  The host polls the tx pointer using a timer, kicking
> itself periodically; if polling yields no packets it disables tx exit
> mitigation.  This saves the guest the bother of programming the timer,
> which presumably requires an exit if the timer is the closest one to
> expiration.
>
> [btw, this can be implemented in virtqueue rather than virtio-net, no?]

Yes, the current patch is a hack (look at the hardcoded constant); wanted to 
see how much it helps, if any.

More sophisticated timer management would be a definite win... funny, I have a 
patch here which helps that....

Rusty.
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