Re: [PATCH 0/6] Kill off the virtio_net tx mitigation timer

[Mark McLoughlin <markmc@redhat.com>]

Hi Avi,

Just thinking about your variable window suggestion ...

On Mon, 2008-11-03 at 14:40 +0200, Avi Kivity wrote:
> Mark McLoughlin wrote:
> > On Sun, 2008-11-02 at 11:48 +0200, Avi Kivity wrote:

> >> Where does the benefit come from?
> >>     
> >
> > There are two things going on here, I think.
> >
> > First is that the timer affects latency, removing the timeout helps
> > that.
> >   
> 
> If the timer affects latency, then something is very wrong.  We're 
> lacking an adjustable window.
> 
> The way I see it, the notification window should be adjusted according 
> to the current workload.  If the link is idle, the window should be one 
> packet -- notify as soon as something is queued.  As the workload 
> increases, the window increases to (safety_factor * allowable_latency / 
> packet_rate).  The timer is set to allowable_latency to catch changes in 
> workload.
> 
> For example:
> 
> - allowable_latency 1ms (implies 1K vmexits/sec desired)
> - current packet_rate 20K packets/sec
> - safety_factor 0.8
> 
> So we request notifications every 0.8 * 20K * 1m = 16 packets, and set 
> the timer to 1ms.  Usually we get a notification every 16 packets, just 
> before timer expiration.  If the workload increases, we get 
> notifications sooner, so we increase the window.  If the workload drops, 
> the timer fires and we decrease the window.
> 
> The timer should never fire on an all-out benchmark, or in a ping test.

The way I see this (continuing with your example figures) playing out
is:

  - If we have a packet rate of <2.5K packets/sec, we essentially have 
    zero added latency - each packet causes a vmexit and the packet is 
    dispatched immediately

  - As soon as we go above 2.5k we add, on average, an additional 
    ~400us delay to each packet

  - This is almost identical to our current scheme with an 800us timer, 
    except that flushes are typically triggered by a vmexit instead of
    the timer expiring

I don't think this is the effect you're looking for? Am I missing
something?

Cheers,
Mark.