RE: [Xen-devel] more profiling

["Santos, Jose Renato G" <joserenato.santos@xxxxxx>]

> -----Original Message-----
> From: James Harper [mailto:james.harper@xxxxxxxxxxxxxxxx]
> Sent: Sunday, March 02, 2008 7:48 PM
> To: Santos, Jose Renato G; Andy Grover
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: RE: [Xen-devel] more profiling
>
> > In my experiments I also see overheads on issuing and
> revoking grants
> > due to the use of atomic operations, but these are much
> less expensive
> > than copying an entire packet as you do on the TX path. I
> am surprised
> > with your results.
> > Can you give more details about your configuration and how you are
> > comparing the cost of copy versus issuing grants on TX.
>
> I think you are right in saying that the issuing and revoking
> of grants is due to the use of atomic operations. Having
> looked into it some more, it looks like KeAcquireSpinlock
> (the windows lock operation) is fairly expensive.
>
> Under windows, it is the code that gets the next free ref
> that is protected by spinlocks. I believe that if we only get
> the ref once, but then reuse that ref over and over, then
> we'd get a lot better performace.
>

  Yes. Avoiding the spinlock should improve performance. Definetely, it should 
be a win on the RX path. But is it worth in the TX path, if you now have to 
copy the packet? Do you have experimental data showing that copying is better 
than the spinlock? I don't have much experience with Windows but I think this 
would be very surprising...

Regards

Renato

> James
>

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