RE: [Xen-devel] more profiling

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> 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.

James

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