Re: [Xen-devel] Interesting observation with network event notification and batching

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Jun 17, 2013 at 10:38:33AM +0100, Ian Campbell wrote:
> On Sun, 2013-06-16 at 10:54 +0100, Wei Liu wrote:
> > > > Konrad, IIRC you once mentioned you discovered something with event
> > > > notification, what's that?
> > > 
> > > They were bizzare. I naively expected some form of # of physical NIC 
> > > interrupts to be around the same as the VIF or less. And I figured
> > > that the amount of interrupts would be constant irregardless of the
> > > size of the packets. In other words #packets == #interrupts.
> > > 
> > 
> > It could be that the frontend notifies the backend for every packet it
> > sends. This is not desirable and I don't expect the ring to behave that
> > way.
> 
> It is probably worth checking that things are working how we think they
> should. i.e. that netback's calls to RING_FINAL_CHECK_FOR_.. and
> netfront's calls to RING_PUSH_..._AND_CHECK_NOTIFY are placed at
> suitable points to maximise batching.
> 
> Is the RING_FINAL_CHECK_FOR_REQUESTS inside the xen_netbk_tx_build_gops
> loop right? This would push the req_event pointer to just after the last
> request, meaning the net request enqueued by the frontend would cause a
> notification -- even though the backend is actually still continuing to
> process requests and would have picked up that packet without further
> notification. n this case there is a fair bit of work left in the
> backend for this iteration i.e. plenty of opportunity for the frontend
> to queue more requests.
> 
> The comments in ring.h say:
>  *  These macros will set the req_event/rsp_event field to trigger a
>  *  notification on the very next message that is enqueued. If you want to
>  *  create batches of work (i.e., only receive a notification after several
>  *  messages have been enqueued) then you will need to create a customised
>  *  version of the FINAL_CHECK macro in your own code, which sets the event
>  *  field appropriately.
> 
> Perhaps we want to just use RING_HAS_UNCONSUMED_REQUESTS in that loop
> (and other similar loops) and add a FINAL check at the very end?
> 
> > > But it was odd and I didn't go deeper in it to figure out what
> > > was happening. And also to figure out if for the VIF we could
> > > do something of #packets != #interrupts.  And hopefully some
> > > mechanism to adjust so that the amount of interrupts would
> > > be lesser per packets (hand waving here).
> > 
> > I'm trying to do this now.
> 
> What scheme do you have in mind?

Basically the one you mentioned above.

Playing with various event pointers now.


Wei.

> 

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