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Re: [libvirt] [Qemu-devel] [PATCH v3 2/2] net: introduce command to query rx-filter information



On Fri, 24 May 2013 12:05:12 -0600
Eric Blake <eblake redhat com> wrote:

> On 05/24/2013 10:12 AM, Michael S. Tsirkin wrote:
> >>>>>
> >>>>> Event message contains the net client name, management might only want
> >>>>> to query the single net client.
> >>>>
> >>>> The client can do the filtering itself.
> >>>
> 
> >> I'm not sure I buy the responsiveness argument.  Sure, the fastest I/O
> >> is no I/O, but whether you read and parse 100 bytes or 1000 from a Unix
> >> domain socket once in a great while shouldn't make a difference.
> 
> And the time spent malloc'ing the larger message to send from qemu, as
> well as the time spent malloc'ing the libvirt side that parses the qemu
> string into C code for use, and the time spent strcmp'ing every entry to
> find the right one...
> 
> It really IS more efficient to filter as low down in the stack as
> possible, once it is determined that filtering is desirable.
> 
> Whether filtering makes a difference in performance is a different
> question - you may be right that always returning the entire list and
> making libvirt do its own filtering will still not add any more
> noticeable delay compared to libvirt doing a filtered query, if the
> bottleneck lies elsewhere (such as libvirt telling macvtap its new
> configration).
> 
> >>
> >> My main concern is to keep the external interface simple.  I'm rather
> >> reluctant to have query commands grow options.
> >>
> >> In a case where we need the "give me everything" query anyway, the "give
> >> me this particular part" option is additional complexity.  Needs
> >> justification, say arguments involving throughput, latency or client
> >> complexity.
> >>
> >> Perhaps cases exist where we never want to ask for everything.  Then the
> >> "give me everything" query is useless, and the option should be
> >> mandatory.
> 
> For this _particular_ interface, I'm not sure whether libvirt will ever
> use an unfiltered query -

If having the argument is useful for libvirt, then it's fine to have it.

But I'd be very reluctant to buy any performance argument w/o real
numbers to back them up.


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