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(a)redhat.com&gt; 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.