Re: [libvirt] [Qemu-devel] [PATCH v5 6/6] block: Enable qemu_open/close to work with fd sets

Am 26.07.2012 15:13, schrieb Eric Blake: > On 07/25/2012 09:21 PM, Corey Bryant wrote: >> >> >> On 07/25/2012 03:25 PM, Eric Blake wrote: >>> On 07/25/2012 02:22 AM, Kevin Wolf wrote: >>>>>> Hm, not a nice interface where qemu_close() needs the filename and >>>>>> (worse) could be given a wrong filename. Maybe it would be better to >>>>>> maintain a list of fd -> fdset mappings in qemu_open/close? >>>>>> >>>>> >>>>> I agree, I don't really like it either. >>>>> >>>>> We already have a list of fd -> fdset mappings (mon_fdset_fd_t -> >>>>> mon_fdset_t). Would it be too costly to loop through all the >>>>> fdsets/fds >>>>> at the beginning of every qemu_close()? >>>> >>>> I don't think so. qemu_close() is not a fast path and happens almost >>>> never, and the list is short enough that searching it isn't a problem >>>> anyway. >>> >>> I agree - just do the loop to do the reverse lookup yourself, rather >>> than making qemu_close() have a different signature than close(). >>> >> >> Great, I'll do this then. > > You may want an optimization of using a bitset for tracking which fds > are tracked by fdset in the first place, so that the fast path of > qemu_close() will be a check against the bitset to see if you even have > to waste time on the reverse lookup in the first place. The bitset will > typically be small (bounded not only by the maximum possible fd, but > further by the fact that we don't usually open that many fds in the > first place), but I'm not sure if you can get away with static sizing. Premature optimisation, in my opinion. The list is really small. Kevin