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

[Eric Blake <eblake@redhat.com>]

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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.

-- 
Eric Blake   eblake@redhat.com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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