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

[Kevin Wolf]

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