Re: Audio device mmap and kevents

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

> It's better to use write(2) for this purpose.

Often, yes, but not, I think, always.

> [...], mmap'ing audio device does not lead to improve any performance
> or latency.

Then something is wrong.

(a) It's the rare port on which copying to mmap()ped memory is no
faster than a user/kernel crossing plus a copyin.

(b) Once bytes are write()ten, userland cannot change them.  If the
kernel does a lot of buffering, userland has to know the data it wants
played a significant time in advance; if the kernel does little
buffering, userland has a relatively hard real-time constraint in that
it has to write() on a tight schedule or output audio drops out.

Writing into an mmap()ped ring buffer takes advantage of the speed
differential and fixes the latency issue without requiring real-time
behaviour out of userland.  (The API would need fixing, though; having
to do a syscall to find out where the audio is playing defeats at least
some of the performance advantage.  Those numbers should appear in the
mmap()ped buffer, or perhaps a different mmap()ped buffer.)

Of course, if you're just playing a canned audio data stream and doing
nothing else, the latency issue doesn't matter; you can have the kernel
do lots of buffering and you're happy.  But things such as games that
want to play audio but also want to be able make that audio react very
fast (for human values of "very fast") to unpredictable events may be
willing to accept the inconvenience of an mmap()ped ring buffer for the
sake of getting both lots of buffering and low latency.

Also, if you have a repeating audio track, then at least in principle
you can just copy it into the ring buffer and forget it, rather than
having to constantly write() - though, of course, this depends on your
repeat length dividing the ring buffer size.

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