[alsa-devel] Too many snd_pcm_mmap_readi calls

Jan Homann jh at janhomann.de
Wed May 7 11:52:45 CEST 2014

Arg, i am sorry for the enormouse amount of errors in eight lines of
code. Is there any "Errors per line"-List? Maybe there is a new leader
by now.

I added the sleep to see the start of one while-loop in the strace
output. Not every function appears in the strace. Just to see where the
loop starts i added a function and sleep came into my mind first.
In my real function i am checking the return values of snd_pcm_wait and
snd_pcm_mmap_readi. The return value of snd_pcm_mmap_readi is used to
encode the amount of captured samples with opus.

In my opinion mmap_begin just tells the alsa-lib that someone is about
to access the buffer. With avail_update all buffer states (whatever they
are) are updated and mmap_readi gets the samples to
network_data.data_UC. After that, mmap_commit tells the alsa-lib that i
am ready with the buffer.
Maybe i am wrong with that. Ok, for sure i am wrong with that.

I am using mmap because of its good performance. Under normal condition
with snd_pcm_mmap_readi my program uses about 1% of cpu-time where
snd_pcm_readi uses about 75%.

Do you know a good tutorial about using mmap_readi?

Am 07.05.2014 11:01, schrieb Clemens Ladisch:
> Jan Homann wrote:
>> while(1) {
>>     sleep(1);
>>     snd_pcm_wait( pcm_handle, REC_BUFFER_ELEMENT_SIZE_MS + 10);
>>     snd_pcm_mmap_begin(pcm_handle, &my_areas, &offset, &frames);
>>     snd_pcm_avail_update ( pcm_handle );
>>     snd_pcm_mmap_readi( pcm_handle, network_data.data_UC, ALSA_READ_FRAMES);
>>     snd_pcm_mmap_commit(pcm_handle, offset, frames);
>> }
> The sleep will result in an overrun, which will throw off the timing
> of all following calls.
> You are not checking the return value of any of these functions.
> Why are you calling snd_pcm_avail_update?  And why are you doing this
> _after_ calling mmap_begin?
> You are ignoring the number of frames returned by mmap_begin.  This
> value could be smaller or larger than ALSA_READ_FRAMES.
> Why are you using mmap in the first place, when you don't even try to
> access the buffer directly?
> Regards,
> Clemens

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