Hello Giuliano, My suggestion is to repost your question with Cc: to Jaroslav Kysela and Takashi Iwai. I *think* Takashi Iwai deals more with kernelspace and Jaroslav Kysela is the one that can help you with alsa-lib. This is a high volume list and sometimes mail will get ignored. But Cc: the Maintainers and they will take notice and hopefully you'll have your optimization feedback. Thanks, Geraldo Nascimento On Mon, Aug 2, 2021 at 9:26 AM Giuliano Zannetti - ART S.p.A. <giuliano.zannetti@xxxxxxxxxxxxxxxx> wrote: > > Hi, > > I'm trying to optimize the dmix because I'm working with a big number of channels (up to 16) and in this case the dmix has not a negligible impact on performance. > > I'm working with ALSA 1.1.9. I gave my first look to the generic_mix_areas_16_native function (https://github.com/alsa-project/alsa-lib/blob/v1.1.9/src/pcm/pcm_dmix_generic.c#L130). > > I would ask you if I can avoid to check, for each loop iteration, if the current dst sample is not 0. > > for (;;) { > sample = *src; > if (! *dst) { > *sum = sample; > *dst = *src; > } else { > sample += *sum; > *sum = sample; > if (sample > 0x7fff) > sample = 0x7fff; > else if (sample < -0x8000) > sample = -0x8000; > *dst = sample; > } > if (!--size) > return; > src = (signed short *) ((char *)src + src_step); > dst = (signed short *) ((char *)dst + dst_step); > sum = (signed int *) ((char *)sum + sum_step); > } > > Could it be possible check for the first sample of the period only, as reported in the code below? My assumption is that if dst[0] is 0 also dst[1] ... dst[period-1] will be 0, and I don't need to check every time. This is already an optimization, but it could be also a starting point for other optimization based on my HW. But, first of all, I would ask to you if my assumption is right. > > if (! *dst) { > for (;;) { > sample = *src; > *sum = sample; > *dst = *src; > > if (!--size) > return; > > src = (signed short *) ((char *)src + src_step); > dst = (signed short *) ((char *)dst + dst_step); > sum = (signed int *) ((char *)sum + sum_step); > } > > } else { > for (;;) { > sample = *src; > sample += *sum; > *sum = sample; > > if (sample > 0x7fff) > sample = 0x7fff; > else if (sample < -0x8000) > sample = -0x8000; > *dst = sample; > > if (!--size) > return; > > src = (signed short *) ((char *)src + src_step); > dst = (signed short *) ((char *)dst + dst_step); > sum = (signed int *) ((char *)sum + sum_step); > } > } > > Thank you! > > Best Regards, > Giuliano
