Re: dmix optimization

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@artgroup-spa.com 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_generi...).

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