On Mittwoch, 19. August 2020 14:51:52 CEST Geoffrey McRae wrote: > >> > What latencies do you achieve BTW with Windows guests? > >> > >> Never tested, it's not the reason why I use jack. > > > > Surpring that you never checked the min. latency there, as you nailed > > quite an > > ambitous jack driver into QEMU which I just realize now. Must have been > > splipped my awareness due to traffic. > > Sorry, I should have been clearer. I have tested windows and the latency > is excellent, but I have never performed any empirical measurements.
/* * ensure the buffersize is no smaller then 512 samples, some (all?) qemu * virtual devices do not work correctly otherwise */ if (c->buffersize < 512) { c->buffersize = 512; } So min. latency is 12ms @44.1 kHz. > >> I get no stuttering issues like is commonly > >> reported for ALSA and PA, and allows for a high degree of > >> reconfigurability. The guest VM overall performs far better also as > >> windows is never waiting on the audio device due to the decoupling > >> provided by the ring buffer in my implementation. > > > > Yeah, looks good indeed! The ringbuffer implementation looks a bit wild: /* read PCM interleaved */ static int qjack_buffer_read(QJackBuffer *buffer, float *dest, int size) { assert(buffer->data); const int samples = size / sizeof(float); int frames = samples / buffer->channels; const int avail = atomic_load_acquire(&buffer->used); if (frames > avail) { frames = avail; } int copy = frames; int rptr = buffer->rptr; while (copy) { for (int c = 0; c < buffer->channels; ++c) { *dest++ = buffer->data[c][rptr]; } if (++rptr == buffer->frames) { rptr = 0; } --copy; } buffer->rptr = rptr; atomic_sub(&buffer->used, frames); return frames * buffer->channels * sizeof(float); } On both sides there is no check whether one side is over/underrunning the other side (rptr vs. wptr). I would really recommend using an existing ringbuffer implementation instead of writing one by yourself. And question: static size_t qjack_write(HWVoiceOut *hw, void *buf, size_t len) { QJackOut *jo = (QJackOut *)hw; ++jo->c.packets; if (jo->c.state != QJACK_STATE_RUNNING) { qjack_client_recover(&jo->c); return len; } qjack_client_connect_ports(&jo->c); return qjack_buffer_write(&jo->c.fifo, buf, len); } So you are ensuring to reconnect the JACK ports in every cycle. Isn't that a bit often? Best regards, Christian Schoenebeck