[LAD] JACK + PA Latency

Hi, Moving this to the LAD list as it seems to be getting to a more technical and challenging stage now. This may be a simple answer and if so that is great but suspect it is a bit more complex than that so might require the brain power of the LAD list rather than just my capabilities. To recap I am trying to get hard data on the best case latency while running a combination of JACK + PA. There are various reasons for and against doing this but discussing them is not the purpose of this thread. - I am using jack_delay to measure the round trip latency between jack -> PA -> JACK - I also have been testing a complete loop including system i/o but for the purposes of this thread we can ignore those results. - I have seen some interesting numbers come up today. If I run the following command: pactl list sink-inputs I get numbers similar to this: Buffer Latency: 8000 usec Sink Latency: 3166 usec The combination of the two shows the actual PA Stream Buffer latency (approx 10 - 11ms). This number stays approximately static while polling with this command. To be clear there is also the additional latency that occurs as the data is transferred from PA to the output but I am told it is negligible although it might turn out to be a key player. The results from jack_delay are somewhat different. I have included a snapshot below*. For reference sake it was also suggested that I open /dev/cpu_dma_latency and give it a value of 0. /dev/cpu_dma_latency allows to write the latency you can tolerate (in ms). The kernel will translate this to the deepest C-state the processor can enter. I have been running my tests with this value at zero in case it helps. I have checked with powertop and turbostat but it doesn't appear to make any difference on this machine. Possibly because it is an older cpu. Armed with that preliminary information there are a couple of issues that I am interested in at this juncture. 1: Why PA is reporting 10ms for the stream buffer but jack_delay is giving the results below. 2: Why PA is reporting 10ms for the stream buffer when I am running jack at 64 frames/period and ecasound too. 3: Where the fluctuating measurements from jack_delay are coming from in the graph as PA Stream Buffer is static at 10ms and ecasound is basically in pass through with a 64/48k buffer same as JACK. A back of the envelop estimation suggests latency should be stable well under 20ms including the 10ms set aside for the PA Stream buffer. --------- RESULTS --------- jack_delay -> pa -> ecasound -> pa -> jack_delay hda_intel driver dual core Intel(R) Core(TM)2 CPU T5600 @ 1.83GHz jack is run at 64/48000/2 ecasound has a period size of 64 PA is configured to run at 48khz too [*] Here's the snapshot from jack_delay: 62784.000 frames 1308.000 ms total roundtrip latency extra loopback latency: 62784 frames use 31392 for the backend arguments -I and -O 62784.000 frames 1308.000 ms total roundtrip latency extra loopback latency: 62784 frames use 31392 for the backend arguments -I and -O ?? Inv 63168.000 frames 1316.000 ms total roundtrip latency extra loopback latency: 63168 frames use 31584 for the backend arguments -I and -O 63743.999 frames 1328.000 ms total roundtrip latency extra loopback latency: 63743 frames use 31871 for the backend arguments -I and -O 64639.999 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64640.000 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64639.999 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64640.000 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64640.000 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64639.999 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64640.000 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64640.000 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 64640.000 frames 1346.667 ms total roundtrip latency extra loopback latency: 64639 frames use 32319 for the backend arguments -I and -O 65088.000 frames 1356.000 ms total roundtrip latency extra loopback latency: 65088 frames use 32544 for the backend arguments -I and -O 65088.000 frames 1356.000 ms total roundtrip latency extra loopback latency: 65088 frames use 32544 for the backend arguments -I and -O 65087.999 frames 1356.000 ms total roundtrip latency extra loopback latency: 65087 frames use 32543 for the backend arguments -I and -O 65152.000 frames 1357.333 ms total roundtrip latency extra loopback latency: 65151 frames use 32575 for the backend arguments -I and -O 65152.000 frames 1357.333 ms total roundtrip latency extra loopback latency: 65151 frames use 32575 for the backend arguments -I and -O 65152.000 frames 1357.333 ms total roundtrip latency extra loopback latency: 65152 frames use 32576 for the backend arguments -I and -O 65152.000 frames 1357.333 ms total roundtrip latency extra loopback latency: 65151 frames use 32575 for the backend arguments -I and -O 65216.000 frames 1358.667 ms total roundtrip latency extra loopback latency: 65216 frames use 32608 for the backend arguments -I and -O 64.000 frames 1.333 ms total roundtrip latency extra loopback latency: 63 frames use 31 for the backend arguments -I and -O 64.000 frames 1.333 ms total roundtrip latency extra loopback latency: 63 frames use 31 for the backend arguments -I and -O ?? Inv 256.000 frames 5.333 ms total roundtrip latency extra loopback latency: 256 frames use 128 for the backend arguments -I and -O 256.000 frames 5.333 ms total roundtrip latency extra loopback latency: 256 frames use 128 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 640 frames use 320 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 640 frames use 320 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 639 frames use 319 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 640 frames use 320 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 640 frames use 320 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 639 frames use 319 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 639 frames use 319 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 639 frames use 319 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 640 frames use 320 for the backend arguments -I and -O 640.000 frames 13.333 ms total roundtrip latency extra loopback latency: 640 frames use 320 for the backend arguments -I and -O 768.001 frames 16.000 ms total roundtrip latency extra loopback latency: 768 frames use 384 for the backend arguments -I and -O 832.000 frames 17.333 ms total roundtrip latency extra loopback latency: 832 frames use 416 for the backend arguments -I and -O 832.000 frames 17.333 ms total roundtrip latency extra loopback latency: 832 frames use 416 for the backend arguments -I and -O 832.000 frames 17.333 ms total roundtrip latency extra loopback latency: 831 frames use 415 for the backend arguments -I and -O 832.000 frames 17.333 ms total roundtrip latency extra loopback latency: 832 frames use 416 for the backend arguments -I and -O 1024.000 frames 21.333 ms total roundtrip latency extra loopback latency: 1024 frames use 512 for the backend arguments -I and -O ?? 1024.000 frames 21.333 ms total roundtrip latency extra loopback latency: 1024 frames use 512 for the backend arguments -I and -O 1024.000 frames 21.333 ms total roundtrip latency extra loopback latency: 1023 frames use 511 for the backend arguments -I and -O 1024.000 frames 21.333 ms total roundtrip latency extra loopback latency: 1023 frames use 511 for the backend arguments -I and -O 1344.000 frames 28.000 ms total roundtrip latency extra loopback latency: 1344 frames use 672 for the backend arguments -I and -O -- Patrick Shirkey Boost Hardware Ltd