should jackd's bufsize still be a power of 2?

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bluebell
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should jackd's bufsize still be a power of 2?

Post by bluebell »

I read that there were plugins that need a power of 2. But on my system xruncounter shows better results when I have multiples of 1 ms latency, that is with buffer sizes of 48, 96,192 ...

Code: Select all

configuring for 48000Hz, period = 48 frames (1.0 ms), buffer = 2 periods
Xrun 1 at DSP load 83.591034

configuring for 48000Hz, period = 64 frames (1.3 ms), buffer = 2 periods
Xrun 1 at DSP load 52.569481

configuring for 48000Hz, period = 96 frames (2.0 ms), buffer = 2 periods
Xrun 1 at DSP load 95.608429

configuring for 48000Hz, period = 128 frames (2.7 ms), buffer = 2 periods
Xrun 1 at DSP load 68.922791
My Qtractor projects seem to run fine with non-powers of 2. Are there any reasons to use only powers of 2?

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Re: should jackd's bufsize still be a power of 2?

Post by merlyn »

It seems not. I've heard that USB interfaces work better with 3 periods. You may be able to get to ~98% DSP before an Xrun with 3 periods.
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Re: should jackd's bufsize still be a power of 2?

Post by bluebell »

I have this effect even with n=3:

Code: Select all

configuring for 48000Hz, period = 96 frames (2.0 ms), buffer = 3 periods
Xrun 1 at DSP load 95.736221

configuring for 48000Hz, period = 128 frames (2.7 ms), buffer = 3 periods
Xrun 1 at DSP load 65.408142
At least xruncounter runs best when buffer_size/sample_rate gives whole milliseconds. The number of buffers seems to be unimportant.

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Re: should jackd's bufsize still be a power of 2?

Post by tramp »

You leave out the most important part, the circle num.
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Re: should jackd's bufsize still be a power of 2?

Post by merlyn »

We could clarify this. Buffer sizes that are a multiple of (samplerate/1000) work best for USB devices that use isochronous transfer, and have an interval between packets of 1ms. Drumfix went into some detail on this in this post.

To attempt to put this into English: These USB devices send packets at a constant time interval, whether they need to or not. If they send a power of two buffer this will get out of step with the stream of packets. To keep the buffer and the device in sync use a buffer that will be filled every 1ms, or multiple of.

lsusb -v gives the necessary information to find out the interval between packets, although I must admit I can't see where it is.

@Bluebell it was a happy accident that your device also uses synchronous isochronous transfer, and has an interval of 1ms. If the interval was different the buffer size would be different.

So these non-power of two buffers only apply to specific devices. I have a PCI card and use a power of two buffer.
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Re: should jackd's bufsize still be a power of 2?

Post by bluebell »

tramp wrote:You leave out the most important part, the circle num.

Code: Select all

Samplerate 48000 
Buffersize is 96 
jack running with realtime priority
Xrun 1 at DSP load 91.893112
Xrun 2 at DSP load 92.355820
Xrun 3 at DSP load 92.438957
Xrun 4 at DSP load 89.244476
Xrun 5 at DSP load 89.244476
Xrun 6 at DSP load 93.297241
Xrun 7 at DSP load 95.261810
Xrun 8 at DSP load 95.261810
Xrun 9 at DSP load 96.080902
Xrun 10 at DSP load 91.765457
Xrun 11 at DSP load 95.607727
in complete 11 Xruns in 9310 circles
first Xrun happen at DSP load 91.893112 circle 8909

Code: Select all

Samplerate 48000 
Buffersize is 128 
jack running with realtime priority
Xrun 1 at DSP load 65.227478
Xrun 2 at DSP load 67.056404
Xrun 3 at DSP load 68.142494
Xrun 4 at DSP load 67.623383
Xrun 5 at DSP load 68.880531
Xrun 6 at DSP load 69.067474
Xrun 7 at DSP load 69.067474
Xrun 8 at DSP load 66.998108
Xrun 9 at DSP load 66.998108
Xrun 10 at DSP load 66.901154
Xrun 11 at DSP load 66.901154
Xrun 12 at DSP load 66.702637
Xrun 13 at DSP load 67.559875
Xrun 14 at DSP load 67.559875
Xrun 15 at DSP load 66.281815
Xrun 16 at DSP load 66.281815
Xrun 17 at DSP load 66.281815
Xrun 18 at DSP load 66.299194
Xrun 19 at DSP load 66.299194
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Xrun 21 at DSP load 67.133095
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Xrun 26 at DSP load 71.322845
Xrun 27 at DSP load 69.326096
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Xrun 209 at DSP load 56.872566
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in complete 437 Xruns in 11190 circles
first Xrun happen at DSP load 65.227478 circle 8584

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merlyn
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Re: should jackd's bufsize still be a power of 2?

Post by merlyn »

Bluebell wrote:

Code: Select all

first Xrun happen at DSP load 91.893112 circle 8909
That looks healthy.

To test a theory I'll predict the speed of your processor. I'll use your 128 result because I have a 128 result for my 2.6 GHz system.

Code: Select all

first Xrun happen at DSP load 99.948204 circle 8809
You got 65% at cycle 8584. First scale that up to 100%. 8584 * 100/65 = 13206.

Now apply that to processor speed. 2.6 * 13206 / 8809 = 3.90.

As far as I know 3.9 Ghz processors don't exist, so I'd say your processor is 3.8 GHz.
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Re: should jackd's bufsize still be a power of 2?

Post by bluebell »

merlyn wrote:
Bluebell wrote:

Code: Select all

first Xrun happen at DSP load 91.893112 circle 8909
That looks healthy.

To test a theory I'll predict the speed of your processor. I'll use your 128 result because I have a 128 result for my 2.6 GHz system.

Code: Select all

first Xrun happen at DSP load 99.948204 circle 8809
You got 65% at cycle 8584. First scale that up to 100%. 8584 * 100/65 = 13206.

Now apply that to processor speed. 2.6 * 13206 / 8809 = 3.90.

As far as I know 3.9 Ghz processors don't exist, so I'd say your processor is 3.8 GHz.
cat /proc/cpuinfo: Intel(R) Core(TM) i7-7700T CPU @ 2.90GHz

cpufreq-info: current CPU frequency is 3.67 GHz

Good estimate.

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