Is JACK considered paUtilFixedHostBufferSize?

[mgeier]

I was looking at the latency of PortAudio streams when using the JACK host API.

When setting framesPerBuffer to 0 (a.k.a. paFramesPerBufferUnspecified), no latency is added, which is good and expected.

For other values of framesPerBuffer, however, that value seems to be added to the latency.

I found this code, which might be relevant:

portaudio/src/common/pa_process.c

Lines 166 to 186 in 57aa393

	if( hostBufferSizeMode == paUtilFixedHostBufferSize )
	{
	unsigned long frameShift =
	CalculateFrameShift( framesPerHostBuffer, framesPerUserBuffer );
	if( framesPerUserBuffer > framesPerHostBuffer )
	{
	bp->initialFramesInTempInputBuffer = frameShift;
	bp->initialFramesInTempOutputBuffer = 0;
	}
	else
	{
	bp->initialFramesInTempInputBuffer = 0;
	bp->initialFramesInTempOutputBuffer = frameShift;
	}
	}
	else /* variable host buffer size, add framesPerUserBuffer latency */
	{
	bp->initialFramesInTempInputBuffer = 0;
	bp->initialFramesInTempOutputBuffer = framesPerUserBuffer;
	}

I didn't find any mention of paUtilFixedHostBufferSize in the code of the JACK host API, which could be the reason that framesPerBuffer is always added to the latency.

This latency is generally larger than necessary, so I guess my question is: is this intentional?

I understand that JACK has a jack_set_buffer_size_callback() function, which could be used to react to changes in the buffer size, but AFAIU, this is rarely supported by applications.

Does PortAudio/JACK support changing the buffer size, or should it be designated as paUtilFixedHostBufferSize?