On 1/25/07, Pieter Palmers <[email protected]> wrote:
I'd like to make one note here:
We should have a way to use smaller DMA buffers than one page size. If I
remember correctly, the page size on my system is 4096 bytes, being 1024
quadlets. If we assume a 4 channel audio stream, this corresponds to 256
audio samples. This means that the controller generates an interrupt
every 256 samples, making that we can achieve a latency of 512 samples
at best. This is unacceptable in a pro-audio environment.
The current stack exhibits this problem, and I solve it by recalculating
the max packet size, based upon the stream composition (i.e. expected
packet size) and the requested audio buffer size, such that the
interrupts are generated at a high enough frequency.
I'm not a kernel hacker, but when looking through the code I had the
impression that smaller DMA buffers were possible (aren't smaller
buffers used in packet-per-buffer mode?).
I am using isochronous receive in RAW1394_DMA_PACKET_PER_BUFFER mode
because I am closing a simulation loop around the data that is
received/transmitted. Just for giggles I cranked up a test
isochronous stream from a bus analyzer at 1kB per packet at 8kHz at
the S400 rate (i.e., one packet on each cycle start: 8MBps ), set the
machine up to listen, and was able to maintain 8kHz interrupts at ~12%
CPU utilization on a 2.8GHz Opteron.
1744719 interrupts int 218.112 seconds is 7999.193 ints/sec
I wasn't doing anything with the data for this test, but I have had
the aforementioned sim running steady at a somewhat lower rate. This
test ran under 2.6.20-rc5-rt10, but the more "productiony" system is
on 2.6.16-rt29.
So hopefully you can get markedly lower latencies. Myself, I'm
tickled pink by the performance that can be achieved.
--
Robert Crocombe
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