Messages in this thread | | | Date | Fri, 26 Jan 2007 06:07:29 -0700 | From | "Robert Crocombe" <> | Subject | Re: In-tree version of new FireWire drivers available |
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On 1/25/07, Pieter Palmers <pieterp@joow.be> 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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