Re: Latency Paper
Re: Latency Paper
- Subject: Re: Latency Paper
- From: Bill Stewart <email@hidden>
- Date: Fri, 03 Aug 2001 16:32:32 -0700
on 3/8/01 11:50 AM, Karl MacMillan wrote:
>
For those of you that saw my latency paper
>
(http://mambo.peabody.jhu.edu/~karlmac/publications/latency-icmc2001.pdf) I
>
made a minor update to the MacOS X numbers. The latency in the original paper
>
was incorrect. The correct value is 3.97ms, which is still the best system
>
with load. I'm surprised no one noticed since the original number was below
>
the minimum settings that CoreAudio seems to allow (at least on my system).
>
>
Thanks,
>
Karl
>
Actually I'd noticed. However this figure is too high, the theoretical round
trip latency is 2.902 msecs with the minimum buffer allowed in 10.0.x
systems. Any other overhead is introduced by co-ordinating the copy from the
input to the output buffers of the two different devices and how these two
IO Procs might "beat" against each other.
However - for hardware that publishes both an in and out stream, then the
throughput latency is 2.902 msecs (at 44.1KHz) plus whatever latency is
introduced by the actual D-A process of the hardware layer (in the case you
tested of built-in hardware, that should be barely detectable).
I've also heard several concerns raised about your figures for Mac OS 9 -
they seem far worse than they should actually be. Did you test other
hardware solutions on Mac OS 9 - like a DigiDesign system for instance, an
RME card, a MoTU system? These are systems that are used in the professional
space and have a very high reputation for latency performance, but your
paper didn't present any figures on those systems that I could see.
Mac OS 8/9 is in a similar position to the Linux based systems your paper
discusses, in that you are profiling in effect custom solutions that involve
extensive work at the very lowest levels of the operating system (for
instance you substitute a custom kernel in Linux, or with a Digi system you
substitute a custom driver stack on OS 9), but on that point the comparison
would be fair and compares the "best of breed", customised solution
delivered on a given platform.
I think the telling point for Mac OS X in all of this (which also includes
the figures you quote for Windows) is that it is a latency figure that is
generated by a shipping, non customised, come as it is, out of the box
solution. Furthermore, the audio device CAN BE SHARED even at this
low-latency figure between multiple applications. On page 2, 2nd column, you
make a considerable point of this capability with one of the Linux systems
without mentioning Mac OS X in that context - (unfortunately:)) that is
mentioned in a separate paragraph on page 3.
Still we were pleased to see the figures you quoted being within the
ballpark of the results we'd have expected.
Bill
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tel: +1 408 974 4056
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