Re: Safety Offset (Was: I/O Latency (Was: Layman with a mission))
Re: Safety Offset (Was: I/O Latency (Was: Layman with a mission))
- Subject: Re: Safety Offset (Was: I/O Latency (Was: Layman with a mission))
- From: Jeff Moore <email@hidden>
- Date: Thu, 14 Oct 2004 14:37:31 -0700
Unless the speed of the CPU is going to affect the quality of the time
stamps the driver drops, the safety offset is orthogonal to the CPU
speed or the number of CPUs. This is not to say that it won't need to
vary by machine. It very well be the case that some future machine will
have a slightly different DMA or cache architecture that affects the
way data moves around and thus require a larger or smaller safety
offset.
Finally, none of the other things you describe affect the device's
safety offset at all. They may very well require the app to do more
buffering or add latency or whatever, but they definitely do not affect
the individual pieces of hardware and how close to "now" the HAL can
get for doing IO.
On Oct 14, 2004, at 1:35 PM, Herbie Robinson wrote:
At 12:11 PM -0700 10/14/04, Jeff Moore wrote:
Because it's not an exact science, driver writers need to examine
their requirements carefully and do some profiling and
experimentation to figure out the best number to report as the safety
offset for their hardware.
Actually, for decent hardware, the optimum value is probably going to
vary quite a lot depending on the CPU speed and the number of CPUs.
Also, note that applications must have more buffers to do any of the
following:
1. Support multiple interfaces.
2. Support audio processing on multiple CPUs (networked or local).
This is because they won't be able to do all the processing in a
single callback.
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Jeff Moore
Core Audio
Apple
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