Re: [OT] Mail hangs real-time processes
Re: [OT] Mail hangs real-time processes
- Subject: Re: [OT] Mail hangs real-time processes
- From: Jeff Moore <email@hidden>
- Date: Wed, 7 Jan 2004 10:22:59 -0800
On Jan 7, 2004, at 12:33 AM, Herbie Robinson wrote:
So, it could be that VM paging or Mail's disk IO is preventing iTunes
from getting at the audio data on the disk when it needs it.
There is no real time scheduling for disk I/O; so, the only totally
predictable way to make HD record and playback is put the audio on
it's own spindle (or multiple spindles if there are a lot of tracks).
This shouldn't be a surprise, that's the way everybody else has been
doing it...
True.
Back in the Classic Mac OS days, the advice we gave when I worked on
Deck was to put all your media onto a different mechanism than the the
one the System folder was on. The same is basically true on X. It's
never a good idea to put your media on the same drive mechanism that
the VM backing store uses. The drive can be busy occasionally servicing
somebody else's request which can cause your disk IO to miss a real
time deadline potentially.
VM paging would be the worst since it will additionally hold off the
HAL's IO thread which will cause glitches.
It shouldn't have a big impact unless the HAL I/O thread has neglected
to wire some of the memory it uses. Or are you thinking that the disk
interrupts for the paging will have that big an impact?
This isn't true.
If you take a page fault in any thread, the thread that services it in
the kernel is higher priority than any real time thread. What you can
end up with is a kind of priority inversion where some normal priority
thread is causing work to be done on a very high priority thread.
One possible explanation of this situation is that opening such a large
mail box is thrashing the heck out of the VM system which would prevent
the HAL's IO thread in iTunes from running.
--
Jeff Moore
Core Audio
Apple
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