Re: [Jack-Devel] JACK thread priorities on different OSes

On Thu, May 12, 2011 at 12:20 PM, Stéphane Letz <[hidden]> wrote:

>> I think the process should run in NORMAL_PRIORITY_CLASS by default,
>> and that the process priority should be adjustable via a command line
>> argument.
>>
>> I'm not totally convinced it will improve the situation either, but,
>> given that JACK 2 runs on Windows and is going to continue to run on
>> Windows, I would like to see the situation improve as much as it
>> possibly can.
>
> But you see on the
> http://msdn.microsoft.com/en-us/library/ms685100(v=vs.85).aspx table
> that *only* process REALTIME_PRIORITY_CLASS will actually change
> the actual value of the THREAD_PRIORITY_TIME_CRITICAL right? But
> then *all threads* (even GUI ones...) of this process will by higher that any
> thread of *any* other lover priority process running on the machine...
>
> And the following is clearly written:
>
> "You should almost never use REALTIME_PRIORITY_CLASS, because
> this interrupts system threads that manage mouse input, keyboard input,
> and background disk flushing. This class can be appropriate for
> applications that "talk" directly to hardware or that perform brief tasks
> that should have limited interruptions."

I do see that.  I also see the absolute thread priority levels that
are issued when a process has REALTIME_PRIORITY_CLASS:

    THREAD_PRIORITY_IDLE 16
    THREAD_PRIORITY_LOWEST 22
    THREAD_PRIORITY_BELOW_NORMAL 23
    THREAD_PRIORITY_NORMAL 24
    THREAD_PRIORITY_ABOVE_NORMAL 25
    THREAD_PRIORITY_HIGHEST 26
    THREAD_PRIORITY_TIME_CRITICAL 31

... and can see the absolute thread priority levels that are issued
with MMCSS (from
http://msdn.microsoft.com/en-us/library/ms684247%28v=vs.85%29.aspx -
see the table at the bottom of the page):

    High
    23-26
    These threads run at a thread priority that is lower than only certain
    system-level tasks. This category is designed for Pro Audio tasks.

... and I don't see much difference, save for THREAD_PRIORITY_TIME_CRITICAL.

So, perhaps a mapping adjustment can be made when the process has a
priority of REALTIME_PRIORITY_CLASS:

   PRIORITY_AUDIO: THREAD_PRIORITY_ABOVE_NORMAL
   PRIORITY_CALLBACK: THREAD_PRIORITY_NORMAL
   PRIORITY_MIDI_TERMINAL: THREAD_PRIORITY_HIGHEST

It's true that threads that don't need to have realtime priority will
have elevated priorities too, but I think it's important to take into
account what those specific threads are doing.

So, uh ... what are those specific threads doing? :)

> Using MMCSS in the appropriate manner will makes much more
> sense... I'll work more in that direction

Very cool. :)

-- 
Devin Anderson
devin (at) charityfinders (dot) com

CharityFinders - http://www.charityfinders.com/
synthclone - http://synthclone.googlecode.com/

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