pthreads and Quality of Service Classes

Question

Jones OP

Created Jun ’16

Replies 6

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Hi,

I'm facing an interessting effect within my application when using multiple pthreads that execute computation intensive tasks. When the threads start processing the data, the main thread function takes ~20 ms for a single loop of processing. This time stays constant for a couple of seconds. After that, the computation time rises to ~80 ms. The thread is sill doing the same kind of work, just with different values, so I would expect the execution time to stay constant.

The initial approach to resolve this was to specify a Quality of Service Class for the pthreads involved. This indeed solved the problem for that time. Now the amount of data to process is getting bigger and the QOS setting seems to not work anymore. I'm now facing the same execution time changes as at the beginning.

Is there anything else I can do except of setting the QOS when the pthreads are created?

That's the API I'm using to set the QOS class for the pthread.

pthread_attr_set_qos_class_np(&qos_attr, QOS_CLASS_USER_INTERACTIVE, 0);

I'm running the application with standard Release build configuration.

THX!

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Answer 1

DTS Engineer OP

Apple

Jun ’16

I'm facing an interessting effect within my application when using multiple pthreads that execute computation intensive tasks. When the threads start processing the data, the main thread function takes ~20 ms for a single loop of processing.

I’m confused by your terminology here. Normally when folks say “main thread” they’re referring to, well, the main thread, the thing running the GUI. In this context, however, it seems like you’re using it to mean the main thread of your background computation code.

Please clarify.

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Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

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Answer 2

Jones OP

Jun ’16

Hi Quinn,
Sorry for the confusion. I refer to the function the pthread was created with. Inside that function we have a while loop calling our data processing algorithm.

Thx,

Andreas

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Answer 3

DTS Engineer OP

Apple

Jun ’16

I refer to the function the pthread was created with.

OK.

With that in mind, re-reading your original post it doesn’t sound like a QoS of

QOS_CLASS_USER_INTERACTIVE

is right here. That QoS is appropriate when the work is directly responsible for keeping the UI responsive. To quote

<sys/qos.h>

:

A QOS class which indicates work performed by this thread
Such work is requested to run at high priority relative to other work on the system. Specifying this QOS class is a request to run with nearly all available system CPU and I/O bandwidth even under contention. This is not an energy-efficient QOS class to use for large tasks. The use of this QOS class should be limited to critical interaction with the user such as handling events on the main event loop, view drawing, animation, etc.

Is that an accurate description of the relationship between this work and your UI?

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—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

WWDC runs Mon, 13 Jun through to Fri, 17 Jun. During that time all of DTS will be at the conference, helping folks out face-to-face. http://developer.apple.com/wwdc/

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Answer 4

Jones OP

Jun ’16

Hi Quinn,
```QOS_CLASS_USER_INTERACTIVE``` is correct here because the calculation is needed to update OpenGL rendering @30 or 60 fps.

Is there some kind of debug interface / logging for anything threading/scheduler related?

thx,

Andreas

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Answer 5

DTS Engineer OP

Apple

Jun ’16

QOS_CLASS_USER_INTERACTIVE
is correct here because the calculation is needed to update OpenGL rendering @30 or 60 fps.

OK then.

Is there some kind of debug interface / logging for anything threading/scheduler related?

Take a look at the System Trace instrument.

Share and Enjoy
—
Quinn “The Eskimo!”
Apple Developer Relations, Developer Technical Support, Core OS/Hardware

let myEmail = "eskimo" + "1" + "@apple.com"

WWDC runs Mon, 13 Jun through to Fri, 17 Jun. During that time all of DTS will be at the conference, helping folks out face-to-face. http://developer.apple.com/wwdc/

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Answer 6

avitzur OP

Dec ’19

I'm encountering the same behavior here in my app in similar circumstance, having tried

QOS_CLASS

DEFAULT, USER_INITIATED, and

USER_INTERACTIVE. Have you found a satisfying resolution to your problem?

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