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c++multithreadingwinapi
Konrad
Konrad

Reputation: 40947

Setting thread priorities from the running process

I've just come across the Get/SetThreadPriority methods and they got me wondering - can a thread priority meaningfully be set higher than the owning process priority (which I don't believe can be changed programatically in the same way) ?

Are there any pitfalls to using these APIs?

Upvotes: 1

Views: 4386

Answers (2)

Tudor
Tudor

Reputation: 62439

Yes, you can set the thread priority to any class, including a class higher than the one of the current process. In fact, these two values are complementary and provide the base priority of the thread. You can read about it in the Remarks section of the link you posted.

You can set the process priority using SetPriorityClass.

Now that we got the technicalities out of the way, I find little use for manipulating the priority of a thread directly. The OS scheduler is sophisticated enough to boost the priority of threads blocked in I/O over threads doing CPU computations (to the point that an I/O thread will preempt a CPU thread when the I/O interrupt arrives). In fact, even I/O threads are differentiated, with keyboard I/O threads getting a priority boost over file I/O threads for example.

Upvotes: 2

Martin James
Martin James

Reputation: 24847

On Windows, the thread and process priorities are combined using an algorthm that decides overall scheduling priority:

Windows priorities

Pitfalls? Well:

Raising the priority of a thread is likely to give the greatest overall gain if it is usually blocked on IO but must run ASAP afer being signaled by its driver, eg. Video IO that must process buffers quickly.

Raising the priority of threads is likely to have the greatest overall negative impact if they are CPU-bound and raised to a high priority, so preventing the running of normal-priority threads. If taken to extremes, OS threads and utilities like Task Manger will not run.

Upvotes: 1

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