Qt-interest Archive, October 2006
thread pool example?
Message 1 in thread
I'm trying to make some code multi-threaded, so each CPU can work on a
portion of the data at once. However, I'm running into some performance
issues with thread overhead - starting up and waiting for the threads
for each chunk of the data is actually slower than working on it on only
one processor.
I'd like to try having a pool of sleeping threads that are already
running, and then send them a packet of data when I want to work on the
data, then wait for them to finish their packet, before continuing on in
the pipeline. I think avoiding the overhead of creating/destroying the
threads for each packet will help speed it up.
Can anyone point me to an example of a working Qt thread pool? I want to
start up a couple of threads (one per core) when the app starts up, have
them wait (using a wait condition of some sort) for a packet of data,
then work on the packet when it comes in, then go back to sleep. I want
the user of the pool to be able to wait until all packets are finished
before continuing. This is a little more difficult than simply creating
new threads and calling wait() on them for them to complete.
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Message 2 in thread
Paul Miller wrote:
> I'm trying to make some code multi-threaded, so each CPU can work on a
> portion of the data at once. However, I'm running into some
> performance issues with thread overhead - starting up and waiting for
> the threads for each chunk of the data is actually slower than working
> on it on only one processor.
>
> I'd like to try having a pool of sleeping threads that are already
> running, and then send them a packet of data when I want to work on
> the data, then wait for them to finish their packet, before continuing
> on in the pipeline. I think avoiding the overhead of
> creating/destroying the threads for each packet will help speed it up.
>
> Can anyone point me to an example of a working Qt thread pool? I want
> to start up a couple of threads (one per core) when the app starts up,
> have them wait (using a wait condition of some sort) for a packet of
> data, then work on the packet when it comes in, then go back to sleep.
> I want the user of the pool to be able to wait until all packets are
> finished before continuing. This is a little more difficult than
> simply creating new threads and calling wait() on them for them to
> complete.
>
Can Qt threads scope off of one CPU? Its been awhile since I looked but
I don't think they can.
Anthony
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Message 3 in thread
Hi,
> Can Qt threads scope off of one CPU? Its been awhile since I looked but
> I don't think they can.
Qt threads just wrap native threads. Why do you think they can't?
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Message 4 in thread
have a look at the mandelbrot-example.
The calculation-thread is a "forever"-loop waiting for the user to ask
for zooming in. It is not exactly what you look for, but using QThread
and QWaitCondition in that combination is a good starting point.
Greetings
Jens
Paul Miller schrieb:
> I'm trying to make some code multi-threaded, so each CPU can work on a
> portion of the data at once. However, I'm running into some performance
> issues with thread overhead - starting up and waiting for the threads
> for each chunk of the data is actually slower than working on it on only
> one processor.
>
> I'd like to try having a pool of sleeping threads that are already
> running, and then send them a packet of data when I want to work on the
> data, then wait for them to finish their packet, before continuing on in
> the pipeline. I think avoiding the overhead of creating/destroying the
> threads for each packet will help speed it up.
>
> Can anyone point me to an example of a working Qt thread pool? I want to
> start up a couple of threads (one per core) when the app starts up, have
> them wait (using a wait condition of some sort) for a packet of data,
> then work on the packet when it comes in, then go back to sleep. I want
> the user of the pool to be able to wait until all packets are finished
> before continuing. This is a little more difficult than simply creating
> new threads and calling wait() on them for them to complete.
>
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Message 5 in thread
On Sun, 2006-10-01 at 13:33 -0500, Paul Miller wrote:
> Can anyone point me to an example of a working Qt thread pool? I want to
> start up a couple of threads (one per core) when the app starts up, have
> them wait (using a wait condition of some sort) for a packet of data,
> then work on the packet when it comes in, then go back to sleep.
If you're desperate take a look at my evolvotron project (Qt3)
http://sourceforge.net/projects/evolvotron/index.htm
or
http://evolvotron.cvs.sourceforge.net/evolvotron/evolvotron/libevolvotron/
if you want to get straight to the code:
There's a MutatableImageComputer subclassing QThread,
and a MutatableImageComputerFarm aggregating a number of them
and dishing out MutatableImageComputerTasks from a prioritized queue.
The whole thing got a bit overcomplicated when I decided worker threads
could put part-completed tasks back on the queue if something higher
priority came in. Still, it seems to be stable and effective enough.
It's been a while since I touched the code. I remember being frustrated
by the lack of an equivalent of win32's WaitForMultipleObjects in the
Qt3 API. But looking at it now I'm wondering why I didn't just have
idle threads wait on QWaitCondition rather than polling for work at a
low rate. Hmmm... maybe it's a bad example!
Tim
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