GUI in Perl/Tk runs slow unless threads sleep periodically

Smile-n-Nod has asked for the wisdom of the Perl Monks concerning the following question:

I have a GUI app (written in Perl/Tk) that uses a few dozen threads and the Boss/Worker model to manage background tasks. The app runs very slowly unless the threads sleep periodically (using the select command), in which case the GUI runs very quickly. What is going on?

Here is an untested code snippet:

    while (1) {
        eval {
            if (my $key = $Queue->dequeue_nb) {
                do_something();
            }
        };

        if ($@) {
            LOG("problem");
        }

        select undef, undef, undef, 1.00;
    }
[download]

If I comment out the select command, the GUI runs an order of magnitude more slowly. What is the cause of this behavior?

Comment on GUI in Perl/Tk runs slow unless threads sleep periodically Select or Download Code

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Re: GUI in Perl/Tk runs slow unless threads sleep periodically by GrandFather (Saint) on Mar 23, 2011 at 23:06 UTC
If your non-GUI threads are processor bound they will consume as much of the processor time as they are allowed by the OS and that will impact on any other processor bound thread such as your GUI thread. By chucking in some "down time" in your processor bound threads you give other threads a chance to get some work done. Probably 1 second is excessive. Most likely you could reduce that to 1ms and still have a responsive GUI thread without impacting the processing threads much. True laziness is hard work	[reply]
Re: GUI in Perl/Tk runs slow unless threads sleep periodically by BrowserUk (Patriarch) on Mar 24, 2011 at 00:12 UTC
`if (my $key = $Queue->dequeue_nb) {` [download] Solution: avoid `dequeue_nb()` unless you have a reason to use it. Ie, something else useful to do if there is nothing in the Q. As coded, (without the sleep), your thread will consume 100% of the processing time of 1 cpu. If you have as many (or more) of those threads as cores, they will be consuming all your cpu whenever they can, which will not only make your GUI thread sluggish, but your entire system. So don't do that. Recast your queue processing loop as: `while( my $key = $Queue->dequeue ) { do_something(); }` [download] And when there is something for your thread to do, it will wake up, do it and go back to sleep, consuming no cpu when there is nothing to do. What errors are you hoping to trap with eval? Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l] [select]
Re^2: GUI in Perl/Tk runs slow unless threads sleep periodically by Smile-n-Nod (Initiate) on Mar 24, 2011 at 16:29 UTC
The errors I'm hoping to trap with eval are in the &do_something() function.	[reply]
Re^3: GUI in Perl/Tk runs slow unless threads sleep periodically by BrowserUk (Patriarch) on Mar 24, 2011 at 16:50 UTC
The errors I'm hoping to trap with eval are in the &do_something() function. Then wrap that function, not the rest. Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply]
Re: GUI in Perl/Tk runs slow unless threads sleep periodically by Anonymous Monk on Mar 24, 2011 at 00:11 UTC
You have to cause the various threads to give up control of the processor on a very frequent basis.	[reply]