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Can code running in a background thread be faster than in the main VCL thread in Delphi?

Asked 2011-07-19T03:54:42.033
9

If anybody has had a lot of experience timing code running on the main VCL thread vs a background thread, I'd like to get an opinion. I have some code that does some heavy string processing running in my Delphi 6 application on the main thread. Each time I run an operation, the time for each operation hovers around 50 ms on a single thread on my i5 Quad core. What makes me really suspicious is that the same code running on an old Pentium 4 that I have, shows the same time for the operation when usually I see code running about 4 times slower on the Pentium 4 than the Quad Core. I am beginning to wonder if the code might be consuming significantly less time than 50 ms but that there's something about the main VCL thread, perhaps Windows message handling or executing Windows API calls, that is creating an artificial "floor" for the operation. Note, an operation is triggered by an incoming request on a socket if that matters, but the time measurement does not take place until the data is fully received.

Before I undertake the work of moving all the code on to a background thread for testing, I am wondering if anyone has any general knowledge in this area? What have your experiences been with code running on and off the main VCL thread? Note, the timing measurements are being done when there is absolutely no user triggered activity going on during the tests.

I'm also wondering if raising the priority of the thread to just below real-time would do any good. I've never seen much improvement in my run times when experimenting with those flags.

-- roschler

multithreading performance delphi vcl timing
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2 Answers

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When your code has control of the VCL thread, for instance if it is in one method and doesn't call out to any VCL controls or call Application.ProcessMessages, then the run time will not be affected just because it's in the main VCL thread.

There is no overhead, since you "own" the whole processing power of the thread when you are in your own code.

I would suggest that you use a profiling tool to find where the actual bottleneck is.

answered 2011-07-19T05:20:02.260
1

Are you asking if a background thread would be faster? If your background thread would run the same code as the main thread and there's nothing else going on in the main thread, you don't stand to gain anything with a background thread. Threads should be used to split and distribute processing loads that would otherwise contend with one another and/or block one another when running in the main thread. Since you seem to be dealing with a case where your main thread is otherwise idle, simply spawning a thread to run slow code will not help.

Threads aren't magic, they can't speed up slow code or eliminate processing bottlenecks in a particular segment not related to contention on the main thread. Make sure your code isn't doing something you don't know about and that your timing methodology is correct.

My first hunch would be that your interaction with the socket is affecting your timing in a way you haven't detected... (I know you said you're sure that's not involved - but maybe check again...)

answered 2011-07-19T05:54:58.490

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