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i have a bit of C++ code that does some pretty heavy-duty image processing work. sadly, the code wasn't running as fast as i'd hoped, so, after doing all the optimizations i could think of, i decided to try multi-threading it.
i broke the job into strips, with each thread taking a horizontal strip of the image. the algorithm is the same, and 99% of the code is the same; the only difference is some bounds-checking code that the multi-threaded version skips. there is no communication between threads.
the multi-threaded version was more than twice as slow as the single-threaded version - i kept the single-threaded version of the code intact, for benchmarking.
i wrote it using VC6 (because I prefer the IDE to the VS.Net IDEs). so, i opened the project in VS05, built and ran it - hoping the VS05 compiler had some better optimization. but no, the single-threaded version ran exactly as fast as it did when built with VC6.
on the other hand, in VS05, the multi-threaded version ran twice as fast as the single-threaded version. again: the multi-threaded code went from twice as slow to twice as fast, simply by changing the compiler, but the single-threaded version didn't change at all.
these projects are all built with the multi-threaded CLR, same optimizations (controlled by pragmas in the code). the single and multi-threaded function live in the same module (same class, in fact).
so, something changed between VC6 and VS05 which affects how fast threads run.
does anybody know what that change was ?
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Hi Chris,
I don't think the IDE is fundamentally involved in the difference of the results. IMO you either are unfortunate or made some major mistake.
Here are some possible mistakes:
- a total disregard of "locality of reference": in the ideal case all data ever gets loaded only once in the data cache(s); having two or more threads working somewhat independently may make that harder to achieve.
- too much need for synchronization;
- an overall alignment problem, e.g. when both threads need new cache loads at the same time, they both sit idle waiting for main memory to respond. At best memory accesses should be staggered somehow by design.
- cache trashing for shared data (when the same data resides in the data cache of two distinct CPU chips and gets written by one processor, it needs updating in the other cache, a very expensive operation); the extreme case is when synchronization is based on spin locks, and two or more spin lock variables reside in the same cache line. (There is an interesting Intel Application Note on keeping no more than one sync variable in a cache line, applicable to dual processor chips, not Core2Duo).
Image processing generally is quite vulnerable to all kinds of such phenomena, mainly because there is a rather high transport/calculation ratio, where data streaming can work well (making the job entirely compute bound) and can work very badly (making it bus bandwidth bound); and the difference may be minor and accidental, as well as major by a faulty approach.
I would suggest you research this case starting with a simple experiment: ignore the middle border pixels, i.e. let one thread handle the left 45% of the image, the other the rightside 45% of the image, and ignore the middle 10%. Have it run on both IDE; if you did everything right you should notice it runs about twice as fast as a single-threaded solution, and it is IDE independent. Then all you have to do is look into the way you handle the middle 10% of the image.
BTW: don't forget to observe CPU load in Task Manager for both the 1 and 2-threaded attempts.
If OTOH the two-thread solution isn't really twice as fast as the single-threaded one, forget about the middle 10% (and all sync stuff) until you got the threading right for the independent image areas.
If you were to want more specific advice, please provide detailed information on the task: system hardware, typical image size, kind of image operation (e.g. filter size, is it inherently sequential?), etc.
Luc Pattyn [Forum Guidelines] [My Articles]
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Luc Pattyn wrote: gnore the middle border pixels, i.e. let one thread handle the left 45% of the image, the other the rightside 45% of the image, and ignore the middle 10%.
i think i updated my post since you started your reply.
the process here isn't the 50/50 split with the heavy inter-thread communication, which i initially described - that was a different part of the job, the one i stayed up all night fighting with. rather, the process is four totally independent threads going after different horizontal strips of an image, doing a Sobel filter (a two kernel convolution) (tried vertical strips, too - slightly slower). the only difference in code for the multi-threaded operation is that threads working on strips which don't touch the top or bottom of the image don't have to worry about bounds checking.
Luc Pattyn wrote: I don't think the IDE is fundamentally involved in the difference of the results.
it is IDE (or at least CRT) dependent.
single-threaded, VC6: 0.31 s
multi-threaded, VC6: 1.0 s
single-threaded, VS05: 0.31 s
multi-threaded, VS05: 0.14 s
VS08 results are the same as the VS05 results.
exact same code, same machine, same input and parameters, tested multiple times. the single-threaded result confirms that the difference must be in how threading is handled by the different VC versions.
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Hi Chris,
yes I was replying to your post in VS forum, however by the time I had finished, your message was gone; I found this one here, but didn't notice it was somewhat different.
Of the four numbers you mention indeed the multi-threaded VC6 is odd, I still don't believe it is due to the IDE, compiler progress is steady but slow, I have never seen a huge gain from one version tot he next.
The single-thread results being equal suggests very similar code was generated, and the two-threaded VS05 being twice as fast tells us your multi-threading seems to work just fine.
I can't imagine they ever wasted up to 1 second in launching some threads. Maybe the runtime libraries were structured quite differently, causing other/more/larger DLL files to be loaded after you started your timing. How did you measure these times? how did you build and run?
Here are some experiment suggestions:
- enclose code in a for(i=0; i<10; i++) loop and measure 10 executions inside a single process, watching for variance due to code loading, memory allocation effects, and the like. (I always do this, I have seen too many weird things going on in a first execution)
- release build
- run outside VS
Luc Pattyn [Forum Guidelines] [My Articles]
The quality and detail of your question reflects on the effectiveness of the help you are likely to get.
Show formatted code inside PRE tags, and give clear symptoms when describing a problem.
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Luc Pattyn wrote: How did you measure these times? how did you build and run?
{
CTimer[^] t("Sobel");
Sobel(....);
}
start the test app, wait a couple of seconds for things to settle down, etc..
release build, yes. tried all the various compiler optimizations, etc..
i didn't put this one in a loop because it took long enough that i wasn't up against the resolution limit of GetTickCount. i understand the need to time multiple iterations in order to get a decent average, but i did run it dozens of times, with consistent results.
weird stuff.
anyway, i guess i'll just build the final DLL in VS05
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I have seen performance increase as much as 25% in certain functions when porting old code up to VS2005. In the days of VC6 I could use __asm and optimize algorithms better than the compiler. I have a hard time beating VC2005/VC2008 compiler optimizations now. I pretty much gave up on trying to beat the latest compilers.
You could use the /FAs compiler switch[^] and compare the assembly generated by each compiler to see the difference.
Best Wishes,
-David Delaune
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Yeah I'd say its hard to beat the machine in this case.
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Hi Chris,
FWIW: there was a time VS was terribly slow at dealing with the first exception in an app. Not sure which versions and languages, but it was not very constant and in the order of seconds. Maybe that is what you're having?
Luc Pattyn [Forum Guidelines] [My Articles]
The quality and detail of your question reflects on the effectiveness of the help you are likely to get.
Show formatted code inside PRE tags, and give clear symptoms when describing a problem.
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hmm...
i don't recall seeing any exceptions in the output window, but i'll check again.
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The heap manager for VS2005 is faster than that for Visual 6. I ran across this a few weeks ago.
How thread local storage is handled is different fore VS2005 as well, though I doubt this would affect performance in a significant way.
You also need to verify you really are testing what you think you're testing. How fast is the second thread even executing? Due to small changes in the CRT, the new thread may not be getting a quanta as quickly under 6.
Another possibility is that some paging is happening under 6 that isn't happening under 05 (seen that also.)
Anyone who thinks he has a better idea of what's good for people than people do is a swine.
- P.J. O'Rourke
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Joe Woodbury wrote: How fast is the second thread even executing?
that's an interesting question. i never thought to time each thread independently.
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Hello Friends,
I am creating a IE toolbar. I want to develop feature like "Google Toolbar" which allow us to add google's Gadgets in the toolbar. How does a toolbar notified by the webpage that user has selected(by clicking on the "Add" button on webpage) a perticular gadget?
Please suggest the way in which this can be achieved.
ritz1234
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Please Help!
I want to pass variables from form1 to form2,
I want to pass structure (struct fff) from form1 to form2.
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Ok, go ahead.  Reading here wouldn't hurt either.
"Old age is like a bank account. You withdraw later in life what you have deposited along the way." - Unknown
"Fireproof doesn't mean the fire will never come. It means when the fire comes that you will be able to withstand it." - Michael Simmons
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Hi
I created an application in which ,there is a RichEdit control.But i am not getting
proper strings for unicode letters which I typed in.(eg: when I type a character it shows
"?").How to solve this.
thanks
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We have a project at work that needs to support outputting various bits of data via SNMP. I've been playing with the WinSNMP APIs a bit, and I'm stuck on one concept. If you want to report these bits of data, through a custom MIB/OIDs do you have to use the extension DLL approach? In other words, the logic for where the data is located/generated is in another running app. Is it possible for that app to register itself somehow so that SNMP GET messages for our custom OID that come into the machine can get handled by that app? Or is the only way to accomplish this to use the extension DLL?
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It's been ages since I did SNMP development, but looking at some old code, my vague memory is that you have to use extension agent DLLs. I did note I was using a shared queue to communicate with other programs, which reinforces this view.
After posting this, I recalled putting a data area inside the extension agent and updating it through the message queue so the agent could respond very quickly with responses and work regardless of whether the other programs were running. I do recall that one reason for this is the requirement by Microsoft's SNMP stack (at least in NT 4.0) required that you respond to a GET in a certain time window and really bad things happened if you didn't (I do have a vague memory that the "really bad things" issue was fixed in W2K, but hey, it's been seven years.)
Anyone who thinks he has a better idea of what's good for people than people do is a swine.
- P.J. O'Rourke
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OK, thanks, I did just find this:
How to develop a SNMP extension agent DLL[^]
which confirms what I was afraid of, that you have to be an extension DLL. Not the end of the world, just requires a bit more set up.
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Hello,
I have a CRichEditCtrl on CMDIChildWnd which is created on the window properly but it(CRichEditCtrl) cannot capture mouse control when the mouse is on the window(CRichEditCtrl).What must be the problem?
Prithaa
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Hello,
This is continuation of the problem. The keyboard cursor is seen on the CRichEditCtrl but the mouse cursor is not seen.
Prithaa
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what is the difference between Simple Mapi and Extended Mapi
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Let me...[^]. 
If the Lord God Almighty had consulted me before embarking upon the Creation, I would have recommended something simpler.
-- Alfonso the Wise, 13th Century King of Castile.
This is going on my arrogant assumptions. You may have a superb reason why I'm completely wrong.
-- Iain Clarke
[My articles]
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The answer's contained in here[^], which is easily accessible through the programmer's best friend[^].
Java, Basic, who cares - it's all a bunch of tree-hugging hippy cr*p
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Pleaseeeeeeeeee can any one help me to declare public struct or variable to use in all the forms in
"Windows Forms Application C++"
ex: i want to declare a connection and use it in all the forms.
i want to declare a structure and use it in all the forms.
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