Problems with Sleep() in thread

[Chris Losinger]

he was talking about accuracy in the 10-20ms range. GetTickCount can't get near that.

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[acerunner316]

well, it doesn't have to be super accurate, but 6ms delay out of the specified 10ms is huge. 1 or 2 ms might be acceptable. Can you explain how I would use SetTimer to solve my problem, where would I call it? Also can you explain how to use a high priority thread. Would I be able to access member variables/functions of the main class? Does it operate differently than a worker thread.

[Stephen Hewitt]

acerunner316 wrote:

Can you explain how I would use SetTimer to solve my problem, where would I call it?

Calling SetTimer wouldn't solve your problem. I was just pointing out that using SetTimer instead of posting messages from a worker thread would be simpler and only marginally less accurate.

acerunner316 wrote:

Also can you explain how to use a high priority thread. Would I be able to access member variables/functions of the main class? Does it operate differently than a worker thread.

When I said a "high priority" thread I simply meant a worker thread with a high priority. A thread's priority can be set using SetThreadPriority. Normally having a thread with a really high priority is a bad idea but since our thread spends most or its life asleep and when it is awake just takes a sample then goes back to sleep it’s ok. In this context the high priority is just to reduce the scheduling overhead so that our thread is chosen by the scheduler earlier than others when the sleep time has expired. As to accessing member variables of a class that’s a complex question which would require more detailed knowledge of your application.

Steve

[ThatsAlok]

acerunner316 wrote:

I am using Sleep(10) in my thread to post a windows message every 10ms.

Sleep(10) doesn't sleep your thread fro 10 ms, as windows is not realtime system... actually it sleep time range between 10 ms to 54 ms depending on current load on the system and processor speed

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[Joe Woodbury]

Generally Windows NT and later have a timer interrupt of 10ms. However the HAL for some systems has set it to use 15ms. This means you can't sleep for less than that amount using the timeouts for Sleep() and Wait...() calls.

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[kasturi_haribabu]

1. Are you sure your function which is posting message takes '0'ms to execute ? I raise this question because, if your calling code has a lot to do and then sleep 10 ms, the interval between subsequent calls would be (Sleep time + Time taken to execute other part of the function). If you are very specific about the response time, i dont think post message is the right way to address such issuesbecause the latency of the response is not guaranted. You can use Events or a Semaphore-give-release technique to decrease the latency further. but the technique you need to choose depends on your applcation. Haribabu.

[S Douglas]

acerunner316 wrote:

but 6ms delay is still way too slow

To get an understanding of how time works read Time is the Simplest Thing[^]

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[Jorgen Sigvardsson]

So.. uh, the system timer resolution is too low? Lucky me that I don't have to do anything multimedia related, because I was under the impression that a timer which is given the attribute system would be the most accurate one. Thanks for the heads up - who knows? I might have to deal with issues like this some day...

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[Chris Losinger]

it's updated every ~15ms to 50ms (depending on the version of Windows). so, it is measuring time in the ms range, but the difference between two consecutive calls will never be less than ~15ms. QueryPerformanceCounter is much more accurate.

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[acerunner316]

I don't know if it takes 0ms, but there is nothing else to my code other than repeatedly posting messages. UINT CNOx1000MonitorDlg::FThreadLoop (LPVOID PptrToClass) { CMainWindowDlg *LptrClass = static_cast(PptrToClass); HWND *ptrWindowHandle = new HWND; *ptrWindowHandle = LptrClass->GetSafeHwnd(); while(LptrClass->mbolThreadExecute) { ::PostMessage(*ptrWindowHandle, WM_UPDATE_CONTROLS, 0, 0); ::Sleep(10); } delete ptrWindowHandle; return 0; }

[acerunner316]

How will I use QueryPerformanceCounter as a way of sampling data? I will still need a thread to loop a function call. And in that function will check QueryPerformanceCounter. But if posting a message takes longer than 6ms. Then QueryPerformanceCounter can only be as accurate as that. Can you provide a code sample? Im fairly new to windows programming. I am basically learning from sample code that I find online. Thanks.

[Chris Losinger]

acerunner316 wrote:

I will still need a thread to loop a function call. And in that function will check QueryPerformanceCounter.

right

acerunner316 wrote:

But if posting a message takes longer than 6ms. Then QueryPerformanceCounter can only be as accurate as that.

right maybe this article can help... scroll down to the bottom for a message-loop / QueryPerfCounter sample.

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[acerunner316]

sorry, i'm still having trouble understanding. In code sample at the link you gave, I can see that the function is monitoring for any messages and checking for the time. But there is the message being generated from, and how?

[Chris Losinger]

acerunner316 wrote:

But there is the message being generated from, and how?

the message comes from code outside the loop. this is pretty close to an old-style C Windows message loop. Windows is passing messages (button down, repaint, keydown, etc) to the application, which the app grabs, when it sees one, and starts processing with that Peek/Translate/DispathMessage bit - other code is responsible for handling the specific msgs. otherwise, every 40ms, the app does a run through its Render/Move/etc stuff, then goes back to looking for messages.

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[acerunner316]

Isn't this similar to what I'm already doing, posting messages every 10ms? Because what I'm doing is posting the message repeatedly in a loop every 10ms. And then I have a function mapped to handle that message ON_MESSAGE(WM_UPDATE_DATA, OnUpdateData). So the only difference I see is instead of using ON_MESSAGE, I will have my own function that loops to check for the message posted an respond to it. Is this right? Will this make the difference of the 6ms delay? Would I have to post this message like every 1ms in order to get the 1ms resolution?

[Chris Losinger]

acerunner316 wrote:

Isn't this similar to what I'm already doing, posting messages every 10ms?

pretty much. but Sleep simply isn't that accurate. Sleep uses the system clock, which has a resolution closer to 15ms per tick in most modern versions of Windows. and, it's entirely possible for your sleeping thread to get preempted by another thread, further delaying its awakening. i'm not sure what the overhead in PostMessage / ON_MESSAGE amounts to. but they're asynchronous anyway; apps respond to messages when they get around to it, not the instant they appear in their message. the time from PostMessage to the actual response is (i believe) indeterminate.

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[acerunner316]

Chris Losinger wrote:

pretty much. but Sleep simply isn't that accurate. Sleep uses the system clock, which has a resolution closer to 15ms per tick in most modern versions of Windows. and, it's entirely possible for your sleeping thread to get preempted by another thread, further delaying its awakening.

In that case, what do I use to loop posting of a message?

[Chris Losinger]

honestly, i don't know. Windows really isn't a real-time OS, and as far as I know, there's not a lot of high-level support for the kind of timing precision you seem to want. maybe you could check an open source game or audio application (ex. Audacity) that you could plunder for ideas.

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