Help on Multi-Threaded applicaiton [modified]

Kiran Satish

Thanks, I feel like a complete dumbo. I always use CEvent::WaitForSingleObject(,) in many threads so that they wont lock the CPU and never thought of using them here. Maybe I need to start thinking slowly ;P

PKNT

Mark Salsbery

:)

Kiran Satish wrote:

I always use CEvent::WaitForSingleObject(,) in many threads so that they wont lock the CPU

Yes!. It also throttles your thread loops so they don't sit there spinning while waiting for the flag to be set, eating all your CPU for 1 processor (each). Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Mark Salsbery

plus...you can add a terminate event if needed so the threads can be shutdown gracefully. Then you'd wait on multiple objects. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

I tried to implement this in simple way to test and rather than waiting for the event to occur, it runs continuously even I define the timeout value to be INIFINITE

getsnap()
{
start camera thread if not running
PulseEvent(getnewframe)
waitForSingleObject(newframeevent, INFINITE)
return;
}

camera thread
do
{
waitForSingleObject(getnewframe, INFINITE)
get new frame - takes 20ms
PulseEvent(newframeevent)
}while(camrunning);

Here the camera thread continues running even after taking a frame. I am sure 'm missing something here. Sorry for bothering :) . -thanks

PKNT

Mark Salsbery

I would use (and recommend) SetEvent() on auto-reset events and not PulseEvent(): getnewframe = ::CreateEvent(NULL, FALSE, FALSE, NULL); newframeevent = ::CreateEvent(NULL, FALSE, FALSE, NULL); I can't remember why but something about PulseEvent() is flaky/unreliable if I recall correctly (besides the fact that a thread has to be already waiting on it for it to work, which is bad for this case)....I just know I never use it. Compare that to SetEvent() - "The state of an auto-reset event object remains signaled until a single waiting thread is released". That's the behavior you'd want. Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

I tired to implement using SetEvent, but it still deadlocks, as I dont see any processor activity at all and just a busy mouse icon on the application interface. Here is how I did it - I defined the event handles at the start of the application in your way rather than using CEvent objects like I did before

getnewframe = ::CreateEvent(NULL, FALSE, FALSE, NULL);
newframeevent = ::CreateEvent(NULL, FALSE, FALSE, NULL);

when the user clicks snapshot, I call a function (snapshot)

snapshot(exposure)
{
set camera exposure
start snapshotthread if not running
SetEvent(getnewframe)
::WaitForSingleObject(waitfornewframe, INFINITE);
display the frame
}
.
.
.
camthread
{
do
{
::WaitForSingleObject(getnewframe, INFINITE);
take a 20ms snapshot
SetEvent(waitfornewframe);
}while(camrunning)
}

I still dont get where I am doing wrong :confused:. I even tried using a while loop in snapshot function and using a 1ms timeout value for WaitForSingleObject and checking for WAIT_OBJECT_0.

PKNT

Mark Salsbery

I'm not seeing a deadlock situation in the pseudo code... If you run it in the debugger and break, are both threads sitting in the Wait function? If so, why?

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

Here is what happening in debug more

snapshot(exposure)
{
set camera exposure
start snapshotthread if not running
SetEvent(getnewframe)
::WaitForSingleObject(waitfornewframe, INFINITE); -- set breakpoint(1)
display the frame
}
.
.
.
camthread
{
do
{
dummy variable -- set breakpoint(2)
::WaitForSingleObject(getnewframe, INFINITE);
take a 20ms snapshot
SetEvent(waitfornewframe);
}while(camrunning)
}

when I hit snapshot button, the program stops at breakpoint(1) and then goes to breakpoint(2) and thats it from there it does nothing while the C++ interface shows as the program is in [run] mode. From the definitions of SetEvent and WaitForSingleObject I too dont see any thing wrong here. Do they need to be in actual two different threads, I dont think thats the case, interesting!

PKNT

Mark Salsbery

hmm the only reason I see that could happen is if "take a 20ms snapshot" never returns so the waitfornewframe event gets set.

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

I even tired that too by commenting it out, but it does the same. will keep tryin different ways and see. Do you think I may need to define those event handles in CMutex objects :~ .

PKNT

Kiran Satish

does it matter how I am defining my thread?? I am using DWORD WINAPI camclass::camthread(LPVOID) form.

PKNT

Mark Salsbery

Kiran Satish wrote:

Do you think I may need to define those event handles in CMutex objects

No that's a different type of synchronization object for mutual exclusion.

Mark Salsbery Microsoft MVP - Visual C++ :java:

Mark Salsbery

How are you creating your threads? If you're using MFC and using MFC class objects in your threads you should be using AfxBeginThread(), and the prototype for the thread proc is UINT __cdecl ThreadProc(LPVOID pParam); If you're not using MFC but using CRT functions anywhere in your threads (including new/delete!) then you should be using _beginthread() or _beginthreadex(), and the prototype for the thread proc is unsigned __stdcall ThreadProc(void *lpParameter); For a pure C/C++ thread on Windows which doesn't use MFC or the CRT, you can use ::CreateThread(), and the prototype for the thread proc is DWORD WINAPI ThreadProc(LPVOID lpParameter); For MFC and/or CRT, make sure you're linking to the multithread version of those libraries.

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

I got it running. Unfortunately it doesnt run when I do it the way we discussed earlier, but when I run it in two different threads it works like it should. I am not sure why it did like tht before, but it doesnt work when I have it in between a function and thread.

PKNT

Mark Salsbery

I always thought it was two threads!

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

Sorry abt that confusion... maybe I wasnt clear enough.... :), anyway now I know how to use Events in an efficent way... thanks for your help and patience on this...

PKNT

Mark Salsbery

Kiran Satish wrote:

maybe I wasnt clear enough

I think I just assumed :) Glad you got it working! Cheers, mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

I am back with another issue in this application :) , saving frames in real-time without dropping and in the same order. I tried using BipBuffer[^]. This is how I implemented, the actual thread keeps running and signals an event to 'SaveImage' thread for every ~20msec where it appends the image data to BipBuffer memory while another thread keeps running all the time saving images from BipBuffer until the BipBuffer memory is NULL. I am allocating 64x(512x512)bytes of memory to BipBuffer. While running like this, the close loop (main thread) starts off at 50Hz and after a while (like 6-7 sec) depending on the size of BipBuffer memory, it slows down to like 39Hz and then goes back to 50Hz slowly. Is there any other way I can save all the images in real-time at 50Hz with out dropping the actual frames coming in from the camera (i.e I have ~17msec to save a particular frame)?? -thanks

PKNT

Mark Salsbery

Kiran Satish wrote:

I am allocating 64x(512x512)bytes of memory to BipBuffer

Are you sure that's a big enough buffer? If the data is 24bpp RGB, you may want 64x(512x512x3). Which thread is slowing down? Are you locking something (or staying in a critical section) too long? Mark

Mark Salsbery Microsoft MVP - Visual C++ :java:

Kiran Satish

My images are only 8bpp(0-255) and I use unsigned char pointer for image data. I am not sure which thread is slowing down and as fas as I know I dont have any critical sections. Here is the order- BipBuffer ImageBuffer; ImageBuffer.AllocateBuffer(128*m_FrameBufferSize) //m_FrameBufferSize = 512x512 -Main Thread (camera thread)- // this controls the processing thread while (camthread) take image (~20.5msec) copy image data into shared buffer set event to processing thread -Processing thread- while (processing thread) wait for event from camera copy image data from shared buffer to processing buffer if(saveimage) copy image data from shared buffer to buffer for saving image Set event to save image thread do the processing on image data in processing buffer Update displays for results -Save Image thread- BYTE* pData wait for save image event pData = ImageBuffer.Reserve(m_FrameBufferSize, iResult) memcpy(pData, m_pImageBuffer, iResult) ImageBuffer.Commit(iResult) //iResult will be equal to m_FrameBufferSize -Store Image thread- //this thread runs continuously until BipBuffer == NULL while ((pData = ImageBuffer.GetContiguousBlock(iResult)) != NULL){ create filename write image data to a file (I tried writing in two ways to see if there will be any difference in speed, one as a tiff format and also as a binary file) ImageBuffer.DecommitBlock(iResult)} I think I might just use a standard circular buffer of my own rather than BipBuffer as I see there is something going on when using it. When I run the program loop for 15sec, the number of images being stored are increasing from one run to another. Hope I am clear :) . If I comment out writing image data to a file in store image thread, everything runs fine at desired speed. -thanks

PKNT