Multi threading with one processor??

boon kian

Hi I am writing a program to process multiple independent datas. This program is very processing intensive. So i am wondering if multi threading would help to reduce the amount of processing time. I have doubts over it because I have only one processor. So wouldn't the time taken for the processing to be the same in multithreading case (or even more due to addition overhead)?? If it helps to cut down the time taken, how does it do it with only one processor?? Can someone please enlighten me ?? Thank you very much for the kind attention !!

jhwurmbach

boon kian wrote: So wouldn't the time taken for the processing to be the same Maybe, but your App stays responsive.

My opinions may have changed, but not the fact that I am right.

Mike Nordell

boon kian wrote: So i am wondering if multi threading would help to reduce the amount of processing time. Not using a single CPU. Quite the opposite; the overhead will make the processing slower. If the process is e.g. I/O bound the percieved speed of the application might benefit greatly from multithreading, but the total CPU time used will always be greater for a multithreading program than a singlethreded program (even if by such a small amount of time that one wouldn't even bother to try measure it).

Nitron

I agree with the above posts, but there are some things you may want to consider... First, the user usually despises apps that appear "frozen" and unresponsive. If you go calling a time-consuming data reduction function from the main UI thread, your app will appear to "hang" during processing. If you don't pump update messages and redraw the screen, the user can slide another window over your app, and your app will just appear blank and dead. However, don't go off spawning 20 threads to process your stuff either! I would definately consider putting the data reduction algorithms in a seperate "worker" thread, and pumping progress messages from within it. Although you won't gain any speed (or may actually lose some), it is most likely in the best interest of the user. This will keep your UI happy and responsive, and the world will be a better place :rolleyes: - Nitron

"Those that say a task is impossible shouldn't interrupt the ones who are doing it." - Chinese Proverb

Ted Ferenc

As has already been mentioned it could be slower, but you will find that the single CPU will be running at 100% all the time, whereas a single threaded app could be waiting for some input/event whilst trying to process the data. If the data does lend it's self to multithreading then do it, you never know you may get a multi cpu machine, I do have a 2 cpu machine, but be careful as there could be an interaction between processing data on each of the cpu's that could cause the prog to hang/crash

If I have seen further it is by standing on the shoulders of Giants. - Isaac Newton 1676

Nitron

Ted Ferenc wrote: whereas a single threaded app could be waiting for some input/event whilst trying to process the data. umm, don't you mean a multi-threaded app? :~ A single thread will cycle through the processing algorithm and not return until complete. The multi-threaded app can process the data AND recieve messages from the user. - Nitron

"Those that say a task is impossible shouldn't interrupt the ones who are doing it." - Chinese Proverb

Ted Ferenc

What as I was trying to say was on a single CPU machine, multithreaded, the CPU usage will probably be 100% but if only one thread is used it could end up waiting for some event therefore it will not use 100% CPU time, but there is an overhead of course. But if multithreaded the likelyhood is that one thread may be waiting but another should be processing. I fink my grammer was not correct, sorry:confused:

If I have seen further it is by standing on the shoulders of Giants. - Isaac Newton 1676