multiple thread is slower???

[caperover2002]

Hi, All,

I tried to use multiple thread to speed up. Unfortunately, it becomes about 10% slower if I add one more thread, although exactly the same code is used in both cases.

I am sure my computer has two dual-core CPU's and my code is compiled with option multi-thread DLL. The only reason why it slows down, which I can think of, is that all threads are using the same CPU although there are four CPU's. Does anyone know how to test that instead of look at CPU performance?

I don't have any lock or mutex in my code, but four thread reads the same vector through iterator. Will that be a problem?

Besides this, is there any other reason that can cause mutiple thread to be slower?

Thanks,
CR