[Interest] Program runs only on four core processors.... sometimes.

[Guido Seifert]

Hi,
I have here a strange problem. Probably not 100% ontopic for this list. But it is a Qt program. :-)

My problem: I have three programs, which exchange data with a fourth program via tcp and udp. On my i7 and i5 with four cores it runs perfectly. It also runs perfectly on my partner's machines. Now I gave it to a few testers... and it was pure disaster. Bugs I have never seen before. From the description it seems to be some kind of deadlock. The main program just freezes. None of the four programs are multithreaded. At least not directly. The programs use the Qt network classes. Since I don't have an unlimited pool of hardware I tried to run my program in a Windows 7 and Windows 8 installation in a VirtualBox environment. And strangely I could see exactly the reported behaviour. But only when I gave the virtual machine < 4 cores. As soon as I switched the virtual machine to four or more cores everything worked perfectly again. Slow and jerky, but generally as expected.

This behaviour is not 100% consitent. There is a tester with an i7 machine and four cores, who sees the same blocking behaviour.

I am a little at a loss here. Anyone of you seen something similar? Any ideas what the problem could be?

Guido