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

[Thiago Macieira]

On quarta-feira, 20 de março de 2013 23.00.19, Guido Seifert wrote:
> 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?

Hi Guido

It's very hard to make a guess with so little information.

If the number of cores appears to affect the outcome of the execution, it 
usually means a threading problem (where "threading" here can be across 
processes too). If you increase the number of cores and it stops working, it's 
usually a race condition.

Since decreasing the number of cores makes it work, I would guess it's a 
priority inversion problem: you have too many tasks doing a lot of work that 
are hogging the available cores, but depend on data from other tasks. Those 
other tasks aren't getting CPU time.
-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center
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