[Boost-users] two boost.threads taking much longer than one

From: Andrew Chapman (andrew.chapman_at_[hidden])
Date: 2004-10-07 12:14:53

Hi everyone,

I've got an operation which takes 14 seconds on a single thread, but is 
taking up to 45 seconds when running via two boost.threads on a dual 
proc machine (2x 3ghz Xeons, Fedora Core 1).

The thread functor objects are being passed a lot of references (about 
20) to some very large std::vectors. One thread always reads/writes from 
the start of the vectors to their midpoint, and the second thread from 
(midpoint+1) to the end of the vectors.

The only thing I can think of is that the threads are taking a long time 
to fork and get running, perhaps moving a lot of stack data around or 
something?

When I look at the output of 'top' with threads displayed, I see the two 
threads appear, but they only use about 50-60% CPU utilization each. The 
system monitor shows mostly red (Kernel) as opposed to blue (User).

Any ideas?

-- 
Andrew Chapman
Senior Technical Director - Framestore CFC