Re: threads within threads

From: pmedrow (pmedrow_at_[hidden])
Date: 2003-06-23 11:58:02

--- In Boost-Users_at_[hidden], "William E. Kempf" <wekempf_at_c...> 
wrote:
> 
> pmedrow said:
> >
> > Hi
> > I am attempting to create a boost thread within an existing boost
> > thread, i.e. a function called by the operator() function of an
> > existing, running thread is attempting to make more threads. The 
new
> > threads function objects are of the same type, but different from 
the
> > parent function object type.
> >
> > This compiles and runs, but takes time equivalent to the threads
> > running sequencially on a single thread. CPU usage indicates 
that  both
> > threads are running in parallel. Very strange...
> >
> > I am building with gcc v3.2.3 on red-hat linux build 2.4.20.
> > Any ideas gratefully received!
> 
> You don't give anywhere near enough information for a definative 
answer,
> but you've said enough for me to assume that you're under a 
misguided
> assumption.  Threads do not automatically equate into less overall 
clock
> time.  If you've got an MP machine and the scheduler runs the 
threads on
> multiple CPUs, you can usually expect an increase in perceived 
speed.  The
> amount of time a thread spends waiting for something can also 
effect the
> perceived speed.  But the actual CPU time (as opposed to clock 
time) spent
> is not effected by running multiple threads.  If the threads just 
do a lot
> of calculations with out doing any I/O and you run the program on a 
single
> processor machine, you can expect no significant impact to the over 
all
> time (actually, it will be a little slower due to the overhead of
> creating/joining threads, and possibly due to time spent task 
switching,
> but both should be too minimal to detect in most cases).  Do know,
> however, that turning a program into a multi-threaded program not 
only may
> not give you a speed improvement, it may actually perform worse, 
due to
> synchronization.  It all depends on how the application is 
architected,
> what the threads do, and how many CPUs are available to run the 
threads
> on.
> 
> -- 
> William E. Kempf

William

I was enquiring into whether or not there were any known issues in 
boost threads regarding (for example) scoping.
I am aware of basic thread theory, and yes, I do have a multiple CPU 
machine. It looks like I am hitting some system resource bottleneck, 
most likely memory speed/bandwidth.
My intention was not to come across as an idiot so I could waste your 
time, I was merely trying to be polite.
Don't worry about it, I'm sure boost threads are working fine.