Re: [Boost-users] [Graph] Multithreading slowdown on dual core Pentium D

From: Douglas Gregor (doug.gregor_at_[hidden])
Date: 2007-01-19 07:22:58

On Thu, 2007-01-18 at 13:27 -0600, Brian Townley wrote:
> All I have to do in my code to see the slowdown is to use a populated
> graph with johnson_all_pairs_shortest_path in some iterative loop in
> both one and multiple threads. A real kicker is that currently I can
> get around this problem by spawning multiple processes to deal with
> the data load (so I have an interim solution at least) and that gives
> me a performance improvement by 2x which I expect with dual core...
> However, I really want to know why this isn't working in a
> multi-threaded environment. 
> 
>  What could I be doing wrong? Is there some proprocessor #define that
> makes the boost graph library more thread independent? Is there
> something wrong with the way that I am declaring the graph? Any help
> is appreciated! 

The graph library itself doesn't have any such defines; it's entirely
sequential code with no thought given to multithreading.

There are some places in Boost that are sensitive to multithreading,
e.g., shared_ptr. It's also possible that your standard library
implementation is doing some locking. Memory allocation routines are the
most likely suspects, I think: a good multithreaded memory allocator can
give a big performance boost.

  Cheers,
  Doug