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

From: Andrew Chapman (andrew.chapman_at_[hidden])
Date: 2004-10-11 06:20:10

Caleb Epstein wrote:
> On Thu, 07 Oct 2004 18:14:53 +0100, Andrew Chapman > 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.
> 
> 
> Perhaps you could provide some code?

     // resize all the furXXX std::vectors
     furGen.allocate(...);

     boost::thread_group threads;

     CalcCurvesThread thread1Obj(furGen, 0, midPoint,
       furCurves, furRootUVs, furRootPositions, furRootNormals,
       furRootDu, furRootDv, furRootDpDuv, guideCurves,
       guideLookupTable, furDesc, 1.0);

     CalcCurvesThread thread2Obj(furGen, midPoint+1, nCurves-1,
       furCurves, furRootUVs, furRootPositions, furRootNormals,
       furRootDu, furRootDv, furRootDpDuv, guideCurves,
       guideLookupTable, furDesc, 1.0);

     threads.create_thread(thread1obj);
     threads.create_thread(thread2obj);

     threads.join_all();

> Are the threads using any sort of synchronization between themselves?
> Perhaps you're spending a lot of time contending on a mutex if so.
> If they're just operating on some shared resources (the vectors) with
> no locking, they should certainly run faster than the single-threaded
> case on a dual CPU machine.

No, there is no need for any synchronization, as far as I can tell. Each 
thread is only reading from and writing to different areas of the
pre-allocated vectors (thread1 to the first half of the vectors, thread2 
to the second half).

It was my assumption that the new child threads are sharing all the data
with the host process. However, after watching the threads running so
slowly, and it appearing as through they're sleep/wait'ing on system
resources, I was wondering if somehow all the stack data in my large 
data vectors wasn't being copied to and from the threads.

-- 
Andrew Chapman
Senior Technical Director - Framestore CFC