Re: threading question

[J . A . Magallon]

On 20010612 Albert D. Cahalan wrote:
> 
> In that case, this could be a hardware issue. Note that he seems
> to be comparing an x86 PC against SGI MIPS, Sun SPARC, and Compaq
> Alpha hardware.
> 
> His data set is most likely huge. It's DNA data.
> 
> The x86 box likely has small caches, a fast core, and a slow bus.
> So most of the time the CPU will be stalled waiting for a memory
> operation.
> 

Perhaps is just synchronization of caches. 
say you want to sum all the elements of a vector in parallele split in
two pieces:

int total=0;
thread 1:
	for fist half
		total += v[i]
thread 2:
	for second half
		total += v[i]

and you tought: 'well, I need a mutex for access to total. that will slow
down things, lets use separate counters':

int bigtotal;
int total[2];
thread 1:
	for fist half
		total[0] += v[i]
thread 2:
	for second half
		total[1] += v[i]

bigtotal = total[0]+total[1]

The problem ? total[0] and total[1] are nearby one of each other. So in
the same cache line. So on every write to total[?], even if they are
independent, system has to synchrnize caches.

Big iron (SGI, Sparc), has special hardware, but cheap PC mobos...

-- 
J.A. Magallon                           #  Let the source be with you...        
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