[lug] q: multithreading on smp weirdness

[Zan Lynx]

On Tue, 2004-11-30 at 21:14 -0700, karheng at softhome.net wrote:
> that's an interesting guess! i've gotta watch out for
> that.. i don't think it's the reason though, at least
> not for the test app. 
> 
> for the test app, i've replaced the chunk fetching
> function with a routine that simply does some computation
> over a small buffer. that buffer doesn't grow or
> shrink, & all i do is toggle the number of iterations
> over that buffer. so it should exclude issues caused by
> the cpu cache line... 
> 
> actually, the test app i have is part of a bigger app.
> thread 1 of the bigger app fetches data & thread 2
> processes them.
> i've extracted & simplified that code out and made both
> threads only do computation on a small buffer to
> reproduce the problem from the bigger app. 
> 
> anyway, i've also tested the threads in the bigger app,
> & i've found that data returned from thread 1 is
> consistently sufficient to keep thread 2 busy.
> (thread1:thread2 elapse time ratio is approx 6:4, in my
> test app, it's made to be 1:1).
> in theory, i should be getting about 40% or 30%
> performance improvement in my bigger app, & always 40%
> to 50% performance improvement in my test case. 

Something else that I just thought of that could help.  CPU performance
counters.  Intel's Vtune software uses these to profile software in all
sorts of interesting ways.  I don't know if Itanium has the same
performance registers that the P4 has, but if they were good I don't see
why Intel would leave them out.

I also don't know if Intel has anything like Vtune for Itanium.  *goes
to check Google* Ah, yes they do.  And there's another thing called
Thread Checker (http://www.intel.com/software/products/threading/tcwin/)

I don't know if Intel offers an evaluation copy or if you'd have to buy
it.  Geez.  I just looked.  They want $1,198 for the Vtune package (that
includes Thread Checker).

There may be other tools to do this sort of thing out there.
-- 
Zan Lynx <zlynx at acm.org>
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