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Re: Triax profiling on cluster


Dne 23.9.2009 16:32, Bruno Chareyre napsal(a):

All nodes ARE using internal memory attached to a node. You are
confusing something here. Each node is an *ordinary* PC. They are
just connected to each other with a mechanism which allows submitting
jobs (sun grid engine). There is nothing special about them.

Maybe Remi was talking about big CPU cache which is always used
automatically. And indeed is relatively big: a whole 6 MB ! Which
confirms what I already said, if you don't use RAM then Intels are
much faster than AMD (this is the case when you are calculating
digits of pi or mandelbrot set, or such). If you do use RAM, then
AMDs are faster. In our calculations we use RAM. A smallest
simulation uses 100MB. Another simulation could be 1 GB. There is no
point with 6 MB of CPU cache then.

Yes, all nodes have this but some of them have bigger cache (reason for the option "big cache") so you don't use RAM at all if possible (but yeah, if its 6MB then it is useless for DEM...).
L1 cache is certainly not useless even for DEM, it's just that all your data will not fit inside. But still if one part of your data is at one memory location (not chain of shared_ptr's jumping all over the RAM), it makes the computation much faster (e.g. Dem3Dof classes have comparable speed to SpheresContactGeometry even if they copy extra Vector3r+Quaternionr (=28b of data) at every step. There are some papers [1] on that; speeds of the L1 cache are orders of magnitude higher than speed of CPU-RAM bus and of the RAM modules themselves.

Cheers, V.

[1] http://people.redhat.com/drepper/cpumemory.pdf

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