Re: Externalize SLIT table

From: Erich Focht <>
Date: 2004-11-06 04:13:24
Hi Jack,

the patch looks fine, of course.
> 	# cat ./node/node0/distance
> 	10 20 64 42 42 22

> 	# cat ./cpu/cpu8/distance
> 	42 42 64 64 22 22 42 42 10 10 20 20

what exactly do you mean by cpu_to_cpu distance? In analogy with the
node distance I'd say it is the time (latency) for moving data from
the register of one CPU into the register of another CPU:
        cpu*/distance :   cpu -> memory -> cpu
                         node1   node?    node2

On most architectures this means flushing a cacheline to memory on one
side and reading it on another side. What you actually implement is
the latency from memory (one node) to a particular cpu (on some
                       memory ->  cpu
                       node1     node2

That's only half of the story and actually misleading. I don't
think the complexity hiding is good in this place. Questions coming to
my mind are: Where is the memory? Is the SLIT matrix really symmetric
(cpu_to_cpu distance only makes sense for symmetric matrices)? I
remember talking to IBM people about hardware where the node distance
matrix was asymmetric.

Why do you want this distance anyway? libnuma offers you _node_ masks
for allocating memory from a particular node. And when you want to
arrange a complex MPI process structure you'll have to think about
latency for moving data from one processes buffer to the other
processes buffer. The buffers live on nodes, not on cpus.


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Received on Fri Nov 5 12:15:49 2004

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