opteron VS Itanium 2

Mark Hahn hahn at physics.mcmaster.ca
Thu Oct 30 12:00:54 EST 2003


> Of course, there is some truth to what you say, but "this says nothing about 
> It2" seems a tad dramatic (but ... definitely in character ... ;-) ).  Below is 

all the world's a stage ;)

> the memory table for most of the benchmarks.  A few fit in the 6MB cache (although
> some surely should, as some codes do or can be made too fit into cache).  Many 

seriously, the memory access patterns of very few apps are uniform
across their rss.  I probably should have said "working set fits in 6M".

and you're right; I just reread the spec blurb, and their aim was 100-200MB.

> are in the 100 to 200 MB range.  The floating point accumen of the I2 chip is hard 

that's max rss; it's certainly an upper bound on working set size,
but definitely not a good estimator.  

in other words, it tells you something about the peak number of pages that 
the app ever touches.  it doesn't tell you whether 95% of those pages are 
never touched again, or whether the app only touches 1 cacheline per page.

in yet other words, max rss is relevant to swapping, not cache behavior.

> And after all a huge cache does raise the average memory bandwidth felt by the 
> average code ... ;-) (even as average codes sizes grow) ... and a large node count 

even though Spec uses geo-mean, it can strongly be influenced by outliers,
as we've all seen with Sun's dramatic "performance improvements" ;)

in particular, 179.art is a good example.  I actually picked it out by
comparing the specFP barchart for mckinley vs madison - it shows a fairly
dramatic improvement.  this *could* be due to compiler improvements,
but given that 179.art has a peak RSS of 3.7MB, I think there's a real
cache effect here.

> Got any snow up in the Great White North yet?

no, but I notice that the permanent temporary DX units are not working as
hard to keep the machineroom from melting down ;)

oh, yeah, and there's something wrong with the color of the leaves.

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