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Subject: Re: The need to unmake move

Author: Robert Hyatt

Date: 20:53:20 09/01/03

Go up one level in this thread


On August 29, 2003 at 19:56:34, Jeremiah Penery wrote:

>On August 29, 2003 at 18:40:51, Eugene Nalimov wrote:
>
>>On August 29, 2003 at 18:32:46, Jeremiah Penery wrote:
>>
>>>Of course I know that.  My point is that with Opteron, even if you are accessing
>>>non-local memory *always*, you are not accessing it slower than you would with,
>>>say, a traditional SMP machine (2x Xeon, for instance).
>>>Of course you can do a lot better - all I'm saying is that there's no way you're
>>>going to be doing worse.
>>>
>>>Either way you win, even with a crappy NUMA algorithm.
>>
>>I am not so sure. With some NUMA implementations each memory bank has limited
>>bandwith, so if you happened to allocate all the critical data in one node's
>>memory you'll overload its memory controller.
>
>>I had seen a case where SMP application was blindly ported to a 32-CPUs NUMA
>>system (8 nodes, 4 64-bit CPUs per node, 256Gb RAM total). Application run much
>>slower on 32 CPUs than on single CPU.
>
>I'm not talking about "some NUMA implementations".  I'm talking about 2-4
>processor Opteron implementation.  It should never have any of the problems you
>describe.  Indeed, you can see from SPECRate that it scales very nearly as well
>as Itanium, and that still with compilers/OS still not very NUMA aware or very
>good for AMD64.

Look at the SPEC programs.  The look at _the_ problem I mentioned for Crafty.
It is almost guaranteed that _all_ critical search data for _all_ threads will
be allocated in a single processor's local memory.  That is going to be a hot-
spot and the fancy redundant memory controllers will _not_ be able to hide that.

You can't do 4x memory reads to a single bank.  Yet Crafty is going to demand
just that.  And performance is going to suffer.  _significantly_.

It is fixable.  But it isn't fixed in the current implementation.




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