Author: Mridul Muralidharan
Date: 02:13:07 09/03/03
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Hi, On September 02, 2003 at 18:37:05, Jeremiah Penery wrote: >On September 02, 2003 at 07:15:55, Mridul Muralidharan wrote: > <snip> > >If I didn't have some idea what I was talking about, I wouldn't be talking, >unlike a lot of people in these discussions. > This should give you some direction to think about : http://www.talkchess.com/forums/1/message.html?313791 Actually , more references could be give - but considering your set mindset, no thanks :) >> Refer to cray architecture , an opteron 8 way box architecture , and some >>IBM supercomp cc-NUMA based system architecture docs for more info. I'm not > >Those machines are designed and built for *completely* different purposes. You >might as well compare the documentation for a P4 to that of an UltraSPARC, for >all the good it would do you. > If you say a cc-NUMA is built for a entirely different purpose - definitely , I agree with you ! Like Bob Hyatt mentions in the above mentioned post - performace / price / scalability matrix works quiet well for NUMA at higher number of processors. Which is exactly what I said - no point in saying crafty (or any other program for that matter) will scale well on a 16 or 64 proc NUMA box just 'cos it scales well on a 4 proc smp box. NUMA machines are a slightly different breed. >>refering to just theoretical differences , or _only_ architecture differences - >>but as a programmer - what details that need to be taken care of while writing >>apps for such a system. > >And those details would be what, other than the aforementioned theoretical or >architectural differences? > Quiet simple - on a smp or cray box , typically you do not care much about latency for accessing memory being different for different processors , etc. As a programmer , you have to be aware of all these. Why do you think Linux on numa sucks ass ?!!! Also , depending on how the box is configured , number of procs per node , etc - you memory management , thread/process splitting , etc (for a chess program that is) will have to be modified. Just because you might know what the architecture of the box is , does not imply that you will come up with a program which scales well on NUMA ! >>>But in reality, almost nobody uses a machine that big, especially for chess. >> >>The question was - can it be done , is it just a bunch of tweaks - not do you >>have a system. >>Answer : Yes it cn be done , needs lots of rewrite - not just "tweaks". > >Not really. Bob said he already completed the changes, and it didn't really >involve much. Only instead of forking processes he had to manually start >processes on each processor. That really doesn't take much work. > If it was just a bunch of tweaks that you mention here - I would love to see how much performance it will give on a 64/128 proc NUMA box :) I can make a guess - it will suck a**. (No offence to anyone here) >>>For any but the most extremely scalable architectures, there is significant >>>diminishing returns when adding processors for chess playing. I'd say that a >>>very scalable 8-way SMP or NUMA (Opteron) machine will not be very much slower >>>than even a 64-way Alpha/Itanium/xxx machine for chess. >> >>If badly programmed , then yes not much difference between a 8 proc box and a 64 >>proc box (actually it can be lower performing!). >>Which is exactly my point , you need to design a program specifically to run on >>such a system - not expect something that works on a 2 or 4 proc system and >>expect it to work for a 64 proc system ! > >The Alpha-Beta algorithm used for chess is a serial algorithm. There's no >getting around that. The more processors you use, the less efficiency you will >get, unless you use something else than Alpha-Beta. > >No matter how much you want to rewrite and "tweak" for a NUMA machine (or any >kind of machine, for that matter), adding more and more processors is simply >going to stop being beneficial at some point. Just because alpha-beta is serial does not imply that it need not scale well beyond the 4 or 8 or 16 proc boxes that it is shown to scale well to. I _have_ seen results of how well it scales :) sadly I'm not at liberty to reveal them - but in a few months/next year or so , you will also see how well it scales when results are published. I'm not denying the limitations of alphabeta algo - definitely they exist - but not to the extent to which it is believed to exist.
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