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Subject: Re: The need to unmake move

Author: Sune Fischer

Date: 14:22:52 08/25/03

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On August 25, 2003 at 16:50:08, Dan Andersson wrote:

> The issue is the same. Because you can't guarantee that copying will be in
>cache. And you can't guarantee that other data structures won't be close or
>aligned in such a way that it won't trash the cache. The impact might not be
>great but it will be there. So the net cache bandwidth will be lower or even
>much lower than the simple linear relationship. Thus the slow main memory
>bottleneck will appear.

For the whole picture goes, probably yes, but it is not easy to figure that
since there are many factors.

I know this is down to hair splitting now, but IMO the reason that unmaking is
faster than uncopying isn't the one Bob gave, and I quote:

"
>>I was thinking more about how silly it is to copy the empty bitboards for each
>>ply. If you update the boards that are active, they will stay in the cache.
>>Those that are not used might drop out, unless they are copied once every micro
>>second.
>>
>
>That is a reasonable rate for a program that searches 1M nodes per second.  I'm
>going at 2.4M so make that about once every 400 nanoseconds.  :)  Suddenly it
>begins to add up in a big way.  :)"

As though the 2.4 Mnps was the reason.
The reason is that double stacks increase memory trafic _between CPUs_, but that
is _not_ what he said if you follow the thread, and this thread wasn't about SMP
at all, so if that was his point I'm not sure how it related to the discussion.

He gave numbers of 25%, nobody can confirm those numbers (I get ~10%), but I
figure now that he was talking 25% in SMP search, or what?

Suddenly the whole thing is rather confusing because his numbers doesn't compare
with non-smp numbers, and I believe Johan was talking strictly non-smp.

But anyway, this is getting silly. ;-)

-S.
>MvH Dan Andersson



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