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Subject: Re: Another memory latency test

Author: J. Wesley Cleveland

Date: 12:21:35 07/18/03

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On July 17, 2003 at 18:25:51, Robert Hyatt wrote:

>On July 17, 2003 at 17:35:33, Dieter Buerssner wrote:
>
[snip]
>>
>>I cannot find any randomness in the reads of lm-bench (I downloaded latest
>>stable source today, not the experimental version, available, too). If it would
>>do random reads, it would have no way to avoid the problem with the TLBs you
>>explained.
>
>4M pages solves it for at least 250mb worth of RAM.  But then again, _no_ chess
>program depends on purely random memory accesses to blow out the TLB.  The only
>truly random accesses I do are the regular hashing and pawn hashing, which
>both total to significantly less than the total nodes I search.  Which means
>the TLB penalty is not even 1% of my total run time.  Probably closer to
>.01% - .05%.
>
>I ignore that.

Why do you think it is that low? I get ~20-30% of nodes have hash probes with
crafty. If you are getting 1m nodes/sec, then this is a probe every 3-5 usec.
With a very large hash table and 4K pages, the large majority of these will
cause a TLB miss. At 200 nsec each (a guess), this could be up to 5% of your
total run time.

[snip]




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