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Subject: Re: Another memory latency test

Author: Robert Hyatt

Date: 18:17:14 07/18/03

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On July 18, 2003 at 15:21:35, J. Wesley Cleveland wrote:

>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.


Look at the code.  Crafty probes memory _once_ for a hash probe.  That
introduces a memory access penalty once per node in the basic search,
less than once per node in the q-search (I only probe phash there and I
don't probe it but about 25% of the q-search nodes I visit).

As a result, you get less than one probe per node searched.  A node searched
requires something on the order of 3000-5000 instructions.  What percentage
of that 3K-5K instruction timing is that single hash probe?  Almost zero.

Ignore hits and misses, that is not the issue here.  The issue is the cost of
doing the probe itself, which is essentially zero.




>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.

See above.  I don't really probe once for every node.


>
>[snip]



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