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Subject: Re: Another memory latency test

Author: Robert Hyatt

Date: 20:45:16 07/18/03

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On July 18, 2003 at 21:58:18, J. Wesley Cleveland wrote:

>On July 18, 2003 at 21:17:14, Robert Hyatt wrote:
>
>>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.
>I not only looked at the code. I *instrumented it*. I won't have complete
>results until Monday, but it appears that crafty spends 3-5% of its total time
>inside hashprobe on my (slow) machine and a prefetch could reduce that by about
>half.
>
>>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).
>
>If you had read whai I wrote, you would see I said crafty does a hash probe
>20-30% of its total nodes.

OK.  I clearly mis-read what you meant.  the 20-30% was eye-catching as that
is a pretty common hash hit percentage as well...


>
>>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.
>
>Except that a fast machine may do these 3-5K instructions in <1usec. A cache
>miss + a TLB miss may take 300-400 ns. I would not call 30% almost 0.

You are missing my point.  In the position(s) you tested, you saw 20-30%
hash probes.  That means one probe for every 3-5 nodes.  At 1M nodes
per second, that is 200K-300K probes per second.  If you measure the
time spent in searching a single node, multiply that by 3-5X, then compare
that to the hash probe time, the time spent probing the hash table is low.

Note that your 5% is _not_ the total time used to probe the table.  It is
the time to probe the table, and do it _twice_ although the second probe
doesn't have any memory access penalty associated with it in most cases.

So a big percent of that 5% is doing the actual work done in HashProbe(),
rather than being all memory access penalty...






>>
>>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.
>
>See above. I never said you did.



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