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Subject: Re: Another memory latency test

Author: Vincent Diepeveen

Date: 15:53:55 07/17/03

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On July 17, 2003 at 17:08:41, Robert Hyatt wrote:

>On July 17, 2003 at 16:11:33, Gerd Isenberg wrote:
>
>>
>>>Vincent: 256 ns
>>>Note, random access is faster than before
>>>I get similar numbers for smaller sizes bigger than the cache
>>>
>>>
>>>Summary: There is more, than just an lmbench number. Actually
>>>the comment in lmbench source suggests, that they actually
>>>wanted to get the random access times.
>>>
>>>I don't want to argue about defenition of the "real" memory
>>>latency. But for chess programs/hash the Vinent type number
>>>is the most interesting.
>>>
>>>Regards,
>>>Dieter
>>
>>very interesting Dieter,
>>
>>your algorithm confirms roughly Vincent's results!
>>I will try it later at home and learn from your source code ;-)
>>But there is still the question whether this measured times is memory latency
>>per definition - i guess not.
>
>The classic definition of memory latency is the amount of time needed to
>do a random read to any specific memory address.  If you blow the TLB, then

Now that amazes me. All the memory latencies that manufacturers claim are memory
lookups that are sequential lookups. In fact R14000 is clearly optimized for
fortran loops

>you just added another one or two memory accesses which means you are now
>doing two or three accesses rather than one.
>
>Calling this "memory latency" is wrong.

What are you murmuring this time?

The proof Dieter clearly shows is that the further away the memory accesses are
from each other, EVEN WHEN SEQUENTIAL, the more the times add up.

If you first access adress 0 then adres 200, that's way faster than
first access adress 0 then 2000 which again is way faster than first adress 0
then 20000.

That proof is beyond any doubt delivered.

>I've said this to him before.  Again, the terminology is pretty specific
>everywhere I see it.  If you want to define latency to include the MMU
>overhead, then the latency becomes variable, depending on the page table
>segment table and TLB accesses.

So basically you are not interesting in how fast you can get a random cache line
from memory, but you are more interested in what the ram speed can do under
labatory conditions when not mounted in a computer but a special ram machine
that just measures latency 1 mm away from the ram with some kind of connection
thick as a leg to a single DIMM.

>>
>>In worst cases there is more than memory latency (additional TLB-latency and
>>some RAM hardware interface latencies) to get data into a register - maybe a
>>question of definition.
>
>The TLB problem can easily exceed the cost of reading the data.  In fact,
>the PC has a three-level memory map that most systems don't use.  But to do
>so adds yet another 120-150ns to latency because you are fetching yet another
>memory word.

the only thing that matters Bob, is how quick we can get a hashtable entry from
a random place.

And that's around 400ns at a dual.

>>
>>Another interesting point is to measure not only the average but the maximum and
>>minimum access times (processors performance counter?). Are the accesses  about
>>equal, or are there heavy spikes due to some chaotic TLB behaviour?
>>
>>Cheers,
>>Gerd



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