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Subject: Re: Source code to measure it - there is something wrong

Author: Gerd Isenberg

Date: 05:04:49 07/16/03

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On July 15, 2003 at 20:26:28, Vincent Diepeveen wrote:

>On July 15, 2003 at 17:58:01, Gerd Isenberg wrote:
>
>>Ok, i think there is one problem with Vincent's cache benchmark.
>>
>>There are two similar functions DoNrng and DoNreads. DoNrng is used to mesure
>>the time without hashread. But the instructions has the potential of faster
>>execution due to less dependencies and stalls. It may execute parts of two loop
>>bodies of DoNrng interlaced or simultaniesly - that is not possible in DoNreads.
>>Therefore the time for N DoNrng is not the time used inside the N DoNrng loop,
>>and maybe much faster.
>
>If you look to the generated assembly code you will see that nothing is wrong.
>If something goes wrong in the pairing of the instructions that might at most
>make a difference of 1 to 2 clockcycles.
>
>At 2Ghz that's 0.5 nanoseconds.
>
>It is cool if it measures accurate in 0.5 ns of course, but that was never the
>intention of the test. It was intended to measure latencies at big
>supercomputers when n processors read in the memory of just 1 poor node.
>
>the other test latencyC.c is doing a criss cross reference. A 0.5 ns error in
>the measurement there (assuming there is no other system stuff disturbing which
>there is because actually other processes may also use RAM at your node, which
>can give major differences, but still doesn't give a bug in the test) is not
>really relevant.
>
>If you care for 0.5 ns difference then shoot me, but don't ever say it's 130ns
>like bob claims for 133Mhz DDR ram.
>
>You're at new fresh i bet cl2 150Mhz DDR ram if not faster.
>
>the 280 ns was measured for P4 with 533 bus or 133Mhz DDR ram and the 400 ns as
>you see is from the duals. For me 380 ns goes to 400 ns or something when using
>bigger hashtables. I see no significant difference between 380 and 400 so to
>speak. It's simply *huge* random latency.
>

Hi Vincent,

i tried this one ( AllocateHash before) in DoNrng doing some cyclic hash reads
with index 0..3:

int DoNrng(BITBOARD n) {
  BITBOARD i=1,dummyres,nents;
  int t1,t2;

  nents = nentries; /* hopefully this gets into a register */
  dummyres = globaldummy;

  t1 = GetClock();
  do {
    BITBOARD index = RanrotA()%nents;
    dummyres ^= index ^ hashtable[index&3];
  } while( i++ < n );
  t2 = GetClock();

  globaldummy = dummyres;
  return(t2-t1);
}

Here the latency is about 25ns less on a P4 with 256MB only ;-(
180ns instead of 205ns.

Paul Hsieh's comment on Oliver's first approach may also address your approach:
The 64-bit mod is very heavy, maybe it's better to use power of two tables and
use "and"  2^N - 1.

Anyway, i found your test very instructive.

Regards,
Gerd


Paul Hsieh's comment:


You have a couple of problems:
1) RDTSC often doesn't have enough accuracy/resolution to measure such
a small number of clock cycles (indeed it varied from architecture to
architecture.)

2) In an out-of-order architecture, the time of apparent execution of a
single instruction cannot be measured in isolation.

--------------

Usually the way to measure memory latency is build a linked list, and
simply traverse that list, and measure the number of "traversals per
second" or the number of clocks it takes to traverse a linked list of
length n.

The reason this is a good measure of memory latency is because when
traversing a linked list, there is no way to proceed to the next link
without first getting the result of the previous link traversal.
Out-of-order execution is not possible, thus leading to a completely
serialized execution stream.

Usually you heavily unroll the loop and write it in assembly language to
make sure there is nothing else affecting the timing.





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