[HPGMG Forum] what is a "good" or a "better" TOP500 ranking?

Fabrizio Petrini fpetrin at us.ibm.com
Sun Nov 30 23:10:48 UTC 2014


Hi Jeff,

Graph500 is mostly limited by the local memory and network/communication 
layer performance at scale

Every point increase in the problem scale doubles the data set size and 
the degree of imbalance, roughly
doubling the size of the largest vertex. G500 implementations need to deal 
with potential network hot spots
and also steer the type of algorithm, based on the status of the 
exploration. We rely on non-blocking collectives
to determine what to do at run-time.

Integer performance is one of the important parameters, but not the only 
one. For example, the single node
performance of System K is about 3X BGQ, but BGQ has better scaling.

It would be very helpful to have a benchmark that stresses the network 
performance and the scalability
of the overall system in a non-trivial way.

Hope this helps.

Fabrizio Petrini
Manager, High Performance Analytics Department
IBM TJ Watson Research Center
phone: 914-945-2942
e-mail: fpetrin at us.ibm.com





From:   Jeff Hammond <jeff.science at gmail.com>
To:     Horst Simon <hdsimon at lbl.gov>
Cc:     "hpgmg-forum at hpgmg.org" <hpgmg-forum at hpgmg.org>
Date:   11/30/2014 02:55 PM
Subject:        Re: [HPGMG Forum] what is a "good" or a "better" TOP500 
ranking?
Sent by:        "HPGMG-Forum" <hpgmg-forum-bounces at hpgmg.org>



<shameless self-promotion>

We have proposed a Quantum Chemistry 500 because none of the existing 
community benchmarks come close to representing the behavior of atomic 
integrals, Hartree-Fock and related DFT methods. All these PDE-oriented 
benchmarks are trivial by comparison. 

On the other hand, HPL is actually a fantastic proxy for CCSD(T), which is 
dominated by DGEMM and point-to-point bandwidth.

In our case, we decided that a single code was the wrong way to benchmark, 
and have instead defined the physics and numerics scientists need, without 
prescribing an implementation, since there are many already (and, unlike 
many domains, these many different codes can reproduce the exact same 
solution if the problem is sufficiently specified).

There's nothing wrong with having lots of different benchmarks, as long as 
it's relatively easy to obtain and analyze the associated performance 
data.

And I agree that not very much critical thinking is going into some of 
these benchmarks. It's rather ironic that Blue Gene/Q dominates the 
Graph500, because it's not designed for this class of problems and has the 
worst integer performance (relative to other capability) of any leadership 
class machine, and Graph500 was supposed to be a non-numerical benchmark. 

The near-perfect correlation between HPCG and STREAM is another failure in 
the science of benchmarking. 

Jeff

Sent from my iPhone

> On Nov 30, 2014, at 10:34 AM, Horst Simon <hdsimon at lbl.gov> wrote:
> 
> 
> 
> Like many of you I returned from SC14 with many questions to think 
about, among others "what is a 'better' benchmark?"  Here is a summary of 
at least three conversations that I had at SC14 between "M" (that is me), 
and "C" a colleague (a synthesis of several conversations.
> 
> C: Well, I am really glad that HPCG (or HPGMG) is being developed that 
will make the TOP500 more realistic.
> M: What do you mean by more realistic?
> C: It is well known that HPL is not a good benchmark to measure the 
performance of real systems. If we replace HPL with HPCG (or HPGMG) then 
we would not get such a distortion of the performance, for example all 
those GPU based system would not be ranked as high.
> M: But why do you think that HPCG (or HPGMG) is better?
> C: ... long technical argument involving bisection bandwidth, mixture of 
long and short messages, real applications that don't solve dense linear 
systems, streams benchmark etc.
> M: But why do think this is "better"? What do mean by "better"? I think 
that "better" should imply that any new benchmark would in some sense be a 
better approximation to the application workload. Can you prove this?
> ..... etc.
> 
> What was really striking about these type of conversations was the fact 
how little our community is thinking scientifically. If you want to do 
something better, then  you first have to define what you are actually 
measuring. So how do we really measure the applications performance of a 
petascale platform? I can think of many applications where HPCG (and HPGM) 
as irrelevant to the application as is HPL.
> 
> Horst
> 
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