[Beowulf] Rant on why HPC isn't as easy as I'd like it to be.

Lux, Jim (US 7140) james.p.lux at jpl.nasa.gov
Mon Sep 20 18:27:52 UTC 2021

The recent comments on compilers, caches, etc., are why HPC isn’t a bigger deal.  The infrastructure today is reminiscent of what I used in the 1970s on a big CDC or Burroughs or IBM machine, perhaps with a FPS box attached.
I prepare a job, with some sort of job control structure, submit it to a batch queue, and get my results some time later.  Sure, I’m not dropping off a deck or tapes, and I’m not getting green-bar paper or a tape back, but really, it’s not much different – I drop a file and get files back either way.

And just like back then, it’s up to me to figure out how best to arrange my code to run fastest (or me, wall clock time, but others it might be CPU time or cost or something else)

It would be nice if the compiler (or run-time or infrastructure) figured out the whole “what’s the arrangement of cores/nodes/scratch storage for this application on this particular cluster”.
I also acknowledge that this is a “hard” problem and one that doesn’t have the commercial value of, say, serving the optimum ads to me when I read the newspaper on line.

Yeah, it’s not that hard to call library routines for matrix operations, and to put my trust in the library writers – I trust them more than I trust me to find the fastest linear equation solver, fft, etc. – but so far, the next level of abstraction up – “how many cores/nodes” is still left to me, and that means doing instrumentation, figuring out what the results mean, etc.

From: Beowulf <beowulf-bounces at beowulf.org> on behalf of "beowulf at beowulf.org" <beowulf at beowulf.org>
Reply-To: Jim Lux <james.p.lux at jpl.nasa.gov>
Date: Monday, September 20, 2021 at 10:42 AM
To: Lawrence Stewart <stewart at serissa.com>, Jim Cownie <jcownie at gmail.com>
Cc: Douglas Eadline <deadline at eadline.org>, "beowulf at beowulf.org" <beowulf at beowulf.org>
Subject: Re: [Beowulf] [EXTERNAL] Re: Deskside clusters

From: Beowulf <beowulf-bounces at beowulf.org> on behalf of Lawrence Stewart <stewart at serissa.com>
Date: Monday, September 20, 2021 at 9:17 AM
To: Jim Cownie <jcownie at gmail.com>
Cc: Lawrence Stewart <stewart at serissa.com>, Douglas Eadline <deadline at eadline.org>, "beowulf at beowulf.org" <beowulf at beowulf.org>
Subject: Re: [Beowulf] [EXTERNAL] Re: Deskside clusters

Well said.  Expanding on this, caches work because of both temporal locality and
spatial locality.  Spatial locality is addressed by having cache lines be substantially
larger than a byte or word.  These days, 64 bytes is pretty common.  Some prefetch schemes,
like the L1D version that fetches the VA ^ 64 clearly affect spatial locality.  Streaming
prefetch has an expanded notion of “spatial” I suppose!

What puzzles me is why compilers seem not to have evolved much notion of cache management. It
seems like something a smart compiler could do.  Instead, it is left to Prof. Goto and the folks
at ATLAS and BLIS to figure out how to rewrite algorithms for efficient cache behavior. To my
limited knowledge, compilers don’t make much use of PREFETCH or any non-temporal loads and stores
either. It seems to me that once the programmer helps with RESTRICT and so forth, then compilers could perfectly well dynamically move parts of arrays around to maximize cache use.


I suspect that there’s enough variability among cache implementation and the wide variety of algorithms that might use it that writing a smart-enough compiler is “hard” and “expensive”.

Leaving it to the library authors is probably the best “bang for the buck”.

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