JPL Clusters XServes

burgburgburg writes "MacSlash has a brief note how NASA's JPL has put together a cluster of 33 XServes that was able to achieve 1/5 teraflop. The original article notes that the Applied Cluster Computing Group, using Pooch (Parallel OperatiOn and Control Heuristic Application) ran the AltiVec Fractal Carbon demo and achieved over 217 billion floating-point operations per second on this XServe cluster. More importantly, their research indicates that no evidence of an intrinsic limit to the size of a Macintosh-based cluster could be found."

Comments

Where's the GigE switch?

[teridon]

All G4s (including the XServer) have GigE built-in. I wonder if the GigE switch was too expensive?

Re:Where's the GigE switch?

[EricWright]

Maybe the computations were not communications bound... Fractal calculations can be done with a Monte Carlo method, which is highly parallelizable, and requires very little inter-node communication.

Re:Where's the GigE switch?

[Anonymous Coward]

By the way, it's called XServe, not XServer.

But is it pronounced "Eks Serve", "Ten Serve", or "Throatwarbler Mangrove"?

(Gordon Bennett, lucky they didn't call it ServeX!)

Re:Where's the GigE switch?

[Twirlip of the Mists]

No, I don't know, but my uninformed opinion would be that there's no measurable difference in latency between gigabit and 100BASE-T. The vast majority of the latency happens in the TCP stack inside the computer and in the NIC, as packets are generated and whatnot. Actual transmission latency will be so tiny as to make no meaningful difference.

Re:Where's the GigE switch?

[cremes]

The latency question is a good one. I'd say the answer to this lies in the driver for the NIC. I've written an IOKit ethernet driver and experienced pretty decent performance at 100 Mb. The system is processing packets as incoming data causes interrupts in the system.

However, I think the interrupt overhead for a 1000Mb link would be so high as to bring the machine to a screeching halt (okay, slow it down perceptibly). What a lot of driver writers do for gigabit links is to move their driver into polling mode. They essentially set a timer to go off every X milliseconds and process all the packets that have been copied into memory during that timeframe.

This gives a lower bound on the latency. A packet will always take X milliseconds to be noticed and processed by the system. Interrupt overhead stays low, but packet latency goes up a smidge.

It's a good trade off. I would bet that on a saturated link, packet latency at gigabit speeds is equivalent or WORSE than 100Mb. I might have to test that out...

cr

Re:Where's the GigE switch?

[Twirlip of the Mists]

I thought a lot of gig-e cards implemented a good deal of packet processing in hardware to deal with this very problem. Am I mistaken? I remember that the first PCI gig-e card I ever saw was installed in an SGI Origin, and when running full-out it pegged an entire CPU with interrupt handlers. Later versions of the card-- or perhaps an entirely different card, but sold by SGI and used with the same Origin servers-- had hardly any interrupt activity at all, even when moving data at rates exceeding 50 MB/s.

No comparison?

[photon317]

The article doesn't make any comparison between this and other (read x86 linux clusters) solutions. Do the x86 clusters have a problem scaling as well as xserves? I've heard of several-thousand node x86/linux clusters, so I would guess not, but I don't really know. Also no mention of $$/{MIPS/FLOPS/Whatever}, which would be nice to compare against an x86 cluster as well.

Re:No comparison?

[dhovis]

Well, there is also the issue that they were using test code that Apple distributes to show off what the G4 chip can do. So that 217 GFLOP speed is dependent on having highly Altivec optimized code.

OTOH, if you can take advantage of it, that would put this cluster at #250 in the Top 500 list of supercomputers. In fact, it is just a tick behind an IBM NetFinity cluster with 512x733MHz Pentium IIIs. Not bad for 66x1GHz G4s.

Re:No comparison?

[Anonymous Coward]

f you can take advantage of it, that would put this cluster at #250 in the Top 500 [top500.org] list of supercomputers. In fact, it is just a tick behind an IBM NetFinity cluster with 512x733MHz Pentium IIIs. Not bad for 66x1GHz G4s.

No, it is not. The Top500 ranking is based on *actual* parallel performance in *DOUBLE PRECISION* linpack.

The _theoretical_ peak performance of 66*1 GHz G4 boxes in double precision floating point is 66 Gflop. In practice the G4 has large scheduling problems with the normal floating-point unit, so I would be surprised if it could even achieve 30 Gflops. And ethernet is not going to scale very well for LINPACK. The real performance of parallel LINPACK on this machine would probably be in the order of 10 Gflops.

The Xserve is a nice box, and Altivec is cool for some applications, but real scientific applications are VERY different from a single precision fractal demo.

Re:No comparison?

[elphkotm]

The G4 has 3 parallel floating point execution units, so the theoretical peak is actually 198 GFLOPS. Also, The LINPACK performance number from beowulf-style clusters is derived from the aggregate total performance of each node. This puts true single-image systems at a disadvantage, since real applications use quite a bit more latency sensitive and interconnect bandwidth intensive.

Re:No comparison?

[Gropo]

Actually...

33 Machines x ($3999 + $200 ##1.5Gigs Aftermarket DDR##) = $138,567

$138,567 for 217 gigaflops = $638.55/gigaflop

Re:No comparison?

[dhovis]

Apple provides libraries for doing double precision math with the Altivec unit. See here.

The theoretical peak performance for 33 XServes in the test done here was actually 495 GFLOPS, BTW. I don't know what the theoretical performance of double precision on Altivec is, though. LINPACK is all linear algebra (IIRC), so it would see some benefit.

I will admit that there are plenty of applications where the G4 is not the best processor available. I for one will certainly be happy to see the IBM PPC 970, but you shouldn't discount the XServe until the test is actually run.

This was done before with G4

[fulldecent]

Here is a site that does similar things with a G4 cluster for protein folding:

http://www.spymac.com/gallery/showphoto.php?photo= 4665

Obligatory Post with a Splash of Lemon

[Spencerian]

Imagine a Beowulf cluster getting beat up by a Xserve cluster on the playground and stealing its lunch cycles!

Myth... ?

[EyeSavedLatin]

Don't believe the Gigaflop myth! Oh wait, that's "MHz Myth"... sorry, as a Mac owner, I have to whip out that response in every thread. Carry on.

Imagine This...

[EccentricAnomaly]

2004, Jobs WWDC Keynote...

"Today, I'm going to talking about Mac OS 10.3 and a big part of OS 10.3 is our clustering software.... [blah, blah] ...Apple has long prided itself on the easy of use of our products... [blah, blah] (the tv screen behind jobs shows a room with twenty people wearing apple t-shirts and a stack of X-Serve boxes) ...my friends here have several of our next-generation power-4 based X-Serves running OS 10.3... during this keynote they are going to unpack all of the servers and set up a cluster... ...by the end of the keynote we'll give the cluster a spin and see if we can make it into one of the top 50 supercomputers in the world"

Re:"no upper limit"

[00_NOP]

Seriously, there will be an upper limit in a practical sense! Basically the speed of light will impose a practical limit - if it akes longer for information to pass up and down the system than it does to wait for a new instruction to be queued on an existing element of the system then there is no point in extending the system.

In the Top 500 Supercomputers...

[jpellino]

This would put it at #343 on the Top 500 Supercomputers* - right below the University of Edinburgh's Cray and just ahead of the IBM cluster at Williams-Sonoma. Yes, Williams-Sonoma.

Of course I fully expect the employees of the West Hartford Apple store to ceremoniously run three doors down and moon the folks at Williams-Sonoma. Ah, Mall Life.

(*the whole lot of which just got its lunch eaten, dope slapped and girlfriend stolen by the new NEC cluster in Japan - 35,860 GFlops, Los Alamos is 2nd & 3rd at 7,727 with two of their HP server clusters.. sheesh.).