Ask Dan: When is a Xeon not a Xeon?

Date: 19 October 2007
Last modified 18-Sep-2012.


I would like to build four systems each with two Core 2 Quad Q6600 CPUs (eight cores per system) and 2Gb RAM per system, running Windows XP.

All 32 cores will need to be able to run for 7 days nonstop at 100% utilization.

Can you recommend a motherboard and appropriate cooling system?

I would like to minimize hardware costs if possible. The systems will not have video cards or sound cards, or any other expansion cards for that matter. I only need one hard disk per system.

If it is possible to overclock the Q6600 to get more GHz out of it I'd like to do that, if the $/Ghz is worth it.


First, the good news.

Running desktop hardware at 100% CPU utilisation for days, weeks or even months on end is not a big deal at all. If you give hard drives or 3D graphics hardware that thorough a workout then you may receive some nasty surprises, but the world's full of geeks whose CPUs are constantly redlined in the service of finding aliens or folding proteins or finding Golomb rulers, with no problems.

As long as the computer's reasonably well ventilated, you don't even need CPU cooling beyond the stock heatsink and fan, unless you're overclocking rather ferociously.

Plain old Windows XP Professional will also, as I mentioned in my Ask Dan about cheap quad-core CPUs, indeed work just fine on an eight-core system, as long as those eight cores are on no more than two physical processors.

This is because the Windows licensing system works on a per-CPU-socket basis. So as long as you've only got two CPU sockets in your computer, XP Pro doesn't care how many cores there are.

(WinXP Home, Windows Vista Home Basic and Windows Vista Home Premium only support one CPU socket, though. Any further chips won't be used.)

So that's the good news.

The bad news is that you can't put two Core 2 Quads in a PC.

Well, you kind of can, but it's stupidly expensive.

But fortunately, there's a better option.

Allow me to elucidate.

There is no such thing as a dual socket motherboard for Core 2 CPUs - Duo, Quad or whatever. Since these CPUs let you get four cores out of only one socket, there's not even much real market desire for multi-socket motherboards that accept Core 2 Whatevers. Or the earlier Core Whatevers, for that matter.

If you want two CPU sockets for processors using the Core 2 microarchitecture, you therefore have to get server-class Xeon processors instead.

Current Xeon CPUs are based on the Core architecture, and they come in quad-core versions that can be used on dual-socket motherboards, which will indeed give you the eight cores you're looking for.

Until recently, anything that forced you to buy a server processor instead of a desktop one was bad, bad news. Every generation of Xeons, in particular, commanded a huge price premium over the desktop processors on which they were based. This was only slightly justified by their larger amount of cache memory.

If you didn't really really need a computer with a whole bunch of CPUs in it, there was never much reason to spend the extra money on a Xeon.

A single cheap Intel or AMD multicore desktop chip can, today, comfortably handle many server jobs that used to require a hatful of Xeons or Opterons. Or Alphas, for that matter. And the modern Xeon is also a lot closer to the desktop processors - in basic specifications, and in price.

This kind of snuck past me, since I don't pay attention to server-chip prices. When I looked up the current Xeon prices I fully expected to see quad-core models start at $US1000 and head north from there.

But no. You can actually now get a quad core Xeon for as little as $AU341 delivered.

That buys you the 2.13GHz X3210, based on the "Kentsfield" core. The 2.4GHz X3220 is a bit more expensive, but still under $AU400.

But if you buy a couple of those, you're still going to be disappointed, because there's not actually anything Xeon-ish about these CPUs at all.

They are, literally, just relabelled Core 2 Quads. They have exactly the same specifications, and exactly the same standard desktop LGA775 (or "Socket T") processor socket.

You can use those "Xeons" on various ordinary LGA775 motherboards - the only ones they won't work on are the few whose BIOS doesn't recognise the weird Xeon processor ID.

But if you do this, you're still stuck with only one processor at a time.

I presume these change-the-sticker "Xeons" exist purely to satisfy PHBs who can't believe that their very important enterprise software could possibly run on cheap desktop chips. A cunning BOFH is now able to spend the same money on a "Xeon" that they'd rather spend on a Core 2 Quad, and trouser the change from the $2500 per processor the PHB cheerfully authorised.

Readers who're patient enough to have stuck with me through this foolishness are now probably wondering what they should buy if they want a real, actual multiple-physical-processor Xeon system, as of late 2007.

What you want, in that case, is Socket 771, also known as "Socket J".

Yes, Intel has done their part to keep the motherboard and CPU upgrade markets churning, by coming up with yet another bloody socket format. Socket J replaced the similarly magnificently incompatible Socket 604.

So far, Socket J's only been used for dual-processor-capable Xeons, starting with a couple of generations of dual-core models (allowing you to build a computer with two or four cores in it) and now extending to the "Clovertown" quad-core Xeons (allowing four or eight cores).

The Clovertown Xeons are the "5300 Series", the cheapest model in which is the E5310. As I write this, you can get an E5310 from m'verygoodfriends at Aus PC Market for only $AU522.50 delivered.

That's a pretty blooming low price for what only a few years ago would be four processors, each considerably faster than anything else on the market.

But when you actually look across the spec sheets, it turns out that the 5300-series Xeons are, in essence, just more rebadged Core 2 Quads.

There are precisely three significant difference between the $AU522.50 Xeon E5310 and the $AU379.50 (as I write this) Core 2 Quad Q6600.

One: The Xeon uses Socket J instead of LGA775.

Two: You can use two of the Xeons at once, on a two-socket motherboard.

Three: The Xeon's stock speed is only 1.6GHz, versus 2.4GHz for the considerably cheaper Core 2 Quad.

Everything else is the same. They both have two 4Mb blocks of level 2 cache, they both run from a 1066 megatransfer per second bus, they both have the same input voltage range. The Xeon's Thermal Design Power (TDP) figure is only 80 watts versus 95 watts for current Q6600s, but every quad-core Xeon up to the 2.66GHz E5350 is listed as an 80-watt part; TDPs figures are normally applied across a whole range of processors with different clock speeds and supply voltages, in which case they just give you a vague rule of thumb.

There's a reason for the existence of the low-clocked Xeons: Their relatively low power draw and heat output makes them suitable for use in rackmount computers. A bunch of Series 5300 Xeons come in two flavours, either bundled with a normal CPU cooler for use in ordinary-PC-shaped computers, or with a passive heat sink for use in rackmount boxes. There's even a special 1.6GHz "L5310" low voltage variant for blade servers.

You can, of course, rack-mount ordinary desktop motherboards and processors as well. If heat becomes a problem - which it's less likely to do when there's only one CPU per box - then underclocking a CPU is even easier than overclocking it.

If you want to actually match the stock-speed per-CPU performance of the cheap Q6600, though, the low-clocked E5310 obviously ain't gonna cut it. Small clock speed differences aren't worth worrying about, but the Q6600 is clocked 1.5 times as fast as the X5310. Dual E5310s therefore only give you four-thirds of the CPU power of a single Q6600.

The single current Xeon that comes closest to matching the stock speed of a single Q6600 is, I think, the E5345. That runs at only 2.33GHz, but it's got a 1.33 megatransfer per second bus speed, which'll more or less make up the difference.

As I write this, Xeon E5345s for some reason are selling for less than the 2GHz E5335. An E5345 will still cost you $AU1045, though.

Ah - there's that good old Xeon rip-off factor! I was starting to miss it!

Dual-CPU Socket J motherboards aren't cheap, either. Aus PC stock the Supermicro X7DAL-E at the moment, for a mere $AU775.50. And then you've got to buy FB-DIMM RAM to suit it, which costs about twice as much as normal DDR2 modules.

OK, that Supermicro board has six memory slots and lets you install up to 24 gigabytes of memory. Which is great if you need it. But most people don't.

That Supermicro motherboard, plus two E5335s, plus 2Gb of FB-DIMM RAM, will give you an honest twice the CPU power of a single Q6600.

But all up, that package will cost you more than $AU3150.

In contrast, Aus PC Market have a couple of overclockers' combo packs that include a Q6600 CPU, tweaky X38- or P35-chipset motherboard, and a couple of gigabytes of "PC-6400" DDR2 RAM.

The P35-motherboard package costs $AU709.50 delivered. The funkier X38 package costs $AU880.

So for the price of that one dual-E5335 guts-of-a-computer package, you could get four 2Gb Q6600 setups, and change. That'd be twice as much CPU power, and also twice as much system RAM per CPU core, in case that matters for your application.

The low-end Core 2 Quads all overclock well if your motherboard supports it, too. The current SLACR stepping of the Q6600 overclocks like crazy; well over 3GHz is practically guaranteed.

I presume the Series 5300 Xeons are quite overclockable, too. And I suppose there may be a Socket J motherboard out there somewhere that has overclocking features. Don't ask me which one it is, though. The Supermicro certainly doesn't seem to play such juvenile games.


If you ignore overclocking, the Xeon option will cost you about twice as much per unit of CPU power than the Core 2 Quad option.

If you use the comfortable 50% overclock that can be wrung out of most, if not all, current Q6600s, then the Xeon option will cost you about three times as much as the Core 2 Quad route.

An eight-CPU system may, for your purposes, work better than two four-CPU boxes, all other things being equal. If all of the cores are going to be working hard on whatever you're doing, though, it's quite possible that even four cores will already be too many to all be fighting over the same system RAM. (Speed of access to storage is one of the big differences between ordinary PCs and supercomputers.)

In that case, splitting your four eight-core systems up into eight four-core boxes may get your job done a lot faster, as well as save you money.

UPDATE: After this page went up, a couple of readers pointed out to me that Dell in the USA currently have a pretty amazing deal going on their PowerEdge SC 1430 server. You can get an SC 1430 with two Xeon E5310s and 1Gb of memory, but no operating system, for a lousy $US729.

Those two 1.6GHz Xeons only give you about the performance of a single overclocked Q6600, but you really can't complain for the money. The same system currently costs more than twice as much here in Australia, but it could still be worth keeping an eye on Dell's special offers; as I've previously mentioned, nobody should every buy anything from Dell that isn't on special.

Back in 1999, the CPU market wasn't this strongly segmented. You could buy a dual-socket Abit BP6 and stick two dirt cheap Celerons on it, overclock 'em, and get serious server performance (by the standards of the day) out of a computer that, all together, cost less than a Pentium II Xeon all by itself.

That made Intel very cross.

So they choked the symmetric multiprocessing feature out of their processor lines, one by one. First the Celerons, then the Pentiums. And now, they've gone so far that you can even buy a "Xeon" that doesn't support SMP.

(I presume the people responsible for the creation of the two-wheel-drive Jeep had a hand in this somewhere.)

I'd be quite upset about all this, if it weren't for the fact that multiple cores have been marching down the processor market even as SMP capability has been vanishing into the top-end products.

So, Brian, I advise you to bid the dream of multiple physical processors goodbye, and join the single-socket quad-core scrum instead.

It's so much cheaper down here.

Aus PC Market's $AU709.50 overclocking combo gives you a Core 2 Quad Q6600 CPU, 2Gb of Corsair PC-6400 RAM and Gigabyte P35-DS3 Pro motherboard, for $AU33 off the regular combined price.
Australian shoppers can click here to order it.

Their $AU880 combo is the same, but with a Gigabyte X38-DQ6 motherboard.
Aussies can click here to order that one.