PIII vs. PIII Xeon: Who are they, and why is their price so different?  
Ask Ars,
 
 
Am putting together a spec for an HP rack server running NT4. Will be a file/print server for 300+ users. No major app's running, just small Access DB's. Will upgrade to W2K down the road...  
 
I am looking for some hard data to use to help convince upper mgmt to go with the Xeon over the regular PIII. They are attracted to the lower cost of the reg PIII. Is the Xeon the better choice?  
 
I appreciate all input!!  
 
Hal Flynt  
 
 
 
History of the Xeon: A question of cache
To provide a context for discussing the PIII and PIII Xeon, let's take a quick look at the history of the Xeon processor. A number of years ago, in an attempt to get into the higher-end workstation and server space that was dominated by RISC processor machines, Intel launched the Pentium Pro. Prior to the Pentium Pro, the L2 cache for Intel's processors had been located on the motherboard. The Pentium Pro was an all-new processor core that sported options of 256k, 512k, 1 meg, or 2 megs of L2 cache built right on the processor. The simplest way to explain L2 cache, if you are not familiar with it, is that it is a small amount of memory that the processor uses to store information it needs to retrieve quickly/frequently. The faster the L2 cache runs, the faster the processor can get its work done. If you want to know about caching technology, check out our excellent "Understanding CPU Caching" article.  
 
Putting the L2 cache right on the processor was a new move for Intel, and high-end workstations and servers benefited tremendously from the large, full-speed L2 cache on the Pentium Pro. The down side to the Pentium Pro was the fact that it cost around four times as much as equal-MHz Pentiums of the day. The average PC didn't really see enough performance benefit from the on-die L2 cache of the Pentium Pro to justify the cost. There were some other shortcommings to the Pentium Pro, but we'll ignore those for now.  
 
Intel's answer to the costliness of the Pentium Pro came in the form of the Slot-1 cartridge Pentium II processor. The standard PII came with 512k of cache on the processor PCB (not on the processor die), but it ran at 1/2 the speed of the processor. This was certainly an improvement over the days where the L2 cache was located on the motherboard, and ran at the speed of the system clock. For example, a PII 400 would have 512k of L2 cache running at 200 mhz, compared to a Pentium 166 which would have anywhere from 128k to 512k of L2 running at 66mhz (the speed of the system clock). If you recall the Pentium Pro, you'll see something interesting: the Pro 200MHz had from 128k to 2MB of cache, running at the same speed as the processor, so in this case, 200MHz. It's not wonder that Caesar loved it so.  
 
The Pentium II Xeon
For servers and workstations, Intel launched a version of the Pentium II called the PII Xeon. This version of the PII came with options of 512k, 1 meg, or 2 megs of cache on-die (on the processor core). The L2 cache on the PII Xeon ran at the full processor clock, and in addition was very expensive. However, on large servers that handle thousands of simultaneous requests, the addition of a large, fast L2 cache was quite a blessing and well worth the extra cost. Allowing the processors to store frequently-requested data in the high-speed L2 cache prevented them from having to go to the slower system memory to fetch requested data (or even worse: having to go to a page file to find data!).  
 
So, to say the least, the Pentium II Xeon was an important product in the x86 server world. In fact, large 4-way and 8-way PII Xeon servers were what allowed Microsoft to start publishing competitive benchmarks for Windows NT web and database servers (benchmarks comparing Windows NT's performance to Microsoft's RISC-driven competitors). It also provided a high-performance, lower-cost alternative to Unix/RISC graphics and development workstations.  
 
The Pentium III and Pentium III Xeon (or, why the background information is important
By now you are undoubtedly asking yourself "why do I want all this information about the Pentium Pro and Pentium II Xeon? I just want information that proves a Pentium III Xeon is worth all that extra money!". Well I have some surprising news for you: I don't think it is worth all that money! Now, with the history of the Xeon in mind, let me explain why.  
 
The Intel PIII Coppermine (Intel's current PIII), for those who aren't familiar, takes the basic PII/PIII design and puts the cache right on the processor. Just like the Xeon. The difference is, the PIII Coppermine only comes with 256k cache, no larger options. The Pentium III Xeon comes in three flavors: 256k, 1 meg, and 2 megs of cache. Wait, a PIII Coppermine and PIII Xeon with 256k cache? What's the difference?  
 
The PIII Xeon with 256k cache only supports two-way SMP, just like the PIII Coppermine. It comes in speeds up to 1GHZ, with 100mhz and 133mhz front-side-bus options, just like the PIII Coppermine. Same old processor, new high price. (To be sure, Intel's marketing literature claims the PIII Xeon has an advanced associative cache that differs from the PIII Coppermine, but not one of the whitepapers on their website supports this *fact*.) So what is Intel doing with this version of the Xeon? They're counting on all the old positive press and benchmarks surrounding the PII Xeon, and hoping consumers will assume that the Xeon Label=Large Performance Upgrade. Since the processors are basically the same, this is definitely not true.  
 
The PIII Xeons with 1 meg and 2 megs of cache only come in 550mhz and 700mhz parts, with 100mhz front-side-bus. They support four-way and eight-way SMP. Now, I order Compaq servers monthly, and based upon recent server pricing given to me, let's compare cost. A PIII Coppermine running at 1ghz runs around $1400 dollars from Compaq in a server ($1650 if you want that 1GHz to say Xeon on it . A PIII Xeon with 1 meg of cache runs around $3800, and a PIII Xeon with 2 megs of cache runs around $6200 dollars. So that means that I can get two PIII Coppermine 1ghz processors for less than the price of one PIII Xeon with 1 meg of cache.  
 
Here's a fast one you should watch out for: certain OEMs charge an upcharge to go from a PIII Coppermine (256k cache) to a PIII Xeon (256k cache) of equal MHz. This is certainly not a benefit. And, if you want to compare PIII Coppermine to PIII Xeon performance on Intel's site, you're out of luck. All of the Intel SpecOPs and Spec CPU2000 benchmark charts on their site compare PIII Xeon performance to older, slower Xeons, and not to equal MHz PIII Coppermine parts.  
 
So which PIII do I want to purchase?
Hal, this ultimately will depend on your budget and the politics of your corporation. If I could only have one server for a file-server, and it could only have four processors, I'm stuck purchasing the Xeon anyway. Is it worth an extra $3000 per processor to upgrade from the 1-meg Xeon to the 2-meg Xeon? Unless you are running a heavily-utilized database server, I have a tough time justifying the cost/performance difference. For my web and application servers, I would much rather have several dual-1ghz PIII Coppermine servers clustered, or hardware load-balanced, than one big server with a few high-priced PIII Xeons in it. For a file-server, save your dollars and get higher speed hard-drives. Or build a storage area network. With the advent of Windows 2000 Active Directory and the Distributed File System, you could take two (or more) small servers with high-speed PIII Coppermines and cluster them onto one storage area network for your file-services. That way you'd have great performance, and high-availability. You could even take down a server during the day for upgrades!  
 
Conclusion: Bang for your buck
The whole idea behind the Xeon was to move the L2 cache onto the processor die, and have it run at full processor clock. In addition, the cache would be made larger to ease the performance hit incurred by fetching data from system memory. With the advent of the PIII Coppermine, the L2 cache is on-die across the whole PIII line (and the new Athlons are the same way, FYI). So now what we have to ask is where the tradeoff is: Is purchasing a slower PIII Xeon with a lot of cache really going to benefit me more than a faster PIII Coppermine with less cache?  
 
If you are going with a four or eight-way SMP server, you don't have a choice. Xeon it is, in 1 or 2 meg variety. From my experiences, the $3000-per-processor upcharge for the 2 meg Xeon isn't worth the extra performance you'll really get. Save that $12,000 upcharge per machine, and purchase a couple of nice 933mhz dual-processor machines with fast hard-drives and plenty of memory. If you are going for single or dual-processor machines, by no means would I recommend the Xeon. For the Xeon upcharge, add an extra PIII Coppermine or increase the speed of PIII you purchase. Remember, the faster the PIII, the faster that L2 cache is going to run (regardless of size)! In addition, those high-end PIII Coppermines are going to come with a 133mhz FSB, and the 1 and 2 meg Xeons are still running at 100mhz FSB.  
 
Hope that helps answer your questions. Remember to email us with 'Ask Ars Topics' in the subject, and let me know what you'd like to see more detailed information on!  
 
Nota bene: There are, recently available, 16-way and 32-way PIII Xeon systems available. These are highly-specialized hardware and come with Windows 2000 Datacenter Server pre-installed on them. Due to the current market prices of those systems, don't look for one in the Ask Ars! Animal-Free Underground Test Lab anytime soon.