Benefit of Dual Procs

Dual processors also allow the OS to give an application a larger timeslice, meaning that for standard office productivity work with lots of task switching the computer will respond more actively by alot.  Indeed, a dual 500 MHz PIII will “feel” faster than a 1 GHz PIII.  While you won’t take full advantage of dual processors without applications that actually use them, you can do many other things as well that you wouldn’t be able to do anywhere near as effectively with a UP system, such as:
  • Run a game server with affinity set to one CPU while running the game itself on the other.  Great for LAN parties.
  • Have an application with a processing leak that eats all of the processor, yet only have it eat one CPU so you have another CPU to shut down the application rather than having to hard reboot your system.
  • Compress DivX ;-) while playing Quake 3, burning a CD and listening to an MP3.  Multitasking is improved by so much that you can run multiple applications at once, each of which require good CPU latency and some requiring alot of CPU time.
However, there is one minor problem with dual processor systems: the motherboards and processors come at a premium to their uniprocessor counterparts.  For the Athlon end, you can indeed mod an Athlon XP to run in DP mode as if it were an MP, but you then void the warranty on your motherboard and both CPUs, which can be an issue since the mobo can run up to $200.  However, I’ll give you a quick lowdown on the two x86 dual CPU platforms available, and the chipsets for such.  As another note, don’t plan on OCing the system.  The video card? OK, but the CPUs and RAM? forget it.  While it’s easy to do on a UP system, it’s next to impossible to do on an MP system since you now have two CPUs to overclock and a northbridge that, due to its higher design performance, is so complex it does not OC well if at all.
The AMD Athlon MP has two chipsets, the AMD 760MP and the AMD 760MPX.  They’re virtually identical, with the difference being a newer southbridge in the 760MPX that supports 66 MHz 64 bit PCI with two slots, where the 760MP only has 33 MHz 64 bit PCI, but the MP boards out there (the Tyan Tiger MP and the Tyan Thunder K7) have four and six 64 bit PCI slots, respectively.  The maximum bus speed is 266 MHz, but each CPU has an independent frontside bus to the northbridge, giving increased bandwith over the standard Athlon 266 MHz FSB.  The RAM is single channel PC2100 DDR SDRAM for 2.1 GB/s of bandwith.  AGP is 4X and the USB ports are 1.1.  This chipset is being end of lifed by AMD, in preparation for the Opteron (which will initially be well out of your price range, given that the CPUs are likely to cost more than an entire DP rig based on 32-bit x86).  The highest Athlon MP is 2600+, although you may very well see the Barton core in an Athlon MP variant in a week or two.  In addition, this is the market where the AMD Athlon’s exclusive cache (where you can add the numbers of the L2 and L1 cache to get the total amount of cache) really fails.  To perform a cache snoop from one CPU to the other, the CPU executing the snoop must look in both the L2 and L1 cache, where on the Intel processors it need only check the L2.  Also, the independent frontside busses make cache snooping and general scalability much worse, allowing the AMD Athlon MP to only scale about 70% (one CPU is 100% performance, two is 170%) going from one to two CPUs.
The Intel competetor is the Xeon processor.  The Xeon is very similar to a Pentium 4, but uses a different socket.  In addition, above 2.0 GHz they all support HyperThreading, and therefore gives you four virtual processors to work with on one of these systems (providing you’re running Windows XP Pro, Windows 2000 Server or Linux) and really cook with speeds topping out at 3.06 GHz.  The chipsets available are the E7500, E7501, E7505 and the i860.  The E7500 uses dual channel DDR1600 for 3.2 GB/s of bandwith and tops out at a 400 MHz FSB with no AGP slot but four PCI-X busses.  The i860 is the other 400 MHz FSB chipset, but uses dual channel PC800 RDRAM for 3.2 GB/s of bandwith, has an AGP 4X slot and a single 66 MHz 64 bit PCI bus.  The E7501 and E7505 chipsets are Intel’s newer chipsets supporting the 533 MHz FSB, both of them use up to 16 GB of dual channel DDR2100 for up to 4.2 GB/s of bandwith.  The E7501 is the server chipset with four PCI-X busses, while the E7505 has only two PCI-X busses but sports AGP 8X (usually AGP Pro 8X).  The Xeon also has several chipsets by ServerWorks, but these will generally be out of any reasonable price range.  Also, here is where Intel’s shared bus (that gives Intel’s 533 MHz FSB as much bandwith as the Athlon MP’s 266 MHz FSB) and inclusive cache comes in handy.  In true dual processing applications, the Xeons scale at better than 95%.  The shared bus means one fewer hops to get to the other CPU, and the inclusive cache means that one processor only needs to check the other CPU’s L2 cache, not its L1.
Now then, one other thing of note: the AMD 760MP and 760MPX chipsets do not like the Audigy 2.  It’s a finicky device on a finicky PCI bus.  While your SCSI RAID and FC controllers will work fine on the MPX PCI busses, the Audigy 2 does not.  It does, however, work fine on the Intel boards.  In the next installment, I’ll go through some motherboards and assembly tips.