Multiprocessing

Using multiple CPUs could only be done with Intel chips at first, except for the Celeron and some older Pentiums, but Abit has made a dual Socket 370 (Celeron PPGA) board that bypasses the SMP limitations inside the chip. However, the AMD Athlon 4 also supports it (with the AMD 760 chipset), though, oddly, some of the PIII range may not. The Pentium Pro natively supports four CPUs, whilst the Pentium and Pentium II support only two (although there is a 9,000 processor Pentium Pro machine around!).

Glueless Multiprocessing means without special bridges and chipsets.

You may also only do Symmetrical Multiprocessing (SMP), as it's called, with certain operating systems, notably Windows 2000 or NT, XP, OS/2, Sun, SCO, HP, FreeBSD and Linux. Rather than sheer performance, however, except with the newer motherboards, there is more likely to be an improvement in multitasking, as a single threaded application can be run on one CPU and the OS can use the other, so the machine will run more smoothly. In this way, you could burn a CD, download a large file, sort large tables of contents and print, without the machine missing a beat.

A multi-threaded application, on the other hand, can use both CPUs. Note, however, that you don't get a 100% increase as there is an overhead from the various CPUs talking to each other. You also need larger caches to prevent CPUs going after data in memory, which means you need cache coherency, that is, where the data in each CPU's cache is the same.

Where two Athlons talk to memory at 266MHz, the chipset behaves like a switch between the CPUs and memory (in networking terms, a switch provides a dedicated connection between two devices that wish to talk to each other, thus allowing them both use of the maximum bandwidth. The connection is made only for the time required). In this case, AMD's Hypertransport system allows independent access to the whole of memory by each CPU and also ensures cache coherency with reduced latency, done by tagging the state of the data in the cache of one Athlon, then allowing access by the other - the data is not duplicated.

Intel's system, by contrast, allows two CPUs to share one channel - two PIIIs sharing a bus with their 840 chipset will have 1.5 Gb/s. AMD can claim 4.2! In fact, their SmartMP technology can use a second processor to double a system's performance.

You may have to disable Delayed Transactions when using a single CPU OS.