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!).
Multiprocessing means without special bridges and chipsets.
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.
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.
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.
may have to disable Delayed Transactions
when using a single CPU OS.