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Multi-core versus multi-CPU/multi-socket/SMP

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Should you buy a machine with a single multi-core CPU or a multi-CPU
socket machine?

The advantages of the multi-CPU platform are:
- you can have more CPU cores total
- you can have more RAM (but you need registered RAM)
- you always have working ECC RAM
- you have PCI-X in many boards (not that PCI-X is not PCIe)

The advantages of the single-socket but multi-core platform are:
- you use cheaper unbuffered RAM
- you only have the noise from one CPU fan
- the board are smaller, you can use most cases
- you have overclocking support if that's your thing

Almost all multi-CPU mainboards use registered RAM, and that means:
- more expensive per megabyte
- ECC functionality pretty much always works
- you can have twice the number of RAM slots
- and each RAM slots can take modules of twice the capacity
- meaning that with the most common boards you can have 4 times as
  much RAM

With single-CPU boards you use unbuffered RAM, and that means:
- only 4 slots max
- modules can be 1 MB max for DDR and 2 MB max for DDR2 and DDR3
- meaning you have 4 GB max in the machine for DDR
- meaning you have 8 GB max in the machine for DDR2 and DDR3
- ECC functionality pretty much only works on Asus boards, and in the
  Intel works you need the better chipset, 975x.  965 chipsets and p35
  never support ECC (but you can use ECC RAM, ECC will be ignored)

Please make sure you understand that registered/unbuffered and ECC are
entirely different concepts that have nothing to do with each other.
If you have questions, please visit my other guide which explains
these concepts in more detail.

The mainboard size matter quite a bit.  If you buy a dual-CPU board
then you can buy a board in normal ATX format, but that might not be a
good idea.  If you do that then you get less RAM slots, and the board
is so cramped that you cannot mount silent CPU fans.  Only do that if
you have a dedicated machine room.  In most cases you want to use EATX
dual-CPU boards.

The maximum RAM size is also important.  The registered platforms
generally can take 4 times as much RAM (twice the number of slots and
twice the module capacity per slot).  The problem here is that for
unregistered RAM they did not double the capacity per module when they
went from DDR2 to DDR3.  If you could live with a maximum of 4 GB RAM
in DDR times and with 8 GB in DDR2 times, then you are in for a bad
surprise during DDR3's lifetime, because you still only get 8 GB max.

PCI-X slots can be had easily with the multi-CPU boards.  Note that
this is an entirely different concept from PCIe (PCI express), these
are the older 64 bit wide PCI slots with 100 or 133 MHz.  You can use
these slots to buy very fast I/O cards for disks or network used.  The
PCIe variants of these cards are usually very expensive and often
don't work right in many boards.

That brings me to the next item - in general, one of the most severe
problems with the single-CPU boards is that the people writing the
BIOSes often don't make advanced I/O cards work at all.  It's a shame,
but that's what the situation is like.  The Intel socket 775 boards
are even worse on average than AMD based solutions with NVidia chipset
(which have other problems).

Other advantages of the multi-CPU socket/registered-RAM boards
include:
- more sensors
- better service
- BIOSes that can be used on a serial console
- some memory bandwidth advantages can be had, such as with NUMA on
  AMD and with quad-channel RAM in some Intel socket 771 solutions

The multi-CPU//registered-RAM boards generally don't have any built-in
support for overclocking.  If that is your thing you can usually do a
software overclock with clockgen and the like, but then you are still
stuck with no control over CPU voltage.

Exceptions to the rules above:.
- There are some single-CPU unregistered boards with PCI-X from Asus.
- There is one multi-CPU socket platform with unregistered RAM, AMD's
  4x4.  AMD's 4x4 is not a recommended platform.  The only board
  available gives people trouble, they only put 4 RAM slots total on
  there and the whole thing is a power hog.

 
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