[MUD-Dev] MMORPG/MMOG Server design - intel-ligent choices of hardware ; )

ceo ceo at grexengine.com
Sat Feb 22 22:15:09 New Zealand Daylight Time 2003


Keith Brewer wrote:

> Keep in mind that Intel architecture machines use a shared memory
> bus which isn't very friendly to highly contentious multi-threaded

>...

> have SMP support at all).  If you HAVE to go Intel, Solaris is
> about the best there is for this but you'll still run into the
> shared memory bus problem eventually.  >

In the words of a product manager at a major unix vendor (who shall
remain nameless), "Linux on intel makes our server software look
cr*p, and Solaris['s excellent multi-threading] makes it work like
it SHOULD do, so we prefer using it for benchmarks wherever
possible...even though Sun get to slate us for preferring their
product" (this company's Unix didn't have an intel version).

I think you under-exaggerate the problems of the Intel architecture
when it comes to MP's in one box: it's beyond cr*p, it's hellishly
cr*p :). IIRC, the "shared bus" is even worse than one might imagine
- there's NO negotiation between the processors; its TDM, with each
CPU getting a timeslot in turn, and if that CPU doesn't want to do
any memory read/writes, then the slot is wasted (even if another
processor is currently waiting desperately for a chance to
read/write!) In a box with 1 CPU, only 1 CPU can physically
read/write memory at once. In a box with 4 CPU's, only 1 CPU
can...in a box with 32 CPU's, only 1 CPU could...arrrgh!

AFAIAW, this is the reason for the creation of the Xeon intel
processors (slightly non-standard CPUs, main difference is up to 8Mb
of L2 cache where a typical equivalent normal CPU would have about
256kb). In an MP intel box, if you're a CPU and you get a chance at
that memory bus, you do your damned best to make full use of it
while you can!

Although, this is possibly a completely wrong interpretation, IANA
chip designer :).

Anyway, you really really shouldn't try to scale up the number of
intel CPU's in one box. Don't even hope that the situation will get
better over the next few years, and that 16-way intel boxes will
become commonplace: it's unlikely, the architecture is too hobbled.

Adam M


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