🚀 go-pugleaf

Quoth George Bell <MasterofTimeADim@netscape.net>:
> While I generally agree that dual CPU may be inappropriate, this is
> an inaccurate statement.  The allocation of CPU resources is a
> function of the os, not the tasks running(and thus the software).
> Since Linux can support SMP - symetric multiprocessor( each
> processor gets as much "work" as the other ), then twice as much CPU
> time is available to the system and the performance is twice as
> great, period.

This isn't a fair assessment of things.  

Having a second CPU /may/ allow the kernel to push tasks onto one CPU
or another, sharing the load.  If you have a whole bunch of processes
that /can/ run in parallel, that can be useful.

On the other hand, the two CPUs have to share:
 a) The same memory bus;
 b) The same memory caches;
 c) The same I/O buses.

If the CPUs are working on totally different sorts of things, b) can
lead in particular to "sucky performance."

Consider: on a single CPU machine, any given task is going to get a
bunch of cycles to work on whatever is the work of the task.  When a
context switch occurs, different activity will lead to different cache
usage, but at least you get locality of access within the process
context.

On a dual CPU machine, two processes really are running concurrently,
and if their accesses to memory are really quite different, they'll be
stumbling over each other as they concurrently try to access very
different things in memory.

Both CPUs have to coordinate their access to memory lest they get out
of sync and start trampling on each others' memory.  That coordination
effort has a cost, and it is a cost that a single CPU machine does not
need to pay.

If both CPUs are /reading/ the same code, or /reading/ the same memory
locations, there could conceivably be a big /WIN/ as a result; the
first CPU already read the data, so it's in cache so that the second
CPU doesn't have to read it in.  But that is not the typical case; it
is more likely that you'll have XFree86 on one CPU, doing its thing,
and Mozilla on the other, doing something different, so that
maintaining cache coherency is a net /cost/.
-- 
(concatenate 'string "cbbrowne" "@acm.org")
http://www3.sympatico.ca/cbbrowne/oses.html
I was  thinking about how  people seem to  read the Bible a  whole lot
more as they  get older then it dawned on  me...they were cramming for
their finals.