[linux-help] Re: Dual processors

[Carl D Cravens <raven@xxxxxxxxxxx>]

On Sat, 10 Aug 2002, Lowell Premer wrote:

> To a certain extent, this was the point to ny (regretably smart-assed
> sounding) reply below; applications have to be written especially to take
> advantage of dual-or-greater proceesing IIRC...I was wondering which of
> the software being used in your case was dual-processor friendly...

For a single application to see significant change in speed, it does have
to be multi-threaded so it can run on multiple CPU's at the same time...
and there aren't that many of those on the average user's desktop

But you don't need multi-threaded processes to see large benefits from
multiple CPU's... because you're running more than one program at a time.
SETI@Home can have an entire CPU to itself while a second CPU runs
everything else.  Nobody has to be multi-threaded for that.

If they did, there wouldn't be a reason for the 6-cpu HP-UX boxes we run
at work, because none of our applications are multi-threaded. On a
single-CPU box, only one process can run at a time... and each process
gets a tiny slice of CPU time every so often.  On a multi-CPU box, you can
have as many processes running at the same time as you have CPU's... you
double the number of tiny slices of CPU time available.

Drop in a second CPU (and appropriate kernel config) and you'll see a
performance increase immediately, no special software required.  If most
of your applications are CPU-bound (they're waiting for more CPU time, not
waiting to read/write data somewhere), you'll certainly see an increase in
performance.

Probably the best way to visualize what multiple CPU's do to performance
is to run 'top' on a multi-CPU box that's running a lot of CPU-intensive
stuff.

_Now_, all that said, multiple CPU's won't do you a lot of good if you're
box is like mine... sitting at 99% idle most of the time.  Obviously, if
you have a lot of CPU white-space, you don't need another CPU.  Though you
need to look at peak times and not averages... when things are the
busiest, how much of that is CPU-bound programs and how long are programs
waiting for their slice of the CPU time?  (In my case, most of my programs
are IO-bound... apache, mysql, sendmail... stuff whose job is to push bits
around instead of adding them up.)

As an aside... if you fancy being a systems administrator, you don't
_have_ to understand how the kernel works.  But if you _do_, it'll make
your job a lot easier and your skills a lot more marketable.  And it'll
keep you from doing things like promising the boss that a second CPU will
speed up your applications when the problem is IO and not CPU. :)

--
Carl D Cravens (raven@xxxxxxxxxxx)
If at first you don't succeed, try 2nd or shortstop.

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