dual cpu's

jason andrade jason at dstc.edu.au
Thu Jun 13 16:41:00 CDT 2002


On Thu, 13 Jun 2002, Bryce C wrote:

> Hi All,
> 
> I hope this isn't too off-topic or chit-chatty for
> this list, but does anyone have any general comments
> of the effectiveness of dual cpu's in a linux
> environment. The servers are used for apache/mysql/php
> based websites and pretty much active all the time.
> 
> I guess my question is can anyone verify that those
> apps are really going to take advantage of the second
> cpu. I am not too informed about Linux's multiple cpu
> support but I know that on some O.S/apps the second
> CPU is not really used at all?

it depends on whether your application is multithreaded or not
as well as whether it forks multiple instances.

an application which does neither can't really take advantage
of dual cpus but this doesn't mean it can't benefit your system
overall.  your app will run on one cpu freeing the other one up
for system tasks, other applications etc.

in general, dual cpu machines are quite effective in delivering
twice the cpu throughput in the same form factor (around 98%
more, and perhaps in the case of Xeon2 with hyperthreading, perhaps
110-120% more..).  the scaling efficiency drops as you start adding
cpus in most cases and 4cpu is probably the limit for most uses.

linux is pretty good about making the most of SMP machines, in
particular with the 2.4.X kernels (in particular where X is > 18
nowadays).

if you want detailed technical whitepapers rather than ramblings
of a sysadmin, i'll try and dig up some some urls.. or you could
use google to search for "why is dual cpu better.." :-)


regards,

-jason




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