On Wed, Feb 06, 2002 at 09:35:48PM +0100, Jaromir Dolecek wrote:
> 
> If the benchmarking involves only I/O, I imagine system using just
> one processor (and without processor interlocking) _might_ outperform
> a system using all four CPUs. Very much depends on usage pattern.

Certainly it's the case that a system using all four CPUs, with
a multithreaded or otherwise concurrent kernel, will significantly
ourperform the same system using our current giant-lock kludge --
which, as you point out, may not even perform as well as a single-CPU
machine.

> AFAIK FreeBSD SMP is giant-lock-SMP at the moment too. It would
> be interesting to find out how well NetBSD and/or FreeBSD SMP
> performs compared to Linux in quad processor configuration.

For what it's worth, on my 6-CPU machine, I can build the system
faster if I remove three processors, no matter how many build jobs
I use, and I can build the system faster with *one* build job, if
I leave all CPUs spun up, than with six.

The locking has quite a substantial cost.  However, where you really
lose by running giant-lock on a multiprocessor is in the potential
performance you *throw away*; running Solaris or UnixWare, if I
pull three CPUs the NFSops/sec I can run on my machine do, in fact,
drop by about 30%.  Remember, bulk read/write benchmarks are not
a realistic predictor of actual I/O performance for most applications;
there's a certain fixed cost to handling an NFS operation of *any*
size, and with a highly concurrent kernel you can handle up to N of
them at a time when you've got N processors; with a giant-lock
kernel you can handle one of them at a time, period.

-- 
 Thor Lancelot Simon	                                      tls@rek.tjls.com
   But as he knew no bad language, he had called him all the names of common
 objects that he could think of, and had screamed: "You lamp!  You towel!  You
 plate!" and so on.              --Sigmund Freud