On Sat, May 08, 1999 at 02:20:32PM -0400, Steve Willer wrote:

> On Thu, 6 May 1999, Pavel Machek wrote:

> > Are you so sure? 2x SMP on intel means about 30% performance increase,
> > if you are lucky. SMP on intel can even _slow you down_ for certain
> > kind of loads, and web server may be exactly this kind of load.

> That seems _highly_ unlikely, and my own experience is contrary to this.
> Are you sure about this?

It depends highly on the kind of website you're serving. If you are mostly
serving straight files (either html files or other data) your cpu will be
hardly used (relatively speaking) and the bottleneck will be one of the
network, your disks or the throughput your machine can manage. Adding CPUs
will have little impact in such a case. If, however, the machine is used to
run a lot of scripts that do complicated things (and dont do too much i/o,
relatively speaking) the CPU might help.

Basically, from our own (fairly busy) webservers, i've seen that it all
depends on the work the machine has to do. At first, we had to keep adding
memory because the number of different pages that were viewed, grew, and
bigger memory allows for more caching. After we got to 256Mb RAM, we started
seeing disk i/o troubles as well, and we moved some of the data to different
spindels. On a couple other machines, which had most of its documents on an
NFS server, we hit the limit of the number of connections the machine could
keep open and working while serving them large amounts of data. Much of the
connections were coming over congested links (outside of our sphere of
control, as of yet :-) and the machines simply ran out of network buffers.
The only way to scale that, to our knowledge, was to add more machines. 
These machines were all BSDI, not linux, but the same principle holds ;-)

You can do some simple tests though.. run the webserver with a single cpu.
Run something like rc5 or seti-at-home on low priority, and see if it gets
scheduled :-) Also, keep an eye on vmstat -- if your machine starts
swapping, it's in trouble. It's ok to have some background processes in
swap, but if it starts swapping for real, it's in a serious world of hurt.
`vmstat 1` is a very nice tool for finding out what your webserver is
ailing.

Regards,
	Thomas.

-- 
Thomas Wouters <thomas@xs4all.net>

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