Rob van der Heij wrote:
On Fri, May 1, 2009 at 3:30 AM, Leland Lucius <[email protected]> wrote:
Also, if load averages increase sufficiently, doesn't adding another
VCPU help smooth out the load (mainly during spikes)?
If you can manage to distribute resources in an unfair manner so that
you get more than the rest, then you may be able to take advantage of
that. That's the kind of performance measurements that I don't care
for. In real life there is little value in tuning the system to make
one server run faster at the expense of others (unless that is
justified by the business, for example production and development
servers).
If we think that 2 virtual CPUs is better, then it is better for all
of them and all get 2 virtual CPUs. And in that case the contention
gets more and chances of getting both your virtual CPUs dispatched at
the same time are not better than when everyone was having just one
car in the race.
I know one case with Domino where we assume 2 virtual CPUs could be
better than one. Domino does its own dispatching of tasks, and the
idea was that by (bad) design it sometimes would only run the
background replication task and not service the end-users. By giving
it two half CPUs rather than one full CPU, it could not spend more
than half a CPU on the single threaded background process, and thus
was forced to also service the end-users.
Oh, oh...starting to get a bit worried. I think I understood all that. :-)
So I stick to my motto: "When you don't know, one will do. When you
have measured, probably too."
Actually, most of our servers do only have 1 single VCPU. I was just
concerned about a couple of Linux servers where we run 6 WAS instances
per server. Want to make sure that each instance got his turn at a VCPU.
Leland
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