11-10-2010
There's no hard rule on this. Some people over commit.. so the sum total of VM resources is greater than actual physical resources. Usually, one place where this is done is with regards to CPUs. BUT... if your VMs REALLY need a whole bunch of CPU resources, it's possible that you'll reach the limits of what you can actually give and have a "good" performing platform. IMHO, it's pretty safe to say that we waste about 50% of CPU resources (in general, the number is usually much, much higher). So you could use that for a very conservative number of total CPUs you can dole out. And don't discount the hyperthreads... they do help out as well...
If your plan is to virtualize large scale servers (e.g. 4 CPUs + 8G+ memory) ... I think that's a BAD plan. Virtualization is most effective when talking about smaller footprint machines. Then the scalability can be VERY large.. and you can really over commit (but again, there is no RULE).
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LEARN ABOUT CENTOS
libpfm_intel_p6
LIBPFM(3) Linux Programmer's Manual LIBPFM(3)
NAME
libpfm_intel_p6 - support for Intel P5 based processors
SYNOPSIS
#include <perfmon/pfmlib.h>
PMU name: pm, ppro, pii, piii, p6
PMU desc: Intel Pentium M, Intel Pentium Pro, Intel Pentium II, Intel Pentium III, Intel P6
DESCRIPTION
The library supports all Intel P6-based processors all the way back to the Pentium Pro. Although all those processors offers the same PMU
architecture, they differ in the events they provide.
MODIFIERS
The following modifiers are supported on all Intel P6 processors:
u Measure at user level which includes privilege levels 1, 2, 3. This corresponds to PFM_PLM3. This is a boolean modifier.
k Measure at kernel level which includes privilege level 0. This corresponds to PFM_PLM0. This is a boolean modifier.
i Invert the meaning of the event. The counter will now count cycles in which the event is not occurring. This is a boolean modifier
e Enable edge detection, i.e., count only when there is a state transition. This is a boolean modifier.
c Set the counter mask value. The mask acts as a threshold. The counter will count the number of cycles in which the number of occur-
rences of the event is greater or equal to the threshold. This is an integer modifier with values in the range [0:255].
AUTHORS
Stephane Eranian <eranian@gmail.com>
September, 2009 LIBPFM(3)