Happy New
Year!
If you’ve
been following my series then you’ll know that just before the Holidays I said
that I’d deal with what’s the worst that can happen when you oversubscribe.
So what’s the worst that can happen?
Well if you
push things too far, all those things that the Hypervisor can do to try and
keep things running will eventually be overwhelmed.
If you try to
use too much memory you’ll start to see ballooning on a consistent basis, then
swapping. At that point performance will degrade rapidly. Watch active memory values and take
ballooning increasing as the indication things are getting tight.
CPU is as
always a more gentle decay in performance.
CPU also has it’s indicators that the limits are being approached. CPU
Ready and Co-Stop are indicators that VMs are finding it tricky to find CPUs
when they want to do some processing.
The reason
CPU degrades differently to Memory is that it’s used differently. A process is in memory all the time, but only
uses a CPU when it needs so CPU busy is dictated by how frequently the CPU is
required and for how long. The
performance of a transaction will be dictated by the ‘chance’ that a CPU will
not be available when the transaction arrives.
If all the CPUs are busy it’ll enter a queue and this is where queueing
theory comes in.
Contention and
Queuing
Any system
has a finite set of resources. If you
only have a single user trying to use one workstation then there is no
contention for the use of that workstation.
As soon as you have more than one user then there is a chance that they
will want to use the workstation at the same time. That’s contention. It’s perfectly normal and happens inside
every OS all the time. There are lots
more process threads than there are CPUs, and when there is contention, then
the processes queue. Poor performance
only occurs when queueing becomes excessive.
On Friday I'll go in to more detail about the basic ideas of queuing. In the meantime register for our first webinar of 2017 'Performance Management made easy'
Phil Bell
Consultant
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