VMware vSphere vCenter Server 4.0 - Manual (Page 35)

To determine the effectiveness of memory sharing for a given workload, try running the workload, and use
resxtop
or
esxtop
to observe the actual savings. Find the information in the
PSHARE
field of the interactive mode
in the Memory page.
Measuring and Differentiating Types of Memory Usage
The Performance tab of the vSphere Client displays a number of metrics that can be used to analyze memory
usage.
Some of these memory metrics measure guest physical memory while other metrics measure machine memory.
For instance, two types of memory usage that you can examine using performance metrics are guest physical
memory and machine memory. You measure guest physical memory using the Memory Granted metric (for
a virtual machine) or Memory Shared (for an ESX/ESXi host). To measure machine memory, however, use
Memory Consumed (for a virtual machine) or Memory Shared Common (for an ESX/ESXi host). Understanding
the conceptual difference between these types of memory usage is important for knowing what these metrics
are measuring and how to interpret them.
The VMkernel maps guest physical memory to machine memory, but they are not always mapped one-to-one.
Multiple regions of guest physical memory might be mapped to the same region of machine memory (in the
case of memory sharing) or specific regions of guest physical memory might not be mapped to machine
memory (when the VMkernel swaps out or balloons guest physical memory). In these situations, calculations
of guest physical memory usage and machine memory usage for an individual virtual machine or an ESX/ESXi
host differ.
Consider the example in the following figure. Two virtual machines are running on an ESX/ESXi host. Each
block represents 4 KB of memory and each color/letter represents a different set of data on a block.
Figure 3-3. Memory Usage Example
virtual machine
1
guest virtual memory
guest physical memory
machine memory e
e
e
f
f
f
a
a
a a
a
b
b
b b
b
c
c
c c
c
d
d
d
virtual machine
2
The performance metrics for the virtual machines can be determined as follows:
n To determine Memory Granted (the amount of guest physical memory that is mapped to machine
memory) for virtual machine 1, count the number of blocks in virtual machine 1's guest physical memory
that have arrows to machine memory and multiply by 4 KB. Since there are five blocks with arrows,
Memory Granted would be 20 KB.
n Memory Consumed is the amount of machine memory allocated to the virtual machine, accounting for
savings from shared memory. First, count the number of blocks in machine memory that have arrows
from virtual machine 1's guest physical memory. There are three such blocks, but one block is shared with
virtual machine 2. So count two full blocks plus half of the third and multiply by 4 KB for a total of 10 KB
Memory Consumed.
The important difference between these two metrics is that Memory Granted counts the number of blocks with
arrows at the guest physical memory level and Memory Consumed counts the number of blocks with arrows
at the machine memory level. The number of blocks differs between the two levels due to memory sharing
and so Memory Granted and Memory Consumed differ. This is not problematic and shows that memory is
being saved through sharing or other reclamation techniques.
Chapter 3 Managing Memory Resources
VMware, Inc. 35