Installation guide

CPU Performance
Use the vSphere Client CPU performance charts to monitor CPU usage for hosts, clusters, resource pools,
virtual machines, and vApps. Use the guidelines below to identify and correct problems with CPU
performance.
A short spike in CPU usage or CPU ready indicates that you are making the best use of the host resources.
However, if both values are constantly high, the hosts are probably overcommitted. Generally, if the CPU
usage value for a virtual machine is above 90% and the CPU ready value is above 20%, performance is impacted.
Table 22-6. CPU Performance Enhancement Advice
# Resolution
1 Verify that VMware Tools is installed on every virtual machine on the host.
2 Compare the CPU usage value of a virtual machine with the CPU usage of other virtual machines on the host or in
the resource pool. The stacked bar chart on the host’s Virtual Machine view shows the CPU usage for all virtual
machines on the host.
3 Determine whether the high ready time for the virtual machine resulted from its CPU usage time reaching the CPU
limit setting. If so, increase the CPU limit on the virtual machine.
4 Increase the CPU shares to give the virtual machine more opportunities to run. The total ready time on the host might
remain at the same level if the host system is constrained by CPU. If the host ready time doesn't decrease, set the CPU
reservations for high-priority virtual machines to guarantee that they receive the required CPU cycles.
5 Increase the amount of memory allocated to the virtual machine. This decreases disk and or network activity for
applications that cache. This might lower disk I/O and reduce the need for the ESX/ESXi host to virtualize the hardware.
Virtual machines with smaller resource allocations generally accumulate more CPU ready time.
6 Reduce the number of virtual CPUs on a virtual machine to only the number required to execute the workload. For
example, a single-threaded application on a four-way virtual machine only benefits from a single vCPU. But the
hypervisor's maintenance of the three idle vCPUs takes CPU cycles that could be used for other work.
7 If the host is not already in a DRS cluster, add it to one. If the host is in a DRS cluster, increase the number of hosts
and migrate one or more virtual machines onto the new host.
8 Upgrade the physical CPUs or cores on the host if necessary.
9 Use the newest version of ESX/ESXi, and enable CPU-saving features such as TCP Segmentation Offload, large
memory pages, and jumbo frames.
Disk I/O Performance
Use the vSphere Client disk performance charts to monitor disk I/O usage for clusters, hosts, and virtual
machines. Use the guidelines below to identify and correct problems with disk I/O performance.
The virtual machine disk usage (%) and I/O data counters provide information about average disk usage on a
virtual machine. Use these counters to monitor trends in disk usage.
The best ways to determine if your vSphere environment is experiencing disk problems is to monitor the disk
latency data counters. You use the Advanced performance charts to view these statistics.
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The kernelLatency data counter measures the average amount of time, in milliseconds, that the VMkernel
spends processing each SCSI command. For best performance, the value should be 0-1 milliseconds. If the
value is greater than 4ms, the virtual machines on the ESX/ESXi host are trying to send more throughput
to the storage system than the configuration supports. Check the CPU usage, and increase the queue depth
or storage.
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The deviceLatency data counter measures the average amount of time, in milliseconds, to complete a SCSI
command from the physical device. Depending on your hardware, a number greater than 15ms indicates
there are probably problems with the storage array. Move the active VMDK to a volume with more
spindles or add disks to the LUN.
vSphere Basic System Administration
280 VMware, Inc.