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Performance Tuning

Virtual machines generally show similar performance to physical hardware and the overheads are reducing as the virtualisation technology improves. However, in some cases, performance can be affected such as high disk I/O rates or additional network latency. A few hints and tips to investigate are described below. For questions or issues concerning the performance of a VM please use the support portal. .

Define a workload

In order to test different options, it is necessary to define a representative workload such as the time to perform a compilation or transfer a file. With memory caching, these tests can vary the number of times you run them as the disk contents could be already in memory and therefore served more quickly. Tuned-Adm

Tuned-adm is a Red Hat tool for setting kernel parameters, buffer sizes and disk I/O schedulers according to the workload. Red Hat identify some common profiles and find the appropriate setting for these. Thus, there is no need to go into the details of each kernel parameter. By default, a VM is set up with virtual-guest as the setting when a VM is created from the standard images. However, this may need to be set if user defined images are used. The virtual-guest is a good initial starting point for benchmarking the workload. However, for applications such as web servers, it may be that alternative profiles are better suited. tuned-adm list provides a set of profiles to investigate.

tuned-adm profile default
RAM disks

Ramdisk

Some I/O patterns are very intense on a small volume of data which does not need to be permanently stored. These areas are often stored in /tmp or /var/tmp but applications generally allow you to configure the TMPDIR variable or a configuration parameter for other areas. Depending on the size of the temporary data, a RAM disk can be used to mount a section of RAM as a file system. This gives excellent performance at the expense of eating into the RAM available for the application. Selecting a larger flavor of VM may allow a RAM disk to be allocated for this purpose. To mount a RAM file system, use the mount command.

By default /dev/shm/ exists and allows access to 50% of the available memory. If you wish to create your own ramdisk follow theses commands:

mkdir /tmp/ram
mount -t tmpfs -o size=4G tmpfs /tmp/ram/
Care should be taken not to swap memory. This is an expensive operation and would lead to a reduction in overall performance. We recommend disabling swap

Network Offloading

Linux kernels, by default, offload some processing to the network card. In the case of a VM, this does not improve the performance. The effects are most visible on high performance networks.

To disable this, run

ethtool -K eth0 gro off tso off gso off

The current settings can be checked with ethtool -k.

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