Tips for Linux Performance in Hyper-V

[Disclosure:  Microsoft employee]

Every now and then we are asked how well Linux performs on Hyper-V.  We’ve done extensive testing and most of the time it runs quite well, but we’ve found a few cases, where some tuning can make all the difference.

I was recently involved with a scenario where a Linux application on Hyper-V (Windows Server 2016) was showing benchmarks of between 30-35% slower than other hypervisors. We instinctively felt something was wrong, so we rolled up our sleeves and got to work.

After some effort from our outstanding product engineering team, we found that this gap was essentially eliminated by tuning some settings in the BIOS, tweaking some Linux kernel parameters as well as keeping the in-guest tools current. Here’s a quick summary of some considerations if you’re looking to maximize the performance of some of your Linux applications.

TURN THOSE C-STATES OFF!

Many systems have power management profiles in the BIOS, and while many environments correctly have this set to “performance”, many systems will still have additional options for individual C-states.  If you want a primer on what a C-state is, there’s a good article here from Intel, but basically its a power management method. If you want the best possible CPU performance on your host servers, turn it all off!

In this case, we observed a 15% performance improvement after disabling all C-states in the host server’s BIOS.

Upgrade Linux Integration Services (LIS)

Linux Integration Tools consists of optimized synthetic device drivers and other “virtualization helpers” that go into the guest OS.  Windows servers have integration services built in and get can updates via Windows Update. Linux is similar in that the various distributions will build LIS into their images, and — depending on what version your are running — updates are distributed through their respective patch channels.

LIS 4.1.3 was released in December of 2016 (download here) and we found that simply updating to this version of LIS there was a 10% performance increase.

A few notes here — some Linux distributions will not support LIS unless it is released through their patch distribution. Before you manually install LIS, make sure you understand if there are any support implications from your Linux vendor.

On the topic of LIS one final important point to keep in mind is that it makes a big difference what version of Linux you are running, as not all features are supported on all versions of Linux. If you visit the Hyper-V Linux support page, you will see sections for RHEL/CentOS, Debian, Oracle, Ubuntu, SUSE and FreeBSD.  In each of these sections you can find detail on which Hyper-V features are supported in which Linux versions.

Bottom line again, is that in this case, simply upgrading LIS to the current version of 4.1.3 delivered a 10% performance improvement.

Set the Linux Clock Source

Let’s say you didn’t want to upgrade to the latest LIS.  You can still capture much of the performance improvement here simply by tuning the Linux clock.

Changing the Linux kernel’s clock source to “tsc” which stands for “Time Stamp Counter” resulted in a performance improvement of 6%.  This particular application had a lot of context switches and made heady use of clocks and timers, so your mileage may vary here.

A bit of information on this parameter is available here.

How Many CPUs?

Linux kernels have an optional parameter where you limit the number of possible CPUs by setting the “possible_cpus” kernel parameter.  For example, if you have 8 virtual CPUs in the guest, edit the kernel parameters to include “possible_cpus = 8”. By setting this parameter, additional overhead processing was removed from the Linux CPU scheduler.

In this application a performance improvement of 2% was observed after applying this setting.

The Bottom Line

IT is often complicated.  There’s so many distros, Linux versions and workload patterns generated by applications and other variables to consider — your mileage may vary.

If you are concerned that your Linux workloads may be running slower than they could be on Hyper-V check these three basic things:

  • Disable C-States in BIOS
  • Upgrade to current version of LIS (If not an option, change clock source to “tsc”)
  • Set “maximum_cpus” in the Linux kernel

For one application, these three steps resulted in a performance improvement of nearly 30% (with another 5 – 8% from correcting some misconfigurations).

So if you have any concerns about Linux performance in Hyper-V, just remember these tweaks. And of course that….

 

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