Bug 1787442

Summary:	"Configuring Real-Time Compute" lacks guest device configuration tuning
Product:	Red Hat OpenStack	Reporter:	Marcelo Tosatti <mtosatti>
Component:	documentation	Assignee:	Stephen Finucane <stephenfin>
Status:	CLOSED WONTFIX	QA Contact:	RHOS Documentation Team <rhos-docs>
Severity:	unspecified	Docs Contact:
Priority:	unspecified
Version:	unspecified	CC:	jschluet, mwitt
Target Milestone:	---
Target Release:	---
Hardware:	Unspecified
OS:	Unspecified
Whiteboard:
Fixed In Version:		Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2020-01-16 17:44:07 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
Cloudforms Team:	---	Target Upstream Version:
Embargoed:

Description Marcelo Tosatti 2020-01-02 20:57:50 UTC

Description of problem:

The "Configuring RealTime Compute" section of the OpenStack manual lacks
customizations to the guest device configuration necessary for realtime operation.

Version-Release number of selected component (if applicable):
OSP 13 is affected:

https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/13/html/advanced_overcloud_customization/realtime-compute#rtc-instances

Newer versions seem to be affected as well.

How reproducible:
Easily.

Steps to Reproduce:
1. Enable graphics or USB and run cyclictest.
2. Perform graphics operations with Cirrus and VNC.

Actual results:

Cirrus and VNC (and X11 in zoom mode):
# Total: 000026758
# Min Latencies: 00005
# Avg Latencies: 00006
# Max Latencies: 04122

USB and usbtablet:
# Total: 000003784
# Min Latencies: 00005
# Avg Latencies: 00006
# Max Latencies: 00259

Expected results:

no USB and no graphics:

# Min Latencies: 00005
# Avg Latencies: 00006
# Max Latencies: 00014

Additional info:

The manual should be updated with the following information:

Its recommended to disable USB and graphics since the execution of these features can negatively impact realtime workloads.

1. Edit the guest XML configuration file

virsh edit guestname

2. Make the following changes:

Remove non-essential hardware, such as USB devices, serial devices, QXL device, the graphic display, etc

libvirt adds a default USB controller to the xml schema, change its model to none to disable it, eg:

Comment 2 Stephen Finucane 2020-01-16 17:44:07 UTC

This is less a documentation and more an issue with nova. If these features should be disabled for real-time, nova should expose the knobs to do this or do it automatically. One should not modify libvirt's XML behind nova's back. I'm going to close this as WONTFIX. Can you please reopen an RFE against nova where we can track this work?