** Also affects: linux (Ubuntu Focal)
Importance: Undecided
Status: New
** Changed in: linux (Ubuntu Focal)
Status: New => In Progress
--
You received this bug notification because you are a member of Kernel
Packages, which is subscribed to linux in Ubuntu.
https://bugs.launchpad.net/bugs/1876044
Title:
[UBUNTU 20.04] PSI generates overhead on s390x
Status in Ubuntu on IBM z Systems:
In Progress
Status in linux package in Ubuntu:
In Progress
Status in linux source package in Focal:
In Progress
Bug description:
SRU Justification:
==================
[Impact]
* PSI is enabled by default for all architectures in Ubuntu.
* On s390x this leads to performance degradations on popular workloads
like web serving (nginx).
[Fix]
* Leave 'CONFIG_PSI=y', but change 'CONFIG_PSI_DEFAULT_DISABLED=n' to
'CONFIG_PSI_DEFAULT_DISABLED=y'
[Test Case]
* Measure the overhead with 'CONFIG_PSI_DEFAULT_DISABLED=n' and
'CONFIG_PSI_DEFAULT_DISABLED=y' on the same environment with nginx.
[Regression Potential]
* The regression potential can be considered as moderate, since PSI
(Pressure stall information tracking),
* since PSI is just used to collect CPU overcommitted, memory and IO
metrics.
* And it can be enabled again with the kernel argument.
__________
PSI is always enabled in Ubuntu 20.04.
For a test system with 72 guests on 8 cores running a nginx workload this
created an overhead of ~1%.
Can we change this back to
CONFIG_PSI=y
CONFIG_PSI_DEFAULT_DISABLED=y
so that by default the overhead is not there but for debugging or if
needed it can still be enabled via kernel parm?
Maybe there has been a reason for this - so feel free to discuss.
---uname output---
Linux t35lp76 5.4.0-26-generic #30-Ubuntu SMP Mon Apr 20 16:57:22 UTC 2020
s390x s390x s390x GNU/Linux
Machine Type = All s390x architecture
---Debugger---
A debugger is not configured
---Steps to Reproduce---
root@t35lp76:/boot# grep PSI config-5.4.0-26-generic
CONFIG_PSI=y
# CONFIG_PSI_DEFAULT_DISABLED is not set
Stack trace output:
no
Oops output:
no
System Dump Info:
The system is not configured to capture a system dump.
*Additional Instructions for epasch@de,ibm.com:
-Attach sysctl -a output output to the bug.
To manage notifications about this bug go to:
https://bugs.launchpad.net/ubuntu-z-systems/+bug/1876044/+subscriptions
--
Mailing list: https://launchpad.net/~kernel-packages
Post to : kernel-packages@lists.launchpad.net
Unsubscribe : https://launchpad.net/~kernel-packages
More help : https://help.launchpad.net/ListHelp
