** Changed in: ubuntu-z-systems
Status: Fix Committed => Fix Released
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https://bugs.launchpad.net/bugs/1876044
Title:
[UBUNTU 20.04] PSI generates overhead on s390x
Status in Ubuntu on IBM z Systems:
Fix Released
Status in linux package in Ubuntu:
Fix Released
Status in linux source package in Focal:
Fix Released
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.
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