CPU performance - disable bug mitigations

Goran sendmailtogoran at gmail.com
Tue Nov 20 16:05:08 UTC 2018


Hi all,

I have some servers with AMD Opteron 6276 CPUs and doing some number
crunching on these.

I installed Ubuntu 16.04 and was pretty much satisfied (never did an
update - running local only). Some days ago I installed 18.04 and got
a performance penalty of 33%.

My guess is, that it has something to do with Spectre. So I did a

# grep . /sys/devices/system/cpu/vulnerabilities/*
/sys/devices/system/cpu/vulnerabilities/meltdown:Not affected
/sys/devices/system/cpu/vulnerabilities/spec_store_bypass:Mitigation:
Speculative Store Bypass disabled via prctl and seccomp
/sys/devices/system/cpu/vulnerabilities/spectre_v1:Mitigation: __user
pointer sanitization
/sys/devices/system/cpu/vulnerabilities/spectre_v2:Mitigation: Full
AMD retpoline, IBPB

How can I disable these mitigations without restart (without boot
command line)? I just found some examples for Red Hat which are not
working for Ubuntu.

Further I read these pages:
https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SpectreAndMeltdown
https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/Variant4
https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/LazyFP
https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/BCBS

I would like to disable all mitigations related to any of these hardware-bugs.

After all I got the harshest penalty when I switched back to 16.04.5
(fresh new install) and could not get my performance back. My old
16.04 is lost so no way back.

How to get back my performance?

Regards
Goran



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