crcastleAWS engineer reports PostgreSQL perf halved by Linux 7.0, fix may not be easy
https://lore.kernel.org/lkml/yr3inlzesdb45n6i6lpbimwr7b25kqk...
Its worth reading this follow-up LKML post by Andres Freund (who works on Postgres): https://lore.kernel.org/lkml/yr3inlzesdb45n6i6lpbimwr7b25kqk...
AIUI in that thread they're saying "0.51x" the perf on a 96-core arm64 machine and they're also saying they cannot reproduce it on a 96-core amd64 machine.
So it's not going to affect everybody both running PostgreSQL and upgrading to the latest kernel. Conditions seems to be: arm64, shitloads of core, kernel 7.0, current version of PostgreSQL.
That is not going to be 100% of the installed PostgreSQL DBs out there in the wild when 7.0 lands in a few weeks.
For production Postgres, i would assume it’s close to almost no effect?
If someone is running postgres in a serious backend environment, i doubt they are using Ubuntu or even touching 7.x for months (or years). It’ll be some flavor of Debian or Red Hat still on 6.x (maybe even 5?). Those same users won’t touch 7.x until there has been months of testing by distros.
Ubuntu is used in many serious backend environments. Heroku runs tens of thousands (if not more) instances of Ubuntu on its fleet. Or at least it did through the teens and early 2020s.
and they are right, this is because a lot of junior sysadmins believe that newer = better.
But the reality:
a) may get irreversible upgrades (e.g. new underlying database structure)
b) permanent worse performance / regression (e.g. iOS 26)
c) added instability
d) new security issues (litellm)
e) time wasted migrating / debugging
f) may need rewrite of consumers / users of APIs / sys calls
g) potential new IP or licensing issues
etc.
A couple of the few reasons to upgrade something is:
a) new features provide genuine comfort or performance upgrade (or... some revert)
b) there is an extremely critical security issue
c) you do not care about stability because reverting is uneventful and production impact is nil (e.g. Claude Code)
but 99% of the time, if ain't broke, don't fix it.
https://en.wikipedia.org/wiki/2024_CrowdStrike-related_IT_ou...
Yes, I did reproduce it (to a much smaller degree, but it's just a 48c/96t machine). But it's an absurd workload in an insane configuration. Not using huge pages hurts way more than the regression due to PREEMPT_LAZY does.
With what we know so far, I expect that there are just about no real world workloads that aren't already completely falling over that will be affected.
Note that it's just not a single post, and there's additional further information in following the full thread. :)
>If this somehow does end up being a reproducible performance issue (I still
suspect something more complicated is going on), I don't see how userspace
could be expected to mitigate a substantial perf regression in 7.0 that can
only be mitigated by a default-off non-trivial functionality also introduced
in 7.0.
suspect something more complicated is going on), I don't see how userspace
could be expected to mitigate a substantial perf regression in 7.0 that can
only be mitigated by a default-off non-trivial functionality also introduced
in 7.0.