Discussion
AWS Engineer Reports PostgreSQL Performance Halved By Linux 7.0, But A Fix May Not Be Easy
FireBeyond: Once upon a time, Linus would shout and yell about how the kernel should never "break" userspace.Now, the kernel engineer who introduced the brand new mechanism (introduced in Linux 7.0) for handling pre-emption says the "fix" is for Postgres to start using this new mechanism (I think the sister comment below links to what one of the Postgres engineers thinks of that, and I'm inclined mostly to agree).
lfittl: Its worth reading this follow-up LKML post by Andres Freund (who works on Postgres): https://lore.kernel.org/lkml/yr3inlzesdb45n6i6lpbimwr7b25kqk...
dsr_: Nobody sensible runs the latest kernel; nobody running PG in production should be afraid of setting a non-default at either boot time or as a sysctl. So this will, most likely, be another step in building a PG database server (turn off pre-emption if your kernel is 7.0 or later and PG is pre-whatever-version).At worst it might become a permanent part of building a PG server and a FAQ... but if it affects one thing this badly, it will affect others.
stingraycharles: That may be the case, but it’s still not a great situation to be in and one has to wonder: if PostgreSQL is affected, what else is?
galbar: It's not a good look to break userspace applications without a deprecation period where both old and new solutions exist, allowing for a transition period.
Meekro: > Nobody sensible runs the latest kernelFrom the article: "Linux 7.0 stable is due out in about two weeks. This is also the kernel version powering Ubuntu 26.04 LTS to be released later in April."Unfortunately, lots of people will be running it in less than a month. At the moment, it'll take a kernel patch (not a sysctl) to undo this-- hopefully something changes.
jeffbee: Funny how "use hugepages" is right there on the table and 99% of users ignore it.
bear8642: > I'd argue a 50% hit to performance [...] is ... quite the regressionIndeed! Especially if said regression happens to impact anything trade/market related...
longislandguido: Anyone check to see if Jia Tan has submitted any kernel patches lately?
Neywiny: Not nobody but not everybody upgrades to the newest distros immediately. That's the advantage of LTS. I've even found that a lot of programs have poorer support on 24.04 than 22.04 due to security changes, so I'm fine sticking with 22.04 as my main dev system.
stingraycharles: This seems to be brushing off a major performance regression just because you personally don’t upgrade for 4 years. I don’t think that’s common at all.
monocasa: I feel like using spinlocks in user space at all without kernel support like rseq is just asking for weird performance degradations.
bombcar: We need some sensible people running the latest and greatest or we won't catch things like this.
cperciva: This makes me feel better about the 10% performance regression I just measured between FreeBSD 14 and FreeBSD 15.0.
justinclift: > ... not everybody upgrades to the newest distros immediately.While that's true, for new deployments the story is often "deploy on the latest release of things available at the time".So, there will probably be a substantial deployment of new projects / testing projects using the Linux 7.0 kernel and the latest available software packages in a few weeks.
justinclift: Note that it's just not a single post, and there's additional further information in following the full thread. :)
bombcar: That's the big thing - PSQL will be tested, noticed, and fixed (and likely have a version that handles 7.0 by the time it's in common use).But other software won't and may not even be noticed, except as a (I hate using the term) enshittification.Better to introduce the "correct way" in 7.0 but not regress the old (translate the "correct" into the old if necessary) - and then in 8.0 or some future release implement the regression.
TacticalCoder: 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.
zx8080: I've came to this thread to find Rust, but it's surprisingly not mentioned!
cwillu: The option to set PREEMPT_NONE was removed for basically all platforms.
esafak: That's the advantage of LTS? 24.04 is the LTS, not the one you use, 22.04.
SoftTalker: 22.04 is also an LTS release, supported for another year still.https://ubuntu.com/about/release-cycleWe're just now looking at moving production machines to 24.04.
jcalvinowens: Yeah, exactly. "Doctor, help, somebody replaced my wooden hammer with a metal one, and now I can't hit myself in the face with it as many times."If you use spinlocks in userspace, you're gonna have a bad time.
stingraycharles: Exactly, this is how it’s usually done. As the developer on the mailing list mentions, implementing a new low level construct in 7.0 and a performance regression that requires said new low level construct to mitigate is not great. You need a grace period in which both old and new approach is fast.
quietsegfault: This was my immediate thought - kernel doesn’t break software, or at least it didn’t used to.
master_crab: 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.
crcastle: 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.https://devcenter.heroku.com/articles/stack
cortesoft: All even number .04 releases are LTS in Ubuntu
MBCook: So perhaps this is a regression specifically in the arm64 code, or said differently maybe it’s a performance bug that has been there for a long time but covered up by the scheduler part that was removed?
MBCook: It wouldn’t be the first time one of the other maintainers ran afoul of “Linus’s law“.He may simply be waiting until more is known on exactly what’s causing it.
nine_k: Do they upgrade to the new LTS the day it is released?
crcastle: Not historically.
999900000999: Depends on your shop.As someone with a heavy QA/Dev Opps background I don't think we have enough details.Is it only ARM64 ? How many ARM64 PG DBs are running 96 cores?However...This is the most popular database in the world. Odds are this will effect a bunch of other lesser known applications.
arjie: Well, the reason he'd yell about it is that someone did it. If no one ever did it, he'd never yell and we'd never have the rule. So one can only imagine that this is one of those things where someone has to keep holding the line rather than one of those things where you set some rule and it self-holds.Doubtless someone will have to do the yelling.
rvnx: 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...
Seattle3503: If you're running in a docker container you share the host kernel. You might not have a choice.
cdelsolar: https://lkml.org/lkml/2012/12/23/75
db48x: Heh. Did they at least add useful features to balance out that cost?
db48x: Could be either of those, or something else entirely. Or even measurement error.
zamalek: It was later reproduced on the same machine without huge pages enabled. PICNIC?
anarazel: 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.
anal_reactor: Can someone explain to me what's the problem? I have very little knowledge of Linux kernel, but I'm curious. I've tried reading a little, but it's jargon over jargon.
pmontra: A customer of mine is running on Ubuntu 22.04 and the plan is to upgrade to 26.04 in Q1 2027. We'll have to add performance regression to the plan.
rs_rs_rs_rs_rs: They don't need to, there's about a billion bugs they can exploit.
whilenot-dev: Please follow the complete thread: https://lore.kernel.org/lkml/xxbnmxqhx4ntc4ztztllbhnral2adog...> [...] used huge_pages=on - as that is the only sane thing to do with 10s to 100s of GB of shared memory [...] if I disable huge pages, I actually can reproduce the contention [...]
up2isomorphism: Not sure why people have to upgrade to the newest major kernel version as soon as it is released.
conradludgate: It's the performance team's job to test these things. Doesn't mean they're going to deploy it immediately.Someone should be testing these things and reporting regressions
miki123211: On the other hand, I suspect LLMs will dramatically decrease the window between a vulnerability being discovered and that vulnerability being exploited in the wild, especially for open-source projects.Even if the vulnerability itself is discovered through other means than by an LLM, it's trivial to ask a SOTA model to "monitor all new commits to project X and decide which ones are likely patching an exploitable vulnerability, and then write a PoC." That's a lot easier than finding the vulnerable itself.I won't be surprised if update windows (for open source networked services) shrink to ~10 minutes within a year or two. It's going to be a brutal world.
rixed: There is serious as in "corporate-serious" and serious as in "engineer-serious".
shakna: Freund seems to suggest that hugepages is the right way to run a system under this sort of load - which is the fix.> Hah. I had reflexively used huge_pages=on - as that is the only sane thing to do with 10s to 100s of GB of shared memory and thus part of all my benchmarking infrastructure - during the benchmark runs mentioned above.> Turns out, if I disable huge pages, I actually can reproduce the contention that Salvatore reported (didn't see whether it's a regression for me though). Not anywhere close to the same degree, because the bottleneck for me is the writes.But, they can speak for themselves here [0].[0] https://news.ycombinator.com/item?id=47646332
Deeg9rie9usi: Once again phoronix shoot out an article without further researching nor letting the mail thread in question cool down. The follow up mails make clear that the issue is more or less a non-issue since the benchmark is wrong.
