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Linux 7.0 Broke PostgreSQL: The Preemption Regression Explained
- selckin 5mo agohttps://news.ycombinator.com/item?id=47644864 https://news.ycombinator.com/item?id=47644864
- neogodless 5mo agoExpanded: AWS engineer reports PostgreSQL perf halved by Linux 7.0, fix may not be easy (phoronix.com) ~24 days ago, 165+ comments
- PunchyHamster 5mo agoSeems Linus needs to yell at someone again. Especially with containers around you might very well hit the case of running new kernel but older version of PostgreSQL with no code mitigation for the problem
- nobleach 5mo agoI get that folks love a good Linus rant. But as someone who's been at the end of that style of "feedback", nothing can be more humiliating or demotivating. Certainly there are contributors that are making "rookie mistakes". There are folks that aren't willing to ingest the entire context of what was tried back in 2.0.36, 2.2, 2.4... etc. And perhaps it's wise to simply stay away until you're completely certain you've got the chops to contribute. More than half the folks that enjoy that sort of abuse don't have those chops. I can defend someone who is unwilling to yield on quality. Afterall, this truly is his baby. Issuing scathing rebukes to well-intentioned contributors is like slapping my kid when he brings me the wrong type of screwdriver.
- ecshafer 5mo agoI don't think a Linus rant ever hit anyone that was a rookie, they are always AFAIK against people "who should know better". Veteran developers, with multiple commits merged.
- colechristensen 5mo agoIf you're at the level of delivering to Linus, I'm sorry but humiliation and demotivation are earned. You don't talk like this to junior or even senior engineers, but you do reach a level at which gently telling isn't necessary. If you don't like it go fork Linux and try being the nice benevolent dictator and we'll applaud your success.
- themafia 5mo ago> scathing rebukes Would you be able to point one out? > to well-intentioned contributors This is a system used and relied upon by billions of people around the world. Your intentions, while good, are not material to the problem. Put another way we have an endless supply of people with "good intentions" but we don't enjoy the same largess of people with "good skills."
- vogelke 5mo agohttps://lwn.net/Articles/343828/ https://lwn.net/Articles/343828/ describes Alan Cox trying to fix the TTY layer, being trashed by Linus, and removing himself from the maintainer page.
- themafia 5mo agoI find it hard to call this a "scathing rebuke:" https://lkml.org/lkml/2009/7/28/373 https://lkml.org/lkml/2009/7/28/373 It also didn't just happen out of the blue. It's also true that Alan had already been working on the kernel for 15 years, was an employee of RedHat at the time, and his Wife's health was starting to fail. If you follow the thread it goes back and forth across quite a few messages with frustration building on both sides with Alan ultimately deciding to step away from a single (and very hairy) subsystem.
- nijave 5mo agoNot as scathing as some but recent https://lore.kernel.org/all/CAHk-=wiLdmz92CCfu2+-9_UrGSn6Pu6sUVvLs5gjyhGaEov-BA@mail.gmail.com/ https://lore.kernel.org/all/CAHk-=wiLdmz92CCfu2+-9_UrGSn6Pu6...
- slackfan 5mo agoCode quality does not care about your feelings.
- deleted 5mo ago[deleted]
- nijave 5mo agoNo, but code quality can suffer if you piss off all the competent people and they leave
- bonzini 5mo agoNope, there was and will be no yelling.
- panny 5mo agoThis was my first thought. How long was the Linus rant.
- baq 5mo agoTLDR of the LMKL thread: 120GB RAM postgres with hugepages=off, lock contention went from terrible to abysmal. nothing to see here except that amazon for whatever reason runs DB tests with huge pages disabled. (hope I'm not paying for RDS and auroras like that in production!)
- Twirrim 5mo agoHuge pages has had a spotty history, that lead to people being paranoid about it, and no doubt a whole bunch of folks just disable it "because that's what we've always done". It has been stable and reliable for quite a while now, would really hope folks could move away from that perspective.
- nijave 5mo agoI tested it once about 2 years ago on Azure VM and got a nice 10-15% perf boost on pgbench (I want to say at least 64GB shared mem)
- jeltz 5mo agoAre you sure you are not thinking of transparent huge pages? They have a spotty history but you are supposed to run big PostgreSQL instances with huge pages, not transparent huge pages.
- lstodd 5mo agoI remember when support for them just appeared and you had to LD_PRELOAD a shim IIRC to make Postgres actually use them we jumped on it, enabled them immediately and got a pretty significant boost, around 15-20%, yes. That was idk, 2008-9 -ish? I don't know what spotty history you are talking about, if you have multigigabyte address spaces floating on a machine it's stupid not to use hugepages.
- mplanchard 5mo agoAlso was only on ARM, wasn’t it?
- 5mo ago
- dataflow 5mo agoAn X% performance regression is basically a (100 - X)% feature breakage, so whatever that implies in terms of breaking userspace...
- nijave 5mo agoRight on the heels of 6.19 breaking tcmalloc and Mongo
- matharmin 5mo agoYup - interesting to see so much written about Postgres having a performance regression on Linux 7.0, in a scenario that affects almost no-one in practice. Meanwhile MongoDB refuses to run at all on Linux 7.0 due to some issue with tcmalloc. https://jira.mongodb.org/browse/SERVER-121885 https://jira.mongodb.org/browse/SERVER-121885
- duskwuff 5mo agoThe underlying tcmalloc issue is interesting - the library was relying on an implementation detail of the rseq kernel API which was never guaranteed, and which already generated warnings in previous versions. https://lore.kernel.org/all/20260126204745.GP171111@noisy.programming.kicks-ass.net/ https://lore.kernel.org/all/20260126204745.GP171111@noisy.pr...
- nijave 5mo agoImplemented behavior of the interface vs documented behavior of the interface I thought the warnings were only generated when you turned on a kernel config "that no one uses in practice"
- MBCook 5mo agoThis only happened under a very odd configuration. Yeah it wasn’t great but it was not the normal case. The headline implies it broke PG everywhere. It didn’t.
- buster 5mo agoI'd rather like to know if any real world usage broke, before coming to the conclusion that an edge case synthetic benchmark is worth changing the kernel (back or wherever) where supposedly the change that broke the benchmark had real world benefits. Since we will never know it might be a good idea to feature gate the change, change the default and let users decide to change it back. This may give some feedback on the lkml or else to decide if the change is worthwhile?
- nijave 5mo ago"synthetic benchmark" is doing some heavy lifting here. Pgbench just runs a bunch of SQL statements against a real Postgres instance. It's very close to a real world simulation of a production workload
- buster 5mo agoI am not questioning the benchmark. But the benchmark is NOT measuring a real world application in a real world setting. Anyway, I am merely wondering IF there is a company out there affected, at all. I understand that this was only measured on a graviton 4 setting with very heavy lifting, without huge tables. For example, this issue aside, I'd rather split such a workload into multiple smaller instances, naturally. Because the impact of a crash on this single node, heavy load, many cores, many clients scenario would be huge.
- ameliaquining 5mo agoThis post comes uncomfortably close to plagiarizing https://thebuild.com/blog/2026/04/23/preempt_none-is-dead-your-postgres-probably-doesnt-care/ https://thebuild.com/blog/2026/04/23/preempt_none-is-dead-yo..., which it cites as a source; almost all the technical explanation is in there and some of the wording is extremely similar. Compare, e.g., "What Linux 7.0 actually changed" in Pettus's post to "What Is Preemption?" in this one. I think this link should have been to Pettus's post instead.
- jdonaldson 5mo agoThat post comes uncomfortably close to how Opus writes this kind of prose. It's a good idea to acknowledge all stakeholders.
- teivah 5mo agoI used that post as a source yes, and it’s stated explicitly but it’s not the only one. One section in particular is similar since we both present the different preemption modes. However, both audiences are different. thebuild.com has an audience composed of PostgreSQL enthusiasts (if not experts) and I don’t. So a significant part of my post was about explaining things from first principles (what’s a page, a TLB, a spinlock, etc.). I explain way more “basic” things and he goes beyond me in terms of how to cope with the problem. I don’t think the posts are closed.
- galkk 5mo agoAfter your comment I went to original and it really looks like ai assisted rewrite with prompt like “give more explanations about basic concepts”…
- deleted 5mo ago[deleted]
- ozgrakkurt 5mo agoIt is a crime that postgres isn't able to allocate with 1GB huge pages by changing a config parameter in 2026 Also a crime that people are still running databases with 4kb pages. To put it in perspective, this means you will have more than 30 million pages on a server with 128GB RAM. As an example, if there is 16bytes of metadata for memory page. The metadata itself would take more than half a gigabyte.
- bonzini 5mo agoThere is 64 bytes of metadata per memory page indeed.
- dezgeg 5mo agoEven worse, the actual struct page on Linux is 64 bytes, so 4x your example
- andrewstuart 5mo agoSensible defaults would be nice.
- ldargin 5mo agoDatabase systems lock pages when writing to them, to maintain integrity. Using 1GB pages would cause excessive blocking in many if not most transactional databases.
- jlokier 5mo agoIn database engines that use page locks, the locked page size can be different from the file/mapped/allocated page sizes. If you still have excessive locking while using smaller page locks, there are other ways to reduce contention as well, such as CoW to protect concurrent reads, deferred write-merging to assist concurrent writes, and the storage equivalent of RCU.
- andrewf 5mo agoI don't think hardware page size has to match database page size. It would if Postgres was mmap'ing it. https://www.postgresql.org/docs/current/runtime-config-preset.html https://www.postgresql.org/docs/current/runtime-config-prese... says the database page size is 8KB by default.
- ApolloFortyNine 5mo agoI can't help but think of the classic XKCD example of breaking a user's workflow [1]. Doing research though a spinlock actually doesn't seem as unusual a hack as it would first seem, do drivers and the like not have similar issues because they don't trigger a page fault I guess? [1] https://xkcd.com/1172/ https://xkcd.com/1172/
- doubletwoyou 5mo agoFrom what I understand userspace spinlocks are particularly hazardous whereas in-kernel spinlocks are the norm
- ahartmetz 5mo agoPREEMPT_LAZY triggering on page faults seems like a bad idea in light of this. It is probably not a good idea to suspend processes right when they get unexpectedly bogged down. The logic makes a little more sense for syscalls that are expected to take long compared to a scheduling quantum (a few milliseconds). But page faults are mostly invisible and unplannable. It only took a few decades for Linux to get a good CPU scheduler and good I/O schedulers, too. I don't get how such an important area can be so bad for so long. But then, bad scheduling is everywhere. I find it to be a pretty fun area to work in, but, judging by how much it is less than half-assed in much existing software, most developers seem to hate dealing with it?
- bobmcnamara 5mo agoUserspace spinlocks seem like a risky idea too. What if it was on a VM and the core holding the lock got descheduled from the hypervisor?
- AlienRobot 5mo agoOne thing I miss from using Windows is that the desktop didn't just freeze completely if you ran out of RAM. At first I thought that maybe Linux doesn't have ways to give priority to the desktop environment (a.k.a. "graphical shell") which is why running out of RAM means your cursor starts lagging, clicking on things stops working, etc. But maybe Linux is just bad at that in general and a single process eating too much RAM can simply bring the whole system to a halt as it tries to move and compress RAM to a pagefile on an HDD (not SSD). Every time it happens to me I just find it so incredible. Here I am with a PC with a multiple cores, multiple processors, and a single process eating all the RAM can bottleneck ALL of them at once? Am I misunderstanding something? Shouldn't it, ideally, work in such way that so long as one processor is free, the system can process mouse input and render the cursor and do all the desktop stuff no matter what I/O is happening in the background? Since it's Linux maybe it's just my DE/distro (Cinnamon/Mint). Maybe it does allocations under the assumption there will always be a few free bytes in RAM available, so it halts if RAM runs out while some other DE wouldn't. But even then you'd think there would be a way to just reserve "premium" memory for critical processes so they never become unresponsive. I wonder if other people have the same experience as me. This part of Linux just always felt fundamentally poor for me.
- jeltz 5mo agoModerators should change this headline because it is nowhere near true. It only regressed performance on some incorrect configurations.
- nijave 5mo agoWhat is considered incorrect? Edit: It may not be optimal or recommended config but I was under the impression it's very close to default config. As far as I know, most popular distros are shipping with no hugepage pool reservation and shared memory transparent hugepages disabled.
- fulafel 5mo ago> PREEMPT_NONE: The kernel almost never interrupts a running thread This seems confused. These are options for preemptibility of the kernel, which is a relatively modern fearure. Userspace could always be preempted and these options do not change anything there. The kernel must in any case frequently interrupt threads and processes to implement preemptive multitasking which Linux of course had since the beginning. Read more eg at https://lwn.net/Articles/944686/ https://lwn.net/Articles/944686/ or help texts at https://github.com/torvalds/linux/blob/master/kernel/Kconfig.preempt https://github.com/torvalds/linux/blob/master/kernel/Kconfig...
- fabian2k 5mo agoThat regression is maybe most useful as a reminder to people to configure huge pages for PostgreSQL. That's the one recommended basic performance tuning that is just annoying enough to set up that I suspect many people with smaller DBs will skip it. Though I actually don't know how large shared buffers has to be for huge pages to make a noticeable difference.
- cachius 5mo agoThe last time a linux upgrade broke PG was the xz backdoor.
- singron 5mo agoThis has the wrong explanation of the proposed rseq (Restartable Sequences) solution. > a Linux kernel facility that lets userspace code detect whether it was preempted or migrated during a critical section and restart it if so. PostgreSQL's spinlock paths would use rseq to detect preemption and retry, avoiding the scenario where a preempted lock holder stalls all waiting backends. The real proposal is about time-slice extension, which is a feature that uses the abi for rseq but otherwise has nothing to do with retrying critical sections. While a process holds a s_lock, it would set a request bit. If the kernel tries to preempt that thread while the request bit is set, it instead extends the time slice once and returns control back to the thread. It's further explained here: https://docs.kernel.org/userspace-api/rseq.html https://docs.kernel.org/userspace-api/rseq.html
- nubinetwork 5mo agoThey got rid of PREEMPT_NONE? Just a while ago they got rid of slab, and the noop io scheduler. Why do they insist on removing features that don't make sense on a desktop according to some random bozo? Not everyone is running a dyntick laptop.