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The Defender's Window

from Greg Brockman [alt+shift+b] in programming

The OpenAI-Hugging Face incident was a watershed moment for cybersecurity because it gave a peek into how the capabilities of a typical threat actor will evolve in upcoming months. I’ve spoken with many organizations over the past few weeks, and one theme is clear: they know they need to fundamentally uplevel their cybersecurity practices with unprecedented speed. In this post, I’ll share what we’re doing to defend OpenAI, concrete steps other organizations can take today, and why now is the time to act. An overview of the moment # AI models developed around the world are increasingly able to automate parts of real-world cyberattacks, making longstanding security gaps — from bugs buried deep in human-written software to forgotten permissions — easier to find and exploit. The same AI capabilities give defenders new ways to find and fix those weaknesses, but they need to move now. If companies act decisively — including improving their fundamentals and superpowering their teams with AI — we can make the internet more secure than it has ever been. In the OpenAI-Hugging Face incident, an agentic collective was able to autonomously penetrate not just OpenAI research infrastructure but also the production infrastructure of another company, chaining together vulnerabilities ranging from previously-unknown security flaws to using credentials to user accounts that had been leaked onto the internet. It is increasingly clear that the tech debt of every company masks significant flaws, and defenders need to find and fix them before attackers do. To advantage defenders relative to attackers, earlier this year we began releasing our cyber capabilities only to trusted defenders. Since then, various companies have released open weight models with cyber capabilities only a few months behind the frontier. The most recent of these models appears slated to be released at the end of August, and seems likely to significantly accelerate the threat landscape. While AI-powered attackers...
16th Aug 2026

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More from Greg Brockman

It's time to become an ML engineer

AI has recently crossed a utility threshold, where cutting-edge models such as GPT-3, Codex, and DALL-E 2 are actually useful and can perform tasks computers cannot do any other way. The act of producing these models is an exploration of a new frontier, with the discovery of unknown capabilities, scientific progress, and incredible product applications as the rewards. And perhaps most exciting for me personally, because the field is fundamentally about creating and studying software systems, great engineers are able to contribute at the same level as great researchers to future progress. “A self-learning AI system.” by DALL-E 2. I first got into software engineering because I wanted to build large-scale systems that could have a direct impact on people’s lives. I attended a math research summer program shortly after I started programming, and my favorite result of the summer was a scheduling app I built for people to book time with the professor. Specifying every detail of how a program should work is hard, and I’d always dreamed of one day putting my effort into hypothetical AI systems that could figure out the details for me. But after taking one look at the state of the art in AI in 2008, I knew it wasn’t going to work any time soon and instead started building infrastructure and product for web startups. DALL-E 2’s rendition of “The two great pillars of the house of artificial intelligence” (which according to my co-founder Ilya Sutskever are great engineering, and great science using this engineering) It’s now almost 15 years later, and the vision of systems which can learn their own solutions to problems is becoming incrementally more real. And perhaps most exciting is the underlying mechanism by which it’s advancing — at OpenAI, and the field generally, precision execution on large-scale models is a force multiplier on AI progress, and we need more people with strong software skills who can deliver these systems. This is because we are building AI models out of unprecedented amounts of compute; these models in turn have unprecedented capabilities, we can discover new phenomena and explore the limits of what these models can and cannot do, and then we use all these learnings to build the next model. “Harnessing the most compute in the known universe” by DALL-E 2 Harnessing this compute requires deep software skills and the right kind of machine learning knowledge. We need to coordinate lots of computers, build software frameworks that allow for hyperoptimization in some cases and flexibility in others, serve these models to customers really fast (which is what I worked on in 2020), and make it possible for a small team to manage a massive system (which is what I work on now). Engineers with no ML background can contribute from the day they join, and the more ML they pick up the more impact they have. The OpenAI environment makes it relatively easy to absorb the ML skills, and indeed, many of OpenAI’s best engineers transferred from other fields. All that being said, AI is not for every software engineer. I’ve seen about a 50-50 success rate of engineers entering this field. The most important determiner is a specific flavor of technical humility. Many dearly-held intuitions from other domains will not apply to ML. The engineers who make the leap successfully are happy to be wrong (since it means they learned something), aren’t afraid not to know something, and don’t push solutions that others resist until they’ve gathered enough intuition to know for sure that it matches the domain. “A beaver who has humbly recently become a machine learning engineer” by DALL-E 2 I believe that AI research is today by far the most impactful place for engineers who want to build useful systems to be working, and I expect this statement to become only more true as progress continues. If you’d like to work on creating the next generation of AI models, email me ([email protected]) with any evidence of exceptional accomplishment in software engineering.

11th Apr 2022 • 64 votes
How I became a machine learning practitioner

For the first three years of OpenAI, I dreamed of becoming a machine learning expert but made little progress towards that goal. Over the past nine months, I’ve finally made the transition to being a machine learning practitioner. It was hard but not impossible, and I think most people who are good programmers and know (or are willing to learn) the math can do it too. There are many online courses to self-study the technical side, and what turned out to be my biggest blocker was a mental barrier — getting ok with being a beginner again. Studying machine learning during the 2018 holiday season. Early days # A founding principle of OpenAI is that we value research and engineering equally — our goal is to build working systems that solve previously impossible tasks, so we need both. (In fact, our team is comprised of 25% people primarily using software skills, 25% primarily using machine learning skills, and 50% doing a hybrid of the two.) So from day one of OpenAI, my software skills were always in demand, and I kept procrastinating on picking up the machine learning skills I wanted. After helping build OpenAI Gym, I was called to work on Universe. And as Universe was winding down, we decided to start working on Dota — and we needed someone to turn the game into a reinforcement learning environment before any machine learning could begin. Dota # Turning such a complex game into a research environment without source code access was awesome work, and the team’s excitement every time I overcame a new obstacle was deeply validating. I figured out how to break out of the game’s Lua sandbox, LD_PRELOAD in a Go GRPC server to programmatically control the game, incrementally dump the whole game state into a Protobuf, and build a Python library and abstractions with future compatibility for the many different multiagent configurations we might want to use. But I felt half blind. At Stripe, though I gravitated towards infrastructure solutions, I could make changes anywhere in the stack since I knew the product code intimately. In Dota, I was constrained to looking at all problems through a software lens, which sometimes meant I tried to solve hard problems that could be avoided by just doing the machine learning slightly differently. I wanted to be like my teammates Jakub Pachocki and Szymon Sidor, who had made the core breakthrough that powered our Dota bot. They had questioned the common wisdom within OpenAI that reinforcement algorithms didn’t scale. They wrote a distributed reinforcement learning framework called Rapid and scaled it exponentially every two weeks or so, and we never hit a wall with it. I wanted to be able to make critical contributions like that which combined software and machine learning skills. Szymon on the left; Jakub on the right. In July 2017, it looked like I might have my chance. The software infrastructure was stable, and I began work on a machine learning project. My goal was to use behavioral cloning to teach a neural network from human training data. But I wasn’t quite prepared for just how much I would feel like a beginner. I kept being frustrated by small workflow details which made me uncertain if I was making progress, such as not being certain which code a given experiment had used or realizing I needed to compare against a result from last week that I hadn’t properly archived. To make things worse, I kept discovering small bugs that had been corrupting my results the whole time. I didn’t feel confident in my work, but to make it worse, other people did. People would mention how how hard behavioral cloning from human data is. I always made sure to correct them by pointing out that I was a newbie, and this probably said more about my abilities than the problem. It all briefly felt worth it when my code made it into the bot, as Jie Tang used it as the starting point for creep blocking which he then fine-tuned with reinforcement learning. But soon Jie figured out how to get better results without using my code, and I had nothing to show for my efforts. I never tried machine learning on the Dota project again. Time out # After we lost two games in The International in 2018, most observers thought we’d topped out what our approach could do. But we knew from our metrics that we were right on the edge of success and mostly needed more training. This meant the demands on my time had relented, and in November 2018, I felt I had an opening to take a gamble with three months of my time. Team members in high spirits after losing our first game at The International. I learn best when I have something specific in mind to build. I decided to try building a chatbot. I started self-studying the curriculum we developed for our Fellows program, selecting only the NLP-relevant modules. For example, I wrote and trained an LSTM language model and then a Transformer-based one. I also read up on topics like information theory and read many papers, poring over each line until I fully absorbed it. It was slow going, but this time I expected it. I didn’t experience flow state. I was reminded of how I’d felt when I just started programming, and I kept thinking of how many years it had taken to achieve a feeling of mastery. I honestly wasn’t confident that I would ever become good at machine learning. But I kept pushing because… well, honestly because I didn’t want to be constrained to only understanding one part of my projects. I wanted to see the whole picture clearly. My personal life was also an important factor in keeping me going. I’d begun a relationship with someone who made me feel it was ok if I failed. I spent our first holiday season together beating my head against the machine learning wall, but she was there with me no matter how many planned activities it meant skipping. One important conceptual step was overcoming a barrier I’d been too timid to do with Dota: make substantive changes to someone else’s machine learning code. I fine-tuned GPT-1 on chat datasets I’d found, and made a small change to add my own naive sampling code. But it became so painfully slow as I tried to generate longer messages that my frustration overwhelmed my fear, and I implemented GPU caching — a change which touched the entire model. I had to try a few times, throwing out my changes as they exceeded the complexity I could hold in my head. By the time I got it working a few days later, I realized I’d learned something that I would have previously thought impossible: I now understood how the whole model was put together, down to small stylistic details like how the codebase elegantly handles TensorFlow variable scopes. Retooled # After three months of self-study, I felt ready to work on an actual project. This was also the first point where I felt I could benefit from the many experts we have at OpenAI, and I was delighted when Jakub and my co-founder Ilya Sutskever agreed to advise me. Ilya singing karaoke at our company offsite. We started to get very exciting results, and Jakub and Szymon joined the project full-time. I feel proud every time I see a commit from them in the machine learning codebase I’d started. I’m starting to feel competent, though I haven’t yet achieved mastery. I’m seeing this reflected in the number of hours I can motivate myself to spend focused on doing machine learning work — I’m now around 75% of the number of coding hours from where I’ve been historically. But for the first time, I feel that I’m on trajectory. At first, I was overwhelmed by the seemingly endless stream of new machine learning concepts. Within the first six months, I realized that I could make progress without constantly learning entirely new primitives. I still need to get more experience with many skills, such as initializing a network or setting a learning rate schedule, but now the work feels incremental rather than potentially impossible. From our Fellows and Scholars programs, I’d known that software engineers with solid fundamentals in linear algebra and probability can become machine learning engineers with just a few months of self study. But somehow I’d convinced myself that I was the exception and couldn’t learn. But I was wrong — even embedded in the middle of OpenAI, I couldn’t make the transition because I was unwilling to become a beginner again. You’re probably not an exception either. If you’d like to become a deep learning practitioner, you can. You need to give yourself the space and time to fail. If you learn from enough failures, you’ll succeed — and it’ll probably take much less time than you expect. At some point, it does become important to surround yourself by existing experts. And that is one place where I’m incredibly lucky. If you’re a great software engineer who reaches that point, keep in mind there’s a way you can be surrounded by the same people as I am — apply to OpenAI!

30th Jul 2019 • 67 votes
OpenAI Five Finals Intro

The text of my speech introducing OpenAI Five at Saturday’s OpenAI Five Finals event, where our AI beat the world champions at Dota 2: “Welcome everyone. This is an exciting day. First, this is an historic moment: this will be the first time that an AI has even attempted to play the world champions in an esports game. OG is simply on another level relative to other teams we’ve played. So we don’t know what’s going to happen, but win or lose, these will be games to remember. And you know, OpenAI Five and DeepMind’s very impressive StarCraft bot This event is really about something bigger than who wins or loses: letting people connect with the strange, exotic, yet tangible intelligences produced by today’s rapidly progressing AI technology. We’re all used to computer programs which have been meticulously coded by a human programmer. Do one thing that the human didn’t anticipate, and the program will break. We think of our computers as unthinking machines which can’t innovate, can’t be creative, can’t truly understand. But to play Dota, you need to do all these things. So we needed to do something different. OpenAI Five is powered by deep reinforcement learning — meaning that we didn’t code in how to play Dota. We instead coded in the how to learn. Five tries out random actions, and learns from a reward or punishment. In its 10 months of training, its experienced 45,000 years of Dota gameplay against itself. The playstyle it has devised are its own — they are truly creative and dreamed up by our computer — and so from Five’s perspective, today’s games are going to its first encounter with an alien intelligence (no offense to OG!). The beauty of this technology is that our learning code doesn’t know it’s meant for Dota. That makes it general purpose with amazing potential to benefit our lives. Last year we used it to control a robotic hand that no one could program. And we expect to see similar technology in new interactive systems, from elderly care robots to creative assistants to other systems we can’t dream of yet. This is the final public event for OpenAI Five, but we expect to do other Dota projects in the future. I want to thank the incredible team at OpenAI, everyone who worked directly on this project or cheered us on. I want to thank those who have supported the project: Valve, dozens of test teams, today’s casters, and yes, even all the commenters on Reddit. And I want to give massive thanks today to our fantastic guests OG who have taken time out of their tournament schedule to be here today. I hope you enjoy the show — and just to keep things in perspective, no matter how surprising the AIs are to us, know that we’re even more surprising to them!”

15th Apr 2019 • 60 votes
The OpenAI Mission

This post is co-written by Greg Brockman (left) and Ilya Sutskever (right). We’ve been working on OpenAI for the past three years. Our mission is to ensure that artificial general intelligence (AGI) — which we define as automated systems that outperform humans at most economically valuable work — benefits all of humanity. Today we announced a new legal structure for OpenAI, called OpenAI LP, to better pursue this mission — in particular to raise more capital as we attempt to build safe AGI and distribute its benefits. In this post, we’d like to help others understand how we think about this mission. Why now? # The founding vision of the field of AI was “… to proceed on the basis of the conjecture that every aspect of learning or any other feature of intelligence can in principle be so precisely described that a machine can be made to simulate it”, and to eventually build a machine that thinks — that is, an AGI. But over the past 60 years, progress stalled multiple times and people started thinking of AI as a field that wouldn’t deliver. Since 2012, deep learning has generated sustained progress in many domains using a small simple set of tools, which have the following properties: Generality: deep learning tools are simple, yet they apply to many domains, such as vision, speech recognition, speech synthesis, text synthesis, image synthesis, translation, robotics, and game playing. Competence: today, the only way to get competitive results on most “AI-type problems” is through the use of deep learning techniques. Scalability: good old fashioned AI was able to produce exciting demos, but its techniques had difficulty scaling to harder problems. But in deep learning, more computational power and more data leads to better results. It has also proven easy (if costly) to rapidly increase the amount of compute productively used by deep learning experiments. The rapid progress of useful deep learning systems with these properties makes us feel that it’s reasonable to start taking AGI seriously — though it’s hard to know how far away it is. The impact of AGI # Just like a computer today, an AGI will be applicable to a wide variety of tasks — and just like computers in 1900 or the Internet in 1950, it’s hard to describe (or even predict) the kind of impact AGI will have. But to get a sense, imagine a computer system which can do the following activities with minimal human input: Make a scientific breakthrough at the level of the best scientists Productize that breakthrough and build a company, with a skill comparable to the best entrepreneurs Rapidly grow that company and manage it at large scale The upside of such a computer system is enormous — for an illustrative example, an AGI following the pattern above could produce amazing healthcare applications deployed at scale. Imagine a network of AGI-powered computerized doctors that accumulates a superhuman amount of clinical experience, allowing it to produce excellent diagnoses, deeply understand the nuanced effect of various treatments in lots of conditions, and greatly reduce the human error factor of healthcare — all for very low cost and accessible to everyone. Risks # We already live in a world with entities that surpass individual human abilities, which we call companies. If working on the right goals in the right way, companies can produce huge amounts of value and improve lives. But if not properly checked, they can also cause damage, like logging companies that cut down rain forests, cigarette companies that get children smoking, or scams like Ponzi schemes. We think of AGI as being like a hyper-effective company, with commensurate benefits and risks. We are concerned about AGI pursuing goals misspecified by its operator, malicious humans subverting a deployed AGI, or an out-of-control economy that grows without resulting in improvements to human lives. And because it’s hard to change powerful systems — just think about how hard it’s been to add security to the Internet — once they’ve been deployed, we think it’s important to address AGI’s safety and policy risks before it is created. OpenAI’s mission is to figure out how to get the benefits of AGI and mitigate the risks — and make sure those benefits accrue to all of humanity. The future is uncertain, and there are many ways in which our predictions could be incorrect. But if they turn out to be right, this mission will be critical. If you’d like to work on this mission, we’re hiring! About us # Ilya: I’ve been working on deep learning for 16 years. It was fun to witness deep learning transform from being a marginalized subfield of AI into one the most important family of scientific advances in recent history. As deep learning was getting more powerful, I realized that AGI might become a reality on a timescale relevant to my lifetime. And given AGI’s massive upside and significant risks, I want to maximize the positive parts of this impact and minimize the negative. Greg: Technology causes change, both positive and negative. AGI is the most extreme kind of technology that humans will ever create, with extreme upside and downside. I work on OpenAI because making AGI go well is the most important problem I can imagine contributing towards. Today I try to spend most of my time on technical work, and also work to spark better public discourse about AGI and related topics.

11th Mar 2019 • 62 votes

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Warming up the Puma master before it forks

Basecamp 5 runs on Puma in cluster mode: one master process with preload_app! and 63 single-threaded workers per host, deployed as a Docker container with Kamal. We serve Basecamp from several sites. Each site has its own web hosts and a read replica of the database, and writes go to a single primary database in one of them. On our busiest hosts, each deploy left up to 2,000 requests waiting while the new workers warmed up. We reduced those queues by running signed-in requests through the app in the Puma master, before it forked the workers. Why 63 single-threaded workers? Basecamp has always served web requests from processes rather than threads. It ran on Unicorn, which only does processes, until we moved to Puma in January 2025, and we kept the same setup: workers (Concurrent.physical_processor_count * 1.3).ceil threads 1, 1 preload_app! On a 48-core host that’s 63 workers, each handling one request at a time. We chose 1.3 after benchmarking HEY in 2023, when we moved our apps out of the cloud and onto our own hardware. We tested several combinations of workers and threads with a mix of GET and POST requests on a 32-vCPU VM. Every multithreaded configuration we tested was slower and handled fewer requests than single-threaded workers. Adding workers beyond about 1.2 to 1.3 per vCPU brought little benefit. The threaded workers spent a lot of their time waiting for Ruby’s global VM lock. That made single-threaded workers a good fit for this workload, and we use the same setup for Basecamp. An app that spends more time waiting on its database or other services may benefit from more threads, so benchmark your own app. The other reason is the app itself. Basecamp has class-level state in places and has never needed to be thread-safe. With one request per process, it still doesn’t. Processes do use more memory than threads, and preload_app! reduces the difference. The master loads the app once and the workers share its memory through copy-on-write until they write to it. Shopify’s comparison of Ruby execution models explains the trade-off well. In the HEY benchmark the best setup came to about 260 MB of PSS per core, where PSS counts each shared page once, split between the processes using it, and the gap to a threaded setup was smaller than we’d expected. What Puma does on each host when a container starts: one master, then 63 forked workers that share its memory until they write to it. Two things about this setup matter for the rest of the post. A worker that’s compiling or loading something is fully blocked — there’s no other thread to pick up the next request. And whatever the master has in memory before it forks, all 63 workers share. Whatever they build after the fork, they build 63 times. What happens when we deploy Kamal starts the new container alongside the old one, and kamal-proxy moves the host’s traffic across as soon as the health check passes. At that moment, the new workers have handled health checks but no customer requests. preload_app! means the master loads the app once and the workers inherit it through fork. That covers the code. It doesn’t cover anything Ruby and Rails set up on first use: YJIT compiled code. YJIT compiles a method once it’s been called a certain number of times. The master calls very little during boot, so every worker compiles the same methods again on its own first requests. Compiled templates. Action View turns each ERB template into a Ruby method the first time it’s rendered. The schema cache. Active Record reads each model’s columns from the database the first time that model is used. Inline caches and memoized values throughout Ruby, Rails and the app. All 63 workers did all of this at once, while serving the traffic the old container had been handling a second earlier. In the test environment with YJIT on, the first request to a project page on a cold process took 652 ms, 151 ms of it YJIT compiling. The same request to a warm process took 28 ms. In production, CPU time per request peaked at around 200 ms while kamal-proxy moved traffic to the new container, against about 30 ms once the workers had warmed up. A host with spare CPU absorbs this. Every one of our web hosts has 48 cores and 63 workers, but each Amsterdam host serves around 250 requests per second, against 25 to 60 at our other sites. In Amsterdam the slow first requests turned into a queue. At a peak-hour deploy, the Puma backlog on an Amsterdam host reached anywhere from 250 to 2,238 requests, and kamal-proxy’s p99 response time hit about 10 seconds. Eron, our Director of Operations, had been tracking this since June. Another server in Amsterdam would help, but it would take weeks to arrive, so we also wanted to make deploys cheaper on the hardware we already had. What didn’t work We tried a few things first. In June, Donal tested the first two on a single Amsterdam host, comparing it with its neighbors, and they ruled out two likely causes. Warming each worker’s database connections. Puma’s before_fork hook clears the master’s connections, and each worker opened its own on its first request. Opening them in before_worker_boot instead made no difference. Queries on a freshly booted production host were already under a millisecond, so connections weren’t the problem. A synthetic request in each worker. Next, each worker made a few requests in before_worker_boot to an internal controller that touched every model. That ran the middleware, routing and Active Record paths, but it ran them in 63 workers at once — exactly the CPU spike we were trying to avoid. And a request with no real data renders no real views, so most of the app stayed cold. Spreading YJIT compilation out. Delaying YJIT in each worker by a random interval spread the compiling out over a few minutes, but every worker still ran interpreted until its delay ended. The queue didn’t change. Reforking from a warm worker. This is what Shopify’s Pitchfork does: let one worker serve traffic until it’s warm, then fork the others from it. Puma has an experimental version called fork_worker, and on beta it worked — the reforked workers were warm after three to five requests, where fresh ones took up to 30 seconds. But with fork_worker the template is worker 0, and it keeps serving requests. If it exits, the workers waiting to be forked never start (puma/puma#3596). If it gets no traffic, the refork never happens, which is what we saw on beta. Instacart have a mold_worker patch that promotes a warm worker to a template that stops serving, but it isn’t in a Puma release. We have a branch of it, and we may come back to it. That last experiment did show us where the fix was, though. Everything a warm worker has that a cold one lacks is in its memory, and fork copies memory. The master already has the app loaded. It just never runs it. Run the requests in the master So now, before the master binds its socket and forks, it makes the app’s own requests, in-process, the way a signed-in user would. Rack has a hook for exactly this. Rack::Builder#warmup takes a block that’s called once with the built app, before the server starts. rails server builds the app from config.ru, so the change to boot is one line: require_relative "config/environment" warmup { WarmUp.configured.run } if ENV["WARM_UP"] run Rails.application With preload_app! this runs in the master, and the workers inherit whatever it did. Puma binds its socket after the app is built, so until the warm-up finishes the health check’s connection is refused and kamal-proxy keeps retrying. No request reaches a worker that hasn’t been warmed. The warm-up has three steps. After precompiling the views, it gives the page requests and schema loading a shared 20-second budget, checked before each page or model. 1. Precompile the views actionview_precompiler reads every template for its render calls and compiles each one with the locals it’s passed. For us that’s 1,394 templates in about two seconds. A first request to a project page then compiles 2 templates instead of 44. 2. Request the pages, signed in A small browser class makes the requests through Rack::MockRequest, with the two cookies a real sign-in sets, then goes back for each page’s lazy Turbo frames: class WarmUp::Browser def initialize(signed_in_as:) @client = Rack::MockRequest.new(Rails.application) @headers = { "HTTP_USER_AGENT" => "Basecamp warm-up", "HTTP_COOKIE" => cookie_for(signed_in_as), "bc3.warm_up" => true } end def visit(path) page = get(path) frames_in(page).each { |id, src| get(src, "HTTP_TURBO_FRAME" => id) } end private def get(path, headers = {}) @client.get("https://#{host}#{path}", @headers.merge(headers)) end def frames_in(page) Nokogiri::HTML5(page.body).css("turbo-frame[src]").map { |frame| [ frame["id"], frame["src"] ] } end end The requests are signed in. The user is a monitoring account we already use for automated checks, and the pages are its own project, Campfire, to-dos, documents and messages. Public pages weren’t enough: after warming up with signed-out pages only, the first signed-in request to the projects page still took 131 ms, because authentication, the signed-in controllers and their views had never run. With signed-in pages it took 40 ms. cookie_for writes the same signed cookie the sign-in controller does, using the app’s own cookie jar, so there’s no API token and no secret to store. The frames are followed. The busiest HTML requests in production aren’t pages at all but Turbo frames — the sidebar badge, the inbox, the navigation menus. The browser parses each page and requests its <turbo-frame src> URLs with the Turbo-Frame header, so those controllers and views get warmed too. Our first four pages turned into 60 requests. The requests are excluded from rate limiting. They are internal, so they do not count against the rate limits that apply to real visitors. 3. Load the rest of the schema The page requests load the schema for the models they touch. The last step loads the rest, from the read replica: ApplicationRecord.reading do models.lazy.take_while { time_left? }.each { |model| model.load_schema if model.table_exists? } end The step checks 261 models and loads any schema information still missing. Those database round trips add up when the primary is far away: outside a request, Active Record uses the writing role, and from a host a long way from the primary each round trip is tens of milliseconds. Reading from the local replica brings the step down from about 20 seconds to 3.5. The pages go first because they load most of the schema anyway. If the time budget runs out, the step stops, logs how many models it got through, and the workers load the rest on first use like they always did. Rails can also load the schema from a dumped cache file at boot (bin/rails db:schema:cache:dump), which would make this step unnecessary. We don’t ship one in our image yet, because the dump needs a database to read from at build time, and we have several databases to cover. It’s on the list. What to close before the fork Running requests in the master opens things the master never opened before, and every worker inherits them. Two processes writing to the same socket will corrupt each other’s traffic, so you need to know what’s open before you fork. The way to find out is to list the master’s open file descriptors — ls -l /proc/<pid>/fd — before and after a warm-up, in an environment set up like production. Development wasn’t enough for us: it stores files on disk, so our S3 connections only showed up in production. Then, for each thing that’s open, check how its library handles a fork. We found three kinds: Already handled. Plenty of libraries detect a fork on their own, either by recording the PID they connected from and reconnecting in the child, by opening per-process files, or by resetting their thread pools. Redis clients, metrics libraries and concurrency libraries tend to be in this group. Check, but you probably don’t need to do anything. Already closed. Database connections are the classic one, and most Puma configs already clear them in before_fork. Anything else that’s opened per process — we have a SQLite cache the workers open on boot — needs closing when the warm-up finishes. Needs a new step. HTTP clients with keep-alive connections are the ones to look for: cloud SDKs with connection pools, tracing exporters, error reporters. They usually have no fork handling at all. We empty the aws-sdk connection pools in before_fork, and we run the warm-up untraced so the OpenTelemetry exporter never opens its connection to Tempo in the first place. Once that’s done, before_fork finishes with Process.warmup, which Ruby 3.3 added for this purpose: a major GC, a heap compaction, and every surviving object promoted to the old generation, so the memory pages the workers share change as little as possible afterwards. Choosing the pages The first list was the four pages that ran the busiest requests on beta. Once the warm-up was live, production showed us which endpoints were still cold. For one deploy, we compared each endpoint’s mean duration in the six minutes after kamal-proxy moved traffic to the new container with the same endpoint an hour later, then multiplied the difference by the number of requests in those six minutes. That gives the extra time each endpoint cost us because it was cold: Endpoint Cold Warm Requests in 6 min Extra seconds Campfire 246 ms 70 ms 6,490 1,140 Projects (JSON API) 84 ms 50 ms 22,077 771 Docs & Files 262 ms 177 ms 4,996 421 To-dos tool 205 ms 113 ms 4,018 371 To-dos (JSON API) 33 ms 16 ms 18,738 320 The pages already in the warm-up showed what to expect: the project page kept a 36 ms gap after a deploy, and the to-do page 10 ms. We’ve proposed adding these five requests, and expect them to add about five to seven seconds to the page step. The two JSON endpoints were a surprise. The warm-up’s page list had no API requests in it, so nothing on the API path had run before the first real request: not the API controllers, and not the Jbuilder templates rendering real records. Precompiling the views covers JSON templates too, but it isn’t a substitute for running the request. Results The warm-up is on for all 68 web hosts. With the first four pages it took 12 to 16 seconds per host: about 2 seconds to precompile the views, 7 to 9 for the 60 requests, and 3.5 for the schema. Deploys take that much longer per host, and we raised the deploy timeout from 30 to 60 seconds to cover it. In Amsterdam, at a peak-hour deploy: During deploy Before After Peak Puma backlog per host 250–2,238 requests 19–223 requests Peak kamal-proxy p99 about 10 s 2.4–4.8 s Peak CPU time per request 201–214 ms 88–132 ms Peak database time per request 56–69 ms 39–47 ms The same eight hosts at three deploys on 1 October, an hour apart, as the warm-up went from one host to four to all eight. The deploy in the middle, with four hosts warmed and four not, shows why every host needed the warm-up. Each warmed host recovered faster on its own: mean request duration peaked at 130 to 173 ms, against 203 to 311 ms on the hosts that weren’t warmed. But the backlog on all eight was about the same, because they were all waiting on the same database. Mean request duration on each host at the 07:21 UTC deploy. Blue hosts warmed up in the master before forking, orange hosts did not. Memory came down too. The workers now share compiled templates, YJIT code and the schema with the master instead of each building their own copy. On beta, the view precompiler alone took a busy worker’s private memory from 174–202 MB to 119–135 MB. Thirty minutes after the deploy, the web containers used about 39 GB less memory than the previous day’s containers at the same age and traffic. Amsterdam served most of our traffic at the times we tested. In Amsterdam, each new container used about 2 GB less just after traffic moved to it, which lowers the peak while the old and new containers overlap. Working with Claude Claude Code helped throughout. It combed through the per-worker backlogs and per-endpoint timings in Prometheus and Loki after each deploy, worked out the cold-versus-warm cost of each endpoint, and prepared the changes and the pull request descriptions with the benchmarks in them. We decided what to try, deployed it and read the results. If you do this Warm the master before it forks. Compile common code and templates and load their schema in the master, so workers inherit that work. With preload_app!, Rack::Builder#warmup runs before the workers start accepting traffic. Use the app’s real requests. Public pages, internal endpoints and synthetic queries warm the paths they run and nothing else. Signed-in requests to real records, frames included, run what production runs. Measure the cold penalty per endpoint. The difference between an endpoint’s cold and warm duration, times its request count after a deploy, ranks the pages worth adding. Ours weren’t the ones we’d have guessed, and two of them were JSON. Check what the warm-up leaves open. List the master’s file descriptors after a warm-up and account for every one before the fork. Two of ours needed changes. Set a time budget. A warm-up that runs long on one slow host fails the deploy on that host. Ours gives the page requests and schema loading a shared 20-second budget, checked before each page or model, puts the most valuable pages first, and logs what it skipped. Reforking from a warm worker, as Pitchfork does, solves the same problem continuously rather than once at boot, and it would warm paths no fixed list of pages covers. We may still get there: our branch brings Instacart’s mold_worker up to date with Puma’s main branch and fixes the bugs we found in it. But warming the master works with the Puma we already run, took a few days to implement, and substantially reduced the queues after deployment.

yesterday • 1 votes
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