How should we think about error-handling in concurrent programs? In single-threaded programs, we’ve mostly converged on a standard pattern, with a diverse zoo of implementations and concrete patterns. When an error occurs, it is propagated up the stack until we find a stack frame which is prepared t...

For a while now, I’ve been fascinated by Z3 and by SMT solving more broadly. While on pat leave recently, I was reminded of the existence of regular-expression crossword puzzles, and allowed myself to get nerdsniped by writing a Z3-backed solver. I expected to spend perhaps an afternoon cranking out...

While investigating the performance of the new Python 3.14 tail-calling interpreter, I learned (via this very informative comment from Sam Gross) new (to me) piece of performance trivia: Modern CPUs mostly no longer struggle to predict the bytecode-dispatch indirect jump inside a “conventional” byte...

About a month ago, the CPython project merged a new implementation strategy for their bytecode interpreter. The initial headline results were very impressive, showing a 10-15% performance improvement on average across a wide range of benchmarks across a variety of platforms. Unfortunately, as I will...

Earlier this month, I used Claude to port (parts of) an Emacs package into Rust, shrinking the execution time by a factor of 1000 or more (in one concrete case: from 90s to about 15ms). This is a variety of yak-shave that I do somewhat routinely, both professionally and in service of my personal com...

Suppose we have a large collection of documents, and we wish you identify which documents are approximately the same as each other. For instance, we may have crawled the web over some period of time, and expect to have fetched the “same page” several times, but to see slight differences in metadata,...

I worked at Stripe for about seven years, from 2012 to 2019. Over that time, I used and contributed to many generations of Stripe’s developer environment – the tools that engineers used daily to write and test code. I think Stripe did a pretty good job designing and building that developer experienc...

I was recently introduced to the paper “Seeing the Invisible: Perceptual-Cognitive Aspects of Expertise” by Gary Klein and Robert Hoffman. It’s excellent and I recommend you read it when you have a chance. Klein and Hoffman discuss the ability of experts to “see what is not there”: in addition to ob...

This December, the imp of the perverse struck me, and I decided to see how many days of Advent of Code I could do purely in compile-time C++ metaprogramming. As of this writing, I’ve done two days, and I’m not sure I’ll make it any further. However, that’s one more day than I planned to do as of yes...

I have spent many years as an software engineer who was a total outsider to machine-learning, but with some curiosity and occasional peripheral interactions with it. During this time, a recurring theme for me was horror (and, to be honest, disdain) every time I encountered the widespread usage of Py...

Suppose we’ve got a service. We’ll gloss over the details for now, but let’s stipulate that it accepts requests from the outside world, and takes some action in response. Maybe those requests are HTTP requests, or RPCs, or just incoming packets to be routed at the network layer. We can get more spec...

What’s the “right” level of CPU utilization for a server? If you look at a monitoring dashboard from a well-designed and well-run service, what CPU utilization should we hope to see, averaged over a day or two? It’s a very general question, and it’s not clear it should have a single answer. That sai...

Ever since its introduction in the 2017 paper, Attention is All You Need, the Transformer model architecture has taken the deep-learning world by storm. Initially introduced for machine translation, it has become the tool of choice for a wide range of domains, including text, audio, video, and other...

In my day job at Anthropic, we run relatively large distributed systems to train large language models. One of the joys of using a lot of computing resources, especially on somewhat niche software stacks, is that you spend a lot of time running into the long-tail of bugs which only happen rarely or ...

CPU cycles are cheaper than they have ever been, and cloud computing has never been more ubiquitous. All the major cloud providers offer generous free tiers, and services like GitHub Actions offer free compute resources to open-source repositories. So why do so many developers still build software o...

Last week, Frederic Cambus wrote about building LLVM quickly on some very large machines, culminating in a 2m37s build on a 160-core ARM machine. I don’t have a giant ARM behemoth, but I have been working on a tool I call Llama, which lets you offload computational work – including C and C++ builds ...

People who work with me tend to realize that I have Opinions about databases, and SQL databases in particular. Last week, I wrote about a Postgres debugging story and tweeted about AWS’ policy ban on internal use of SQL databases, and had occasion to discuss and debate some of those feelings on Twit...

Summary: Read about my efforts to solve the game of Ultimate Tic Tac Toe. It’s been a fun journey into interesting algorithms and high-performance parallel programming in Rust. Backstory Starting around the beginning of the COVID-19 lockdown, I’ve gotten myself deeply nerdsniped by an attempt to sol...

Alex Gaynor recently asked this question in an IRC channel I hang out in (a channel which contains several software engineers nearly as obsessed with software testing as I am): uhh, so I’m writing some code to handle an econnreset… how do I test this? This is a good question! Testing ECONNRESET is o...

Suppose we have some codebase we’re considering applying some patch to, and which has a robust and maintained test suite. Considering the patch, we may ask, is this patch acceptable to apply and deploy. By this we mean to ask if the patch breaks any important functionality, violates any key properti...

Last week, I wrote about the mindset that computer systems can be understood, and behaviors can be explained, if we’re willing to dig deep enough into the stack of abstractions our software is built atop. Some of the ensuing discussion on Twitter and elsewhere lead me to write this followup, in whic...

Introduction This post attempts to describe a mindset I’ve come to realize I bring to essentially all of my work with software. I attempt to articulate this mindset, some of its implications and strengths, and some of the ways in which it’s lead me astray. Software can be understood I approach softw...

At this point in my career, I’ve worked on at least three projects where performance was a defining characteristic: Livegrep, Taktician, and Sorbet (I discussed sorbet in particular last time, and livegrep in an earlier post). I’ve also done a lot of other performance work on the tools I use, some o...

This is the second in an indefinite series of posts about things that I think went well in the Sorbet project. The previous one covered our testing approach. Sorbet is fast. Numerous of our early users commented specifically on how fast it was, and how much they appreciated this speed. Our informal ...

Testing and feedback loops This post tries to set out one mental model I have for thinking about testing and the purpose testing serves in software engineering, and to explore some of the suggestions of this model. As mentioned in an earlier post, I think a lot about working in long-lived software p...

In 2017 and 2018, I (along with Paul Tarjan and Dmitry Petrashko) was a founding member of the Sorbet project at Stripe to build a gradual static typechecking system for Ruby, with the aim of enhancing productivity on Stripe’s millions of lines of Ruby, and eventually producing a useful open-source ...

While talking about thinking about tests and testing in software engineering recently, I’ve come to the conclusion that there are (at least) two major ideas and goals that people have when they test or talk about testing. This post aims to outline what I see as these two schools, and explore some re...

With the ongoing move towards “infrastructure-as-code” and similar notions, there’s been an ongoing increase in the number and popularity of declarative configuration management tools. This post attempts to lay out my mental model of the conceptual architecture and internal layering of such tools, a...

Writing a Go/C polyglot Someone on a Slack I’m on recently raised the question of how you might write a source file that’s both valid C and Go, commenting that it wasn’t immediately obvious if this was even possible. I got nerdsniped, and succeeded in producing one, which you can find here. I’ve bee...

Abstract In writer-priority reader/writer locks, as soon as a single writer enters the acquisition queue, all future accesses block behind any in-flight reads. Thus, if any readers hold the lock for extended periods of time, this can lead to extreme pauses and loss of throughput given even a very sm...