Nelson Elhage's blog. I write about software and software engineering.
Visit blog.nelhage.com → RSS feedOn Hacker News Sep 2010 – Mar 2025
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A deep dive into the performance of Python 3.14's tail-call interpreter: How the performance results were confounded by an LLVM regression, the surprising complexity of compiling interpreter loops, and some reflections on performance work, software engineering, and optimizing compilers.
An overview of the tools and environment Stripe engineers used to write and test code, as of 2019 or so.
The longer I spend as a software engineer, the more obsessive I get about testing. I fully subscribe to the definition of legacy code as “code without an automated test suite.” I’m convinced that the best thing you can do to encourage fast progress in a test suite is to design for testing and have a fast, reliable…
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 which I want to run…
How do you find near-duplicates in a massive collection of documents? An exploration of the Jaccard similarity metric, and the MinHash hashing trick used to efficiently approximate it at web scale.
tl;dr “Transparent Hugepages” is a Linux kernel feature intended to improve performance by making more efficient use of your processor’s memory-mapping hardware. It is enabled ("enabled=always") by default in most Linux distributions. Transparent Hugepages gives some applications a small performance improvement (~ 10%…
Most of the projects I've been working on today have fairly strict code review policies. My work requires code review on most of our code, and as we bring on an army of interns for the summer, I've been responsible for reviewing lots of code. Additionally, about five months ago BarnOwl, the console-based IM client I…
So, you’ve got a program that’s using more and more over time as it runs. Probably you can immediately identify this as a likely symptom of a memory leak. But when we say “memory leak”, what do we actually mean? In my experience, apparent memory leaks divide into three broad categories, each with somewhat different…
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 others. Transformers…
Computers and computer systems are build up from deterministic, comprehensible, building blocks. Their operations and behaviors can be understood and reasoned about. I relate my personal beliefs and mindset on this point, and explore some manifestations and ramifications of this philosophy.
At Stripe, we rely heavily on ruby and EventMachine to power various internal and external services. Over the last several months, we’ve known that one such service suffered from a gradual memory leak, that would cause its memory usage to gradually balloon from a normal ~50MB to multiple gigabytes. It was easy enough…
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 of which ended up on…
I really like Twitter. I think it’s a great, fun, service, that helps enable interesting online communities, and is a surprisingly effective way to spread news and information to lots of people online. One of the things that I’ve loved about Twitter is their API, and how open and welcoming they’ve been to developers…
MongoDB is perhaps the most-widely-mocked piece of software out there right now. While some of the mockery is out-of-date or rooted in misunderstandings, much of it is well-deserved, and it’s difficult to disagree that much of MongoDB’s engineering is incredibly simplistic, inefficient, and immature compared to…
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 specific later. What…
If you’re familiar with nearly any mainstream programming language, and I asked you to draw a diagram of an array, the array indices, and the array elements, odds are good you’d produce a diagram something like this: In this post, I want to persuade you to replace that image, or, at least, to augment it with an…
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 said, for a long time…
6.033, MIT’s class on computer systems, has as one of its catchphrases, “Complex systems fail for complex reasons”. As a class about designing and building complex systems, it’s a reminder that failure modes are subtle and often involve strange interactions between multiple parts of a system. In my own experience…
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 benchmarks on…