Daniel Lemire is a software performance expert. He ranks among the top 2% of scientists globally (Stanford/Elsevier 2025) and is one of GitHub's top 1000 most followed developers.
Visit lemire.me → RSS feedOn Hacker News May 2014 – Sep 2026
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In the first half of the XXth century, there were relatively few scientists, and these scientists were generally not lavishly funded. Yet it has been convincingly argued that these scientists were massively more productive. We face a major replication crisis where important results in fields such as psychology and…
Note: This blog post from 2018 predates the Intel Ice Lake (Sunny Core) and AMD Zen 4 processors. These newer processors are not affected by the issues described in this blog post. Our processors typically do computations using small data stores called registers. On 64-bit processors, 64-bit registers are frequently…
In about 10 years, Apple has multiplied by 19 the number of transistors in its mobile processors. It corresponds roughly to a steady rate of improvement of 34% per year on the number of transistors, or a doubling every 2.5 years. In real dollars, an iPhone has roughly a constant price: the price tag of … Continue…
I have a small collection of servers, laptops and desktops. My servers were purchased and configured at different times. By design, they have different hardware and software configurations. I have processors from AMD, Intel, Ampere and Rockchip. I have a wide range of Linux distributions, both old and new. I also…
Many recent Intel processors benefit from a new family of instructions called AVX-512. These instructions operate over wide registers (up to 512 bits) and follow the Single instruction, multiple data (SIMD) paradigm. These new AVX-512 instructions allow you to break some speed records, such as decoding base64 data at…
There are various ways in software to handle error conditions. In C or Go, one returns error code. Other programming languages like C++ or Java prefer to throw exceptions. One benefit of using exceptions is that it keeps your code mostly clean since the error-handling code is often separate. It is debatable whether…
Though most relational databases (like MySQL) are "row oriented", in that they keep rows stored together... experience has taught us that pivoting the data arrow so that columns, and not rows, are stored together can be beneficial. This is an old observation that most experienced programmers know about as the…
If I give a programmer a string such as "9223372036854775808" and I ask them to convert it to an integer, they might do the following in C++: std::string s = .... uint64_t val; auto [ptr, ec] = std::from_chars(s.data(), s.data() + s.size(), val); if (ec != std::errc()) {} // I have an error ! // val … Continue reading…
The concept of property is a social construction. Animals, such as cats, can own a piece of food, or a territory, but only as long as they are able to personally maintain a credible threat of violence. And animals can only defend concrete, physical properties, such as an area, a dead bird or a tree. … Continue reading…
In my blog post, My programming setup, I stressed how important regular expressions are to my programming activities. Regular expressions can look intimidating and outright ugly. However, they should not be underestimated. Someone asked for examples of regular expressions that I rely upon. Here a few. It is commonly…
I have previously reported that there has been a silent revolution in science where countries like China, India, South Korea... that previously contributed few research articles... have started to catch up and even exceed the productivity of the western world. The National Science Foundation (NSF) in the US has…
One of my favorite languages is the Go language. I love its simplicity. It is popular and useful in a cloud setting. Many popular tools are written in Go, and for good reasons. I gave a talk on Go last year and I was asked for a criticism of Go. I do not mind that … Continue reading The Go compiler needs to be smarter
I know that floating-point arithmetic is a bit crazy on modern computers. For example, floating-point numbers are not associative: 0.1+(0.2+0.3) == 0.599999999999999978 (0.1+0.2)+0.3 == 0.600000000000000089 But, at least, this is fairly consistent in my experience. You should simply not assume fancy properties like…
Whenever you enter a URL into a system, it must be parsed and validated. It is a surprisingly challenging task: it may require hundreds of nanoseconds and possibly over a thousand cycles to parse a typical URL. We can use URL parsing as a reasonable benchmark of a system performance. Of course, no single measure ……
When I took Physics courses in college, I learned about how astronauts should age a tiny bit slower than us. Of course, they would be exposed to a lot more radiation so they might develop more cancers. But all in all, I would have been excited about the prospect of living in space. Then the … Continue reading The…
Modern processors have powerful vector instructions which allow you to load several values at once, and operate (in one instruction) on all these values. Similarly, they allow you to have vector constants. Thus if you wanted to add some integer (say 10001) to all integers in a large array, you might first load a…
At my university, one of our internal software systems allows a professor to submit a revision to a course. The professor might change the content or the objectives of the course. In a university, professors have extensive freedom regarding course content. As long as you reasonably meet the course objectives, you can…
When receiving bytes from the network, we often assume that they are unicode strings, encoded using something called UTF-8. Sadly, not all streams of bytes are valid UTF-8. So we need to check the strings. It is probably a good idea to optimize this problem as much as possible. In earlier work, we showed that ……
I learned to program with BASIC back when I was twelve. I would write elaborate programs and run them. Invariably, they would surprise me by failing to do what I expect. I would struggle for a time, but I'd eventually give up and just accept that whatever "bugs" I had created were there to stay. … Continue reading I…
It would take too long to expose all of the flaws of peer review, here are some: some work is just flat wrong because the reviewers cannot analyze all of the mathematical results, and because they cannot redo the experiments; numerous researchers cheat, sometimes in small ways ("2 out of 3 experiments agree with by ……
In software, it is a common problem to want to remove specific characters from a string. To make the problem precise, let us consider the removal of all ASCII control characters and spaces. In practice, it means the removal of all byte values smaller or equal than 32. I covered a related problem before, the … Continue…
I wanted to know how a modern C compiler like clang would process the following C code: #include <stdint.h> int count(uint64_t x) { int v = 0; while(x != 0) { x &= x - 1; v++; } return v; } Can you guess? popcntq %rdi, %rax That is right. A fairly sophisticated C function, … Continue reading The surprising cleverness…
Twitter user opdroid1234 remarked that they are getting more performance out of the ARM nodes than out of the Intel nodes on Amazon's cloud (AWS). I found previously that the Graviton 3 processors had less bandwidth than comparable Intel systems. However, I have not done much in terms of raw compute power. The Intel…
The division instruction is one of the most expensive instruction in your CPU. Thus optimizing compilers often compile divisions by known constants down to a multiplication followed by a shift. However, in some lucky cases, the compiler does not even need a shift. I call the corresponding divisors ideal. For the math…
Modern processors execute instructions in parallel in many different ways: multi-core parallelism is just one of them. In particular, processor cores can have several outstanding memory access requests "in flight". This is often described as "memory-level parallelism". You can measure the level of memory-level…
Most strings found on the Internet are encoded using a particular unicode format called UTF-8. However, not all strings of bytes are valid UTF-8. The rules as to what constitute a valid UTF-8 string are somewhat arcane. Yet it seems important to quickly validate these strings before you consume them. In a previous…
The one constant that I have observed in my professional life is that people underestimate the need to move fast. Of course, doing good work takes time. I once spent six months writing a URL parser. But the fact that it took so long is not a feature, it is not a positive, it is … Continue reading Why speed matters
In many information systems, we work with arrays of integers. For example, maybe you need to keep track of which records in a database contain a given value. This sort of mapping can be expressed as an array of integers. These arrays can end up taking up a large fraction of your memory or disk … Continue reading…
Though most strings online today follow the Unicode standard (e.g., using UTF-8), the Latin 1 standard is still in widespread inside some systems (such as browsers) as JavaScript strings are often stored as either Latin 1, UTF-8 or UTF-16 internally. Latin 1 captures the first 256 characters from the Unicode standard…
Originally, the domain part of a web address was all ASCII (so no accents, no emojis, no Chinese characters). This was extended a long time ago thanks to something called internationalized domain name (IDN). Today, in theory, you can use any Unicode character you like as part of a domain name, including emojis…