The Scalar Select Anti-Pattern

https://matklad.github.io/2025/05/14/scalar-select-aniti-pattern.html

47•goranmoomin•7mo ago

Comments

castratikron•7mo ago

As long as processing one event does not affect any of the other events in the batch. E.g. events are file IO, and processing one event causes another event's descriptor to get closed before that event can be processed.

wahern•7mo ago

If the close routine on an event source, or the low-level (e.g. epoll) registration, deregistration, and dequeueing logic doesn't know how to keep polling and liveness state consistent between userspace and the kernel, they've got much bigger problems. This looks like Rust code so I would hope the event stream libraries are, e.g., keeping Rc'd file objects and properly managing reference integrity viz-a-viz kernel state before the application caller ever sees the first dequeued event in a cycle. This is a perennial issue with event loop libraries and buggy application code (in every language). One can't just deal with raw file descriptors, call the close syscall directly, etc, hoping to keep state consistent implicitly. There's an unavoidable tie-in needed between application's wrappers around low-level resources and the event loop in use.

taeric•7mo ago

I'm not entirely clear on what the proposal is at the end? Seems that the long term answer as to "which of these implications to pursue" is "all of them?" Simply taking in a batch of instructions doesn't immediately change much? You still have to be able to do each of the other things. And you will still expect some dependencies between batches that could possibly interact in the same ways.

In a sense, this is no different than how your processor is dealing with instructions coming in. You will have some instructions that can be run without waiting on previous ones. You will have some that can complete quickly. You will have some that are stalled on other parts of the system. (I'm sure I could keep wording an instruction to match each of the implications.)

To that end, part of your program has to deal with taking off "whats next" and finding how to prepare that to pass to the execution portion of your program. You can make that only take in batches, but you are almost certainly responsible for how you chunk them moreso than whatever process is sending the instructions to you? Even if you are handed clear batches, it is incumbent on you to batch them as they go off to the rest of the system.

lmz•7mo ago

I guess the proposal is "instead of fetching and acting on one event at a time, consider fetching all available events and look for opportunities to optimize which ones you process (e.g. by prioritization or by skipping certain events if superseded by newer ones)".

taeric•7mo ago

I mean, I got that. But you could as easily say "instead of fetching and acting on one event at a time, fetch and triage/route instructions into applicable queues."

In particular, there is no guarantee that moving to batches changes any of the problems you may have from acting on a single one at a time. To that end, you will have to look into all of the other strategies sooner or later.

Following from that, the problem is not "processMessage" or whatever. The problem is that you haven't broken "processMessage" up into the constituent "receive/triage/process/resolve" loop that you almost certainly will have to end up with.

malkia•7mo ago

in CPU's - pipelining!

jchw•7mo ago

I believe something similar is going on internally in Windows with event queues. It coalesces and prioritizes input events when multiple of them pile up before you're able to pop new events off of the queue. (For some events, e.g. pointer events, you can even go and query frames that were coalesced during input handling.) On the application/API end, it just looks like a "scalar select" loop, but actually it is doing batching behavior for input events!

(On the flip side, if you have a Wayland client that falls behind on processing its event queue, it can crash. On the whole this isn't really that bad but if you have something sending a shit load of events it can cause very bad behavior. This has made me wonder if it's possible, with UNIX domain sockets, to implement some kind of event coalescing on the server-side, to avoid flooding the client with high-precision pointer movement events while it's falling behind. Maybe start coalescing when FIONREAD gets to some high watermark? No idea...)

Stop Designing Languages. Write Libraries Instead (2016)

The Eric and Wendy Schmidt Observatory System

Sergey Brin's Unretirement

Vector graphics on GPU

Formal methods only solve half my problems

Stop Doom Scrolling, Start Doom Coding: Build via the terminal from your phone

Opus 4.5 is not the normal AI agent experience that I have had thus far

Electronic nose for indoor mold detection and identification

Optery (YC W22) Hiring a CISO and Web Scraping Engineers (Node) (US and Latam)

The creator of Claude Code's Claude setup

A 30B Qwen model walks into a Raspberry Pi and runs in real time

Show HN: SMTP Tunnel – A SOCKS5 proxy disguised as email traffic to bypass DPI

The first new compass since 1936

Htmx: High Power Tools for HTML

Vietnam bans unskippable ads

Investigating and fixing a nasty clone bug

On the slow death of scaling

I wanted a camera that doesn't exist, so I built it

Oral microbiome sequencing after taking probiotics

We recreated Steve Jobs's 1975 Atari horoscope program

Calling All Hackers: How money works (2024)

The ISEE Trajectories

CES 2026: Taking the Lids Off AMD's Venice and MI400 SoCs

Launch HN: Tamarind Bio (YC W24) – AI Inference Provider for Drug Discovery

What *is* code? (2015)

Show HN: VaultSandbox – Test your real MailGun/SES/etc. integration

High-Performance DBMSs with io_uring: When and How to use it

Locating a Photo of a Vehicle in 30 Seconds with GeoSpy

Two ways to crack a walnut, per Grothendieck (2025)

Microsoft probably killed my Snapdragon Dev Kit