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The browser's main thread is expensive
- Ayesh 2mo agoSuch an excellently written articles with really nice interactive visualizations!
- ahartmetz 2mo agoYeah, but if your website with, you know, text and stuff, doesn't do any stupid and unnecessary crap, it doesn't matter where the nonexistent unnecessary yet expensive code doesn't run.
- another-dave 2mo agoand if you have no website at all, it takes zero seconds to load. But for people who are developing websites that are more than just text, this is an excellently written article and great advice. It's funny, for mobile app dev it feels like the community are a lot of focused on staying off the main thread, this is the first time I've come across one for webdev.
- Tepix 2mo agoSure, but there are sites like HN and there are sites like https://earth.nullschool.net/ https://earth.nullschool.net/ What's your point? Are you saying all animations and rendering are unnecessary?
- ahartmetz 2mo agoMy point is that many websites are using way too much technology for what they need to do, with correspondingly long load times and choppy runtime behavior.
- dspillett 2mo agoCorrect. But if the page is, at least in part, an art piece rather than an information dump, or needs to do interactive rendering for its purpose (an editor like the markdown example, live updating status displays, games and other interactive toys, etc), then the tech is necessary and this article is useful.
- manphone 2mo agoWhich is something like less than one percent of websites, and this technology is deployed much, much more widely.
- dspillett 2mo agoAgain, correct. But “don't do this if you don't really need to do this” is rather pointless to say, hence is being downvoted, because people already being that sensible don't need to hear it, and those who are not are unlikely to listen. The comment is just noise, as unnecessary as the unnecessary animation and other gumf that it is railing against.
- adzm 2mo agoWell yeah but if you do actually need things like images, animations, high frequency updates, etc, this is actually a pretty good overview of how to approach it. Also learned that react uses posting to a MessageChannel to yield!
- account42 2mo agoGP is right that just like with garbage where "reduce, reuse, recycle" is the mantra, the first question for optimizing something should be if you need to do the calculation at all.
- adzm 2mo agoThis is true as well, yeah, but it came off as dismissive of everything too. Personally I prefer information dense interfaces with minimal chrome, but there really is a certain joy when you get to use a well designed and optimized fancy reactive, immersive interface
- voidUpdate 2mo agoSince when do images need more than just an <img> tag and a bit of styling?
- kodoman 2mo agoAmazing how you are being so down voted, most webpages do utter crap that no one would want the webpages to be doing other then advertisers. Really most things should be text and if we lived in a more cooperative world everything would be easy to parse and manipulate text with some video and images and perhaps various well defined interface types for different services that can be implemented in various ways as to the users liking.
- nicoburns 2mo agoYou'd be surprised how expensive it is to render text, although unless your page is very large you won't see it being slow on modern systems.
- ahartmetz 2mo agoI'm more surprised how cheap it is in browsers because I know it's not that easy - they are really really good at it. Pure text rendering is therefore rarely an issue.
- nairboon 2mo agoIn the first example: doesn't pushing the button while typing cause the textfield lose focus? What's the point of the typing in the example?
- lintfordpickle 2mo agoI think the point is, if you click the '2.0s' button, you cannot re-focus on the input field for those 2 seconds.
- Tepix 2mo agoNice examples, a bit repetetive perhaps. One thing that I found quite interesting to see is how the behaviour of the "4.000 particles" demo changes between the 5ms rendering and the "everything now" rendering.
- piker 2mo agoGreat piece, OP. We work in WASM and are considering web workers to help with rendering documents off the main thread in that context. I was surprised to read that ArrayBuffer is moved by reference only! Might be an option. Thanks for putting this together.
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- gioandthemachin 2mo agohaven't done this for documents, but for canvas rendering transferControlToOffscreen() worked better than moving ArrayBuffers. It hands the canvas to a worker that draws directly, no pixles over postMessage at all.
- bastawhiz 2mo agoArrayBuffer is copied by default unless you pass it as a special transferable object. But only the current owner of the buffer can use it. If you need to read and write from both threads at once, you need to look at SharedArrayBuffer.
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- adzm 2mo agoI wish await in JavaScript could conditionally yield. Instead I end up having to use arounds like if (shouldYield()) await do yield();
- baxuz 2mo agoThere's an RFC or two to spawn threads so I'm looking forward to that. The current way of workers + messages is tedious as hell.
- silverwind 2mo agoIsn't that basically AsyncGenerator?
- adzm 2mo agoNot really, that still yields on every invocation
- xnorswap 2mo agoExcellent article, I went into it thinking the headline is obvious, but the examples are well crafted and it isn't just the obvious stuff.
- mschuster91 2mo ago> So what if off-screen posts were left as empty shells that only take up their height, and got filled with real content as they approach the screen? Yeah fuck everyone doing that, hello Reddit, Outlook for Web or Bluesky. It makes searching on such "feed" pages with the browser's search function an utter pain in the ass, made worse by the fact that the platforms' own search functions are outright braindead.
- prodigycorp 2mo agowhat's your solution to the performance hit that you get when you load up the dom with a zillion things? long, unvirtualized threads will bring every decent computer to its knees. the sites could implement their own search to make it all work if they wanted to. they just dont. but blaming virtualization is not tit.
- charcircuit 2mo agoThe solution is to improve the browser to remove the performance hit. Modern computers are extremely powerful so there is no good reason why they would not be able to handle a zillion things in the DOM.
- mr_toad 2mo agoThe browser can easily handle millions of calculations and then render a million DOM elements so quickly that you won’t even notice it. The problem is when you try and do this hundreds of times very second. The irony is that obsessing over smoothness and frames per second is what causes this in the first place.
- fassssst 2mo agoA zillion things is more than a zillion bytes of RAM :p
- phyzome 2mo agoUsing a lighter weight DOM can help. There's always a point where it starts to bog down, but many pages are just unnecessarily heavy because they're relying on so many nested widgets and whatnot.
- jkhdigital 2mo agoThis article concludes with the following statement: > So much of development is trade-offs, and you have to choose according to the situation, which ultimately comes down to the developer’s experience and judgment. It’s a great article, but I think it is a bit behind the ball in framing scheduling problems as a matter of “experience and judgment”. The problem of allocating work to a scarce resource is one of the oldest and most well-studied in all of computer science. It would make sense to go consult a textbook on the matter before trying to reinvent the wheel.
- TonyStr 2mo agoDo you have any suggestions for books on the topic?
- spockz 2mo agoWell yes. But most of those studies assume/assert control over the environment. In this case you are running in the browser and you have to deal with the primitives it gives you. Both in compute management (threads/workers), communication (RPC, messages, shared memory, sockets), as scheduling (like having yield or not. I’m not sure whether inventing a setup like green threads in JavaScript/workers would even be possible. That is not to say that we should just forget about those learnings though.
- morning-coffee 2mo agoThis! Since it became easier for people to post their thoughts on a problem vs. research and learn from past work, we're now in the era where its harder to find the seminal and really-well-explained material for having to wade through years and years of people's rediscovery blogs. (Don't get me wrong; I too think the article is great; just wish all the folks who went to JS bootcamp and are amazed by this would have cracked any CS or OS text book written since the late 60's and browsed a few chapters.)
- sylware 2mo agoThe javascript web is. Restore the interop with noscript/basic HTML and it will lean towards sanity, something lost a few years ago.
- andai 2mo agoBrilliant, I had no idea about this stuff. Thanks for writing it up. On the price ticket example, on my device (Samsung A15, mediocre phone from 2024) I get 11 fps with the slow example and 30 fps with the fast one. (15 if I scroll!) Is that a case of the back pressure you mentioned? All the other (fast) examples are 60 fps though :)
- tobiu 2mo agoActor Model, dedicated or shared application worker. Full support for multi-window apps. Resolved since 2019. 26000 commits in, and counting. You are welcome! https://github.com/neomjs/neo/blob/dev/learn/benefits/body/OffTheMainThread.md https://github.com/neomjs/neo/blob/dev/learn/benefits/body/O...
- Lebenita 2mo agoAI;DR – too bad as the content seems to be worthwhile, but then why not take the trouble to write it up yourself. Have it proofread/edited/spruced up all you want afterward, but don't have it do all (or most of) the writing.
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- shevy-java 2mo agoSo this is AI slop written? Why would anyone believe I'd want to read AI spam? Also, the issue I see is who controls browsers as much more profound. We need to find a solution here, as the browser is too important to allow private companies to keep mankind hostage. Edit: Wow, and the praise-accounts. Is that new on hackernews?
- ricardobeat 2mo agoWhat signs of slop are people seeing here? Or is it just a knee-jerk reaction to encountering lots of text? I’ve worked in the field for a long time and see a carefully written, correct, calm article. No obvious AÍ tells in wording or sentence construction. It is a bit long - might have been inspired by ciechanow.ski
- Lebenita 2mo ago"and there’s a reason for that" "The code isn’t slow. It just happens to be the code that’s holding the main thread." "dealing with the main thread becomes the important part" "What to remember for now is that holding the main thread for a long time is the same thing as freezing the screen." "One thing not to misread here is" I realize the author may not write native English, but this is not translation slop, it's AI writing slop.
- duskdozer 2mo agoSomewhere along the line "machine translation" became conflated with "machine translation + LLM editing"
- ricardobeat 2mo agoI think you're overindexing here. There are no obvious traces of AI in those full paragraphs (and I read thousands of lines of AI generated text daily). These are not the usual tells, just decent writing. It reads pretty much like every well-written technical article used to, ten years ago; and scores a 0% likelihood of being AI generated in Quillbot.
- larodi 2mo agoTop article, kudos! applied some of these techniques previously, and they do matter a lot. thing is, when you learn/teach JS there is usually limited time left to talk these topics, and they are essential more than it seems. the whole point of cooperative multitasking is to yield now and then (back to the runner), so that it can process the drawing. Besides, at 60fps there's so much that can be computed once every N frames, and the eye does not see it, the animation flows. It becomes even more interesting when webgpu is involved, but the CSS animator is indeed very fast. note: some recent work of mine (newskool digital flyer sites) -> bsf.hmsu.org // nouveauxhivers.dub4powder.xyz
- TIMEFOLD 2mo ago[flagged]
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- monxer 2mo agoOne obvious tool that this article doesn't mention is Chrome's performance monitor. Many techniques in the article only have to be applied once you can verify that they are running slow.
- stwrt 2mo agoThe web inspector is often overlooked. Other important spots to look at are the Network tab and Lighthouse (in Chrome).
- prodigycorp 2mo agoI want to do a blanket response to people calling this AI slop: the post was written in korean and translated to english. Calling this slop is judgment slop.
- duskdozer 2mo agoThe basic Firefox "translate this page" on the original is more readable to me.
- jonathanlydall 2mo agoSuperb article. There was very little new information for me as I have applied some of these techniques myself based on having developed an intuitive understanding on how a rendering "thread" works and blocks, but for a less experienced developer this article should be incredibly enlightening and provide a solid understanding of what's happening. I employed use of yielding on a hobby project [0] I made about 15 years ago, particularly when it had to do lots of draw operations on a canvas. I also experimented with using worker threads to render pieces of it on a background thread, but at the time there was no way to copy the data efficiently between them and the main thread, one had to send the data as a base64 encoded PNG and the overhead of encoding, decoding and then copying it onto the canvas made it perform far worse than just doing it all on the main thread. The website also does Gzip decoding of uploaded files in JavaScript and the library I found at the time did all the work synchronously so could lock up the UI thread easily for 10+ seconds. I tweaked it to be able to yield every 200ms or something, the process of which was very educational, particularly due to it convincing me to never omit the curly braces after an if statement, I spent a very long time trying to understand why it wasn't working until eventually I realized a statement I added wasn't in the if statement's block. It's not that I didn't understand how if statements worked, it's that in my mind the lack of curly braces was initially invisible to me. [0]: https://mordritch.com/mc_rss/ https://mordritch.com/mc_rss/
- TIMEFOLD 2mo ago[flagged]
- martinald 2mo agoGood article and I wish this was much better known. The issue is though that it's too focused on interactivity. In reality, 90%++ of slow sites are not slow because of interactivity really, they are slow because they ship enormous react/nextjs bundles and have extremely heavy hydration work to do. _so many_ sites have bundles >10MB that need to be downloaded, parsed and hydrated. I've even seen (many) sites which have multiple SPAs stacked inside of them. If you're on a slow internet connection and/or CPU the page is basically unusable for many tens of seconds and no amount of yielding post bundle hydrate will really solve that.
- jcelerier 2mo agoI challenge you to browse the web one week on this laptop part of the current top 10 Amazon best sellers on a gigabit fiber connection and tell me if you still think that this is the problem: https://www.amazon.com/HP-Everyday-Processor-Microsoft-Portable/dp/B0H94BMX1K/ref=zg_m_bs_g_565108_m_sccl_5/143-6149589-1760914?psc=1 https://www.amazon.com/HP-Everyday-Processor-Microsoft-Porta...
- cryptonym 2mo agoIt is a problem and plays a role, then the rest of the SPA interactions are as bad. Amplified by 100+ third party scripts. Stop building SPA, go back to HTML. Re-assess every third party. For extra performance and scaling, implement cache. Relax and see web experience healing.
- doix 2mo agoThere's definitely trade-offs. I actually prefer a proper SPA to pure HTML when on a slow connection and interacting with a site a lot. Especially when trying to buy something. I much prefer to have a longer initial load and then have everything just work instead of waiting as I navigate between pages, the multi stage checkout, confirmation etc. But yeah, if I'm just trying read a single article on a blog, preloading everything is pointless. A hybrid site with the initial page being server side rendered + progressive enhancement afterwards is theoretically optimal in my opinion.
- pmkary 2mo agoI learned these lessons from many separate places and have always wished to see them in one place. This was an incredibly great article that I very much enjoyed reading and will be sending to anyone who's in need of ideas about speed. Thanks a lot!
- CommonGuy 2mo agoReminds me of sprinkling DoEvents all over my VBA code to keep it "snappy" when I started to get into programming
- mediumsmart 2mo agogreat article - explains in detail why my sans js websites run so smoothly and so fast.
- bob1029 2mo agoIf you are currently trying to build a web-based clone of something approximating ChatGPT, pretty much everything in this article is absolutely mandatory. The streaming response UI/UX is a total nightmare to make work smoothly. You have to come up with clever heuristics around where to chop up the stream of changes and how to batch the work relative to frame updates to make it not look like confetti during a streaming response.
- bee_rider 2mo agoI really don’t understand why the LLM web chat clients have all converged on this UI where one character appears at a time. It is kind of fun the first time (like you are watching some Harry Potter special effects where an enchanted book writes itself). But really they could send sentence or paragraph long chunks of text, it would be fine. Seems like an artifact of the time when tokens-per-second were low enough that people needed to be informed that the machine was actually doing something.
- bob1029 2mo ago> But really they could send sentence or paragraph long chunks of text, it would be fine. The problem is that the content is multimodal, often recursively. You can be inside markdown and then have a python code section. Simply detecting these boundaries is already challenging.
- duskdozer 2mo agoI suspect one reason is to subtly make the user feel more like they are interacting with a person.
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- vbezhenar 2mo agoThis mistake gets repeated over and over again, in every single GUI framework of any kind. I don't understand why does it have to be that way. Just use multi-threading. Run each library in a separate thread. Use actors approach to pass data between libraries, application and finally to submit data to update GUI. Using single thread for everything and expecting that developer will create separate threads for heavy work is obvious approach, but it never worked. Developer doesn't care. And user gets inferior experience.
- eviks 2mo ago> Press a button: the JS animation and typing freeze, but the CSS animation keeps spinning How do you expect me to type if pressing a button moved text focus away from the text field?
- doginasuit 2mo agoThis was so incredibly helpful and well presented.
- abla2 2mo ago[flagged]
- gwbas1c 2mo agoFYI: This isn't a browser-only thing. All platforms (Windows, Mac, ios, android) typically have a single-threaded UI.
- Asmod4n 2mo agoFor responsiveness that’s what you want, context switches add quite a lot of jitter and perceived slowness’s, making your app feel slow despite running at 120 fps.
- nerdralph 2mo agoI agree with most of the article, but would clarify the following: > For the screen to look smooth, frames have to be drawn at the display’s refresh rate. On the most common 60Hz display, that means 60 frames per second, or about 16.6 milliseconds per frame. It isn't absolutely necessary to match the display refresh rate. With a 144Hz monitor, 72FPS is going to look smooth for the vast majority of the people, and even 48FPS will look smooth for most people. I agree that higher FPS is better, but there are diminishing returns.
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- bgirard 2mo agoIt's a good nitpick. A hypothetical 1000Hz display will still look smooth to the human eye at 60/120Hz. A 1Hz display will never look smooth. But it's more about honoring the user's preference. I personally would rather 60Hz, then 120Hz heating up my room unnecessarily. If someone has a display set to 1000Hz for whatever reason, then you should try your best to honor that.
- jjk166 2mo agoA user's hardware represents the worst case scenario they want to be able to handle, not their preferred nominal settings. Most people aren't changing their screen refresh rate when they switch between playing a game and reading wikipedia.
- duskdozer 2mo agoIt depends. My display being at 1000hz doesn't mean I need your homebrewed accordion menu eating up the CPU to slide out and wiggle at 1000hz
- Me1000 2mo agoBelow a certain threshold things are certainly going to look janky. But you’re right you don’t have to match the screens native refresh rate. More important is that you have a consistent frame rate. If you can’t hit 120hz, it’s better to target 60hz, rather than 120 a miss your frame budget. 30fps will also look better than 60 but regularly dropping frames.
- mikemarsh 2mo agoI remember seeing some JS cryptography library a long time (maybe 10 years?) ago that provided an async hashing function for exactly this reason: to split up what would normally be a simple synchronous computation and yield more often to the main thread. Can't recall the name, but I remember thinking that was very clever once I understood why it was doing that, and am glad to see the concept explained directly here.
- FoundTheIdiot 2mo ago[flagged]
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- nightpool 2mo agoThe chat example here doesn't really seem to work for me—I get 32 FPS consistently no matter what the chat settings are. Maybe a Chrome thing? Or an M3 thing?
- cpt_sobel 2mo agoThat's the kind of content I come to HN for, thanks!
- skobes 2mo agoThe discussion of "FLIP (First, Last, Invert, Play)" would be improved by mentioning the View Transitions API, which is basically there to automate this technique.
- thataccount 2mo agoIt would be better to say that the Browser's Main Thread has been made expensive to run, because browsers have had a bunch of crap piled on them in terms of functionality that is expensive to run.
- Dwedit 2mo agoWhat does "Browser's Main Thread" even mean when there are over 30 processes running?
- chrismorgan 2mo agoA handful of comments on an otherwise fairly reasonable article. > When we run into jank like this as developers, the usual reaction is to wonder “is my code slow?” and start picking apart algorithms or looking for wasted computation. In most cases, though, the speed of the code is not the problem. The code isn’t slow. It just happens to be the code that’s holding the main thread. Eh, most of the time it is slow, because you chose to use React. React used naively scales very badly. You have to jump through lots of sometimes-fiery hoops to make React… not much slower than some of the alternatives. Memoisation (possibly now via React Compiler) and things like that. > [Diagram with rAF, Style, Layout and Paint attributed to the main thread] Paint hasn’t been on the main thread in Firefox since 2017: https://mozillagfx.wordpress.com/2017/12/05/off-main-thread-painting/ https://mozillagfx.wordpress.com/2017/12/05/off-main-thread-... Not sure about other browsers. Firefox has generally been the leader in these areas. > [steering cost demo] The demo is deeply unsatisfying because the simulations are quite different in a very problematic way: in “all at once” mode, it loses most mouse movements, which radically affects the simulation which is attuned to movement events rather than simulating physics based on a constant tick rate. Without examining it closely, my guess is it’s using pointer events and the browser is coalescing the events because of the blockage and it’s only taking the final state rather than going through all of getCoalescedEvents(). In a simulation like that, if you can’t run it at full speed, you have to decide what to do, and you should do it deliberately, because it may be disastrously wrong. Should you fall over, simplify the simulation by dropping some of the calculations, reduce the tick rate, something else? > The demo below is a markdown editor with a long CHANGELOG open. Building the preview means parsing the entire document (about 2,000 lines) and rebuilding its DOM from scratch, which is far too expensive to run on every keystroke. No it doesn’t. This is a clear example of your code being slow, because you’re doing the wrong thing. You want an incremental parser of some kind. Markdown is tolerably well-suited to this kind of thing because for most edits you can just render the current paragraph. The suggestion is bad. “Debounce 300ms” behaviour is obviously worse for small documents. > DOM writes can be batched as well. Appending a hundred nodes in one operation instead of one at a time, or toggling a single class instead of changing style properties individually, turns many changes into one and helps performance. The old technique of assembling an HTML string and assigning it to innerHTML in one shot has the same essence. You gather the writes so the rendering pipeline’s fixed cost is paid once. This is simplified far beyond accuracy. parent.append(one_hundred_nodes) and one_hundred_nodes.forEach(node => parent.appendChild(node)) are unlikely to perform particularly differently. Changing style properties individually isn’t that* much worse than toggling a single class (it amounts to rewriting the style attribute a bunch of times, and the CSS parser and serialiser aren’t particularly slow). Building an HTML string and assigning it to innerHTML is better than constructing children manually in some cases, worse in others. The real thing that matters is avoiding triggering layout unnecessarily by things like accessing clientWidth between changes. > Deferring The lazy rendering trick used here is very problematic because you still need to know how tall each item is. Plenty has been written about hazards of infinite scrolling, that’s what’s being dealt with here. Now if you can have each item be a fixed height, then progressive rendering has far fewer side-effects. I would also say that switching from Notifications back to Feed and having it take an extra few hundred milliseconds is really not that bad. And that I’m not convinced the approach taken was right anyway, you could often just leave the feed rendered and avoid affecting the gross layout. —⁂— On the presentation of the site itself: this makes no sense: .post-content ul { word-break: break-all; }
- kccqzy 2mo agoHere is a problem that I had thought about in my last frontend project: the project needed to parse a user-provided string to provide syntax highlighting. The implementation was to parse the string immediately after a user keystroke so the user always sees correct colors. But I had this nagging thought that if I had designed the grammar wrong some parses might take more than linear time and would hold the main thread during the parse. But I didn’t really want to first render a black string, parse on a service worker and then rerender with color. The effect must have seemed disorienting. Anyways I stopped the project for unrelated reasons and this thought kept nagging me in the back of my mind. The time I work on the project I’ll be sure to try some techniques in the article.
- pedrovhb 2mo agoYou'd use an incremental parser, they're specifically designed to be efficient for small edits. The big name is `tree-sitter`, but its output is compiled so on the web it requires some wasm shenanigans. For webpages, I'm a fan of `lezer`: https://lezer.codemirror.net/docs/guide/ https://lezer.codemirror.net/docs/guide/
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