Dan Luu+

There's no reason for software to be slow anymore: danluu.com/perf-opt/

There's no reason for software to be slow anymore: danluu.com/perf-opt/

There's no reason for software to be slow anymore: https://danluu.com/perf-opt/

There's no reason for software to be slow anymore: https://danluu.com/perf-opt/

The benchmarkpocalypse: danluu.com/benchpocalyp...

The benchmarkpocalypse: danluu.com/benchpocalyp...

The benchmarkpocalypse: https://danluu.com/benchpocalypse/

The benchmarkpocalypse: https://danluu.com/benchpocalypse/

Interesting. I wonder if one of my company's MCP plug-ins is causing something b

Interesting. I wonder if one of my company's MCP plug-ins is causing something bad to happen or if it's more about my workflow. This is all with CLI (I haven't tried to debug this because my restarter script works ok and my original thought was that this should get fixed pretty quickly).

It seems like codex and claude have switched? Since the GPT-5.6 release, I've ha

It seems like codex and claude have switched? Since the GPT-5.6 release, I've had to run a process that looks for signs of a memory leak and then waits for a good time to kill/restart/resume codex. Seems like 10s to 100s of memory leak restarts per day. If I don't do this, growth is unbounded.

How do programming languages impact token efficiency and correctness? danluu.co

How do programming languages impact token efficiency and correctness? danluu.com/pl-tokens/

What's the best programming language for coding agents? https://danluu.com/pl-to

What's the best programming language for coding agents? https://danluu.com/pl-tokens/

Another thing we see on Amazon, for bad non-scam products, is taking over listin

Another thing we see on Amazon, for bad non-scam products, is taking over listings for older (often obviously unrelated) products with better ratings and then selling a bad product. Product dark patterns like this are pervasive and a clear sign of subterfuge, not informed price-quality decisions.

A common pattern is brand makes good product and people switch to it, brand cash

A common pattern is brand makes good product and people switch to it, brand cashes out and declines in quality and people complain, new brand makes good product that people switch to, on repeat. We shouldn't see this pattern in cases where people are trading price for quality.

Max could argue that consumers want to buy products that either never arrive or

Max could argue that consumers want to buy products that either never arrive or literally don't work, but it's more likely the case that people are being tricked and don't actually want to pay money for nothing. The way people's brand preferences switch also doesn't make sense under Max's thesis.

I don't really buy that line of reasoning in part because of the argument in dan

I don't really buy that line of reasoning in part because of the argument in danluu.com/nothing-works/; people just don't know in a lot of cases and are not making an informed decision. An extreme example of this are the scam products I regularly see on FB and Amazon.

The bulk of www.ftc.gov/system/files... is about standard dark patterns, but the

The bulk of www.ftc.gov/system/files... is about standard dark patterns, but there's a fairly good-sized section on the use of 3rd party trackers. It would be really interesting if those end up carrying a real cost the company. Less interesting if this results in another GDPR/EU-banner-like thing.

Earlier this year, the FTC Chair said "you're going to have a hard time keeping

Earlier this year, the FTC Chair said "you're going to have a hard time keeping up with the number of cases we're gonna be bringing" on privacy. It seems like this is starting; the FTC has filed a case against a company for promising privacy while running ad trackers on their site, which share info

Earlier this year, the FTC Chair said "you're going to have a hard time keeping

Earlier this year, the FTC Chair said "you're going to have a hard time keeping up with the number of cases we're gonna be bringing" on privacy. I guess this is starting? See https://www.ftc.gov/system/files/ftc_gov/pdf/Hims-Complaint-Redacted-E-Filed.pdf. The bulk of the complaint is about standard scammy practices, but there's this section where the FTC is going after them because they promised

That 75% fixed it, but I would still feel tired after a long day. After the wate

That 75% fixed it, but I would still feel tired after a long day. After the water filter, the other 25% of the problem went away over the course of a few weeks, which I chalked up to placebo. But after looking at the pre-filter, I'm suspicious this is real. It matches the effects of chronic excess copper and the timeline is right. But the symptoms are vague/general, so who knows? If I wanted to be

For years, I had less mental energy than I used to. This used to be infinite, bu

For years, I had less mental energy than I used to. This used to be infinite, but it got harder to do things and I would get mentally exhausted after a lot of focus, which I can observe is common but didn't really used to happen to me. I talked to doctors, etc., about this and the universal response was basically "you're getting older; it happens", until I paid some "overpriced" private doctor (th

When I moved into this place, I noticed that the water tastes a bit off and it t

When I moved into this place, I noticed that the water tastes a bit off and it tastes better (but still off) if you run the water for a long time (it turns out the standard advice for what to do if you have copper in your water is to run for a long time). Only one other person (out of ~10-15) noticed this taste, so I figured I was just imagining it. Using the filter, I immediately noticed the wate

After a civil engineering prof mention that the people in their department who w

After a civil engineering prof mention that the people in their department who work on water quality/treatment tend to drink out of the water cooler and not drink tap water, I tried getting a water filtration system. "Everyone" says Vancouver has really good water, so I didn't expect much, but the system's pre-filter catches a lot of green flecks (copper). The RDA of copper is < 1mg, which is on t

In another variant of https://danluu.com/learn-what/, I caught up with a former

In another variant of https://danluu.com/learn-what/, I caught up with a former colleague who worked on automated theorem proving. It turns out he's had an interesting career doing all sorts of interesting stuff using the skills he developed by spending a decade writing/using theorem provers. At one point, he said, "if you use X like a theorem prover, it works really well", which surprised me to h

Trying it out again! I've been active on Mastodon, but the community seems to be

Trying it out again! I've been active on Mastodon, but the community seems to be shrinking Also, the AI posts tend to be about how bad AI is and how it rots your brain. Maybe so, but it's more interesting to see how people use it than to hear how bad it is again; there's a lot more variety that way

Realistically, my failure rate when I apply to companies is pretty close to 100%

Realistically, my failure rate when I apply to companies is pretty close to 100% and I have almost exclusively gotten jobs when people reach out to me to hire me because I'm hilariously bad at interviews. Sounds like it would be a fun job, though!

Exercises in benchmarking and evals, part 7: performance napkin math, DeepSWE /

Exercises in benchmarking and evals, part 7: performance napkin math, DeepSWE / Senior SWE-Bench, and winter tires danluu.com/exercise-7/

Exercises in benchmarking and evals: performance math, winter tires, and DeepSWE

Exercises in benchmarking and evals: performance math, winter tires, and DeepSWE / Senior SWE-Bench, https://danluu.com/exercise-7/

Agentic test processes, LLM benchmarks, and other notes on agentic coding from G

Agentic test processes, LLM benchmarks, and other notes on agentic coding from Galapagos Island: danluu.com/ai-coding/

Agentic test processes, LLM benchmarks, and other notes on agentic coding from G

Agentic test processes, LLM benchmarks, and other notes on agentic coding from Galapagos Island: https://danluu.com/ai-coding/

BTW, if there's one feature I'd want from Uber/Lyft it would be the ability to p

BTW, if there's one feature I'd want from Uber/Lyft it would be the ability to pay for a car that doesn't smell like air freshener+cigarettes. You can pay extra. but that doesn't fix it; the car is just fancier. Not sure if this is a Vancouver thing or Δt, but I'm 5/8 for getting a smoking vehicle here. From talking to locals who Uber regularly, > 50% is normal. This might be the top reason I want

Tried using Lyft for the first time in a while and was immediately reminded why

Tried using Lyft for the first time in a while and was immediately reminded why I don't use it. I look at the ride estimate and it's 4 minutes, so I don't request it early. Then when I'm ready to step out the door, I request the ride and it tells me that it will be 10 minutes, but I can get a ride in 6 minutes if I pay $6 extra.

Exercises in benchmarking, evals, and experimental design, part 6: https://www.p

Exercises in benchmarking, evals, and experimental design, part 6: https://www.patreon.com/posts/160473058

It's interesting how much (some) people are adopting AI-isms in writing even whe

It's interesting how much (some) people are adopting AI-isms in writing even when not using AI. It makes sense that people would fall into the same patterns after seeing them everywhere, but it's still very weird to, live, watch someone who never used to write like this, write out something that's full of "It's not X, it's Y", etc. If you actually read it, it's a little too well reasoned and doesn

Yossi has this post, https://yosefk.com/blog/people-can-read-their-managers-mind

Yossi has this post, https://yosefk.com/blog/people-can-read-their-managers-mind.html, where he uses the term "insane employee" to describe someone who fixes problems they're not incentivized to. In his parlance, if you want to be able to buy a car designed to keep you safe or a CPU designed to not crash, you would need an "insane company". There were more of those when competition was less cutthr

I guess this shouldn't be surprising for reasons discussed in https://danluu.com

I guess this shouldn't be surprising for reasons discussed in https://danluu.com/nothing-works/, https://danluu.com/car-safety/, etc. There's *maybe* one car company that tries to do more than the bare minimum on structural safety, but their software quality is horrendous, so ADAS, etc., are untrusthworthy (https://mastodon.social/@danluu/109667147825098339) (and software controls brakes (https://

With CPUs, the problem is that, you really need the company to go the extra mile

With CPUs, the problem is that, you really need the company to go the extra mile if you don't want random crashes. I'm not sure there's a company you can reasonably buy a standalone CPU from that will do that. AMD has always had issues (coincidentally, the last AMD machine I owned regularly crashed due to what turned out to be a CPU bug), Intel has favored velocity over quality since getting spook

Sure, there are the companies that were on fire as hypergrowth startups that gro

Sure, there are the companies that were on fire as hypergrowth startups that grow into companies with ok quality, but none of those (that I know of) are in the same quality league as companies that ship really high quality products. The worst startups that survive fix the issues that are a threat to the business, but if the company culture is to severely cut corners, it seems very difficult, borde

I was thinking about this prediction that we were going to see a lot more CPU bu

I was thinking about this prediction that we were going to see a lot more CPU bugs, danluu.com/cpu-bugs/, as yet another Intel CPU bug story is on the front page of HN. At the time, Intel threw quality under the bus in the name of velocity. I've seen this happen quite a few times now, sometimes with the stated intention of improving quality later. I'm sure it's ever happened, but I don't think I'v

If you read widely cited folks saying AI economics are bad today (e.g., Ed Zitro

If you read widely cited folks saying AI economics are bad today (e.g., Ed Zitron from the quote), they're not credible. Sure, Ed writes rants, swears a lot, and throws the word "scam" around, all of which gets a lot of clicks, but the reasoning seems wrong. If I were going to be skeptical about the economics of AI, it would be some kind of reasoning about how cheap models keep improving, so the f

Why are so many people so sure that the big AI providers are losing money on inf

Why are so many people so sure that the big AI providers are losing money on inference? It reminds me of the comments about how Uber can never make money. Their unit economics were fine and they were only losing money because they chose to do so on customer acquisition. In this case, I don't think it's even obvious that inference should be money losing today. API cost for frontier models has gone

such as adding "sanjay ghemewat" or anyone I've tried to try to seriously improv

such as adding "sanjay ghemewat" or anyone I've tried to try to seriously improve API design or code quality. Maybe expecting that you magically get better API design by doing the moral equivalent of saying beetlejuice three times is obviously silly and shouldn't be expected to work, but this silly thing that shouldn't be expected to work seems to work in a lot of other areas. What areas does it w

I've been experimenting with this more (not rigorous experiments). When you tell

I've been experimenting with this more (not rigorous experiments). When you tell it to use a persona, it often says something like "I'll use a distributed systems lens" or w/e, making it sound like adding a 2nd persona with the same background wouldn't help, but (for example) having "marc brooker" and "kyle kingsbury" seems to help more than 2x of either or 2x generic distributed systems persona.

unless there are a lot of guardrails, which take time to set up. E.g., "linus to

unless there are a lot of guardrails, which take time to set up. E.g., "linus torvalds" seems to nudge things away from spiralling with pointless complexity, "dan luu" nudges things away from claiming improvements without measuring and sometimes comes back with "must avoid a post-hoc rationalization" and prevents bogus reasoning from being carried forward, "kyle kingsbury" stops some bad concurren

I've also heard the suggestion that you should invoke review personas, "SRE", et

I've also heard the suggestion that you should invoke review personas, "SRE", etc., but a name seems to evoke something more specific than you get out of a role. When I use LLMs I'm generally trying to get as much work as possible out of a unit of my time. I often want to type for a few minutes to get something to loop for hours to days. This named persona thing seems to help with that over just a

For people using LLMs for reviews/audits, I'm curious what tricks you've found u

For people using LLMs for reviews/audits, I'm curious what tricks you've found useful. Lately, a lot of people have told me that using multiple models is super great, but IME using one model with a bunch of different personas is more effective (ofc. you can do multi-multi as well), e.g., "use independent agents to review as linus torvalds, kyle kingsbury, tptacek, dan luu" has been working well fo

I was wondering why I was seeing so many "Co-authored-by: Copilot" commits recen

I was wondering why I was seeing so many "Co-authored-by: Copilot" commits recently. It turns out VS Code added a "feature" that inserts that into your commits automatically, even if you're not using Copilot. It looks like people complained about this, which went nowhere until this hit the front page of HN. After this was the top HN story Saturday, an MS engineer submitted a PR to switch this feat

For non-Centaur features, we tried to match Intel since some software would hang

For non-Centaur features, we tried to match Intel since some software would hang or crash if we did something that was correct according to the manual but didn't match an actual Intel processor, so other things should be pretty standard (other than the obvious, like CentuarHauls vendor ID, etc.

Hah. Sorry, I don't have data sheets squirrelled away that aren't just the stuff

Hah. Sorry, I don't have data sheets squirrelled away that aren't just the stuff you can find online. For non-secret Centaur-specific feature flags, I don't know if there's a better still existing resource than looking at the 0xC0000001 section in git.kernel.org/pub/scm/linu...

This is great! Thanks for the pointer!

This is great! Thanks for the pointer!

A version of Missile Command for the Commodore 64 where the bottom of your scree

A version of Missile Command for the Commodore 64 where the bottom of your screen is the game state in memory and missiles cause memory corruption: csdb.dk/release/?id=.... In the video below, a missile broke my controls and caused my cursor to get stuck moving down and to the left.

The commentary I've seen says Teslas are safe so it must be the drivers but, per

The commentary I've seen says Teslas are safe so it must be the drivers but, per danluu.com/car-safety/, maybe it's the cars. The most fatal rated manufacturers (Kia/Hyundai, Dodge, Tesla) all did poorly — Kia/Hyundai, Dodge got the lowest rating and there's a strong case Tesla should have as well.

I find it interesting/surprising that Tesla topped the www.iseecars.com/most-dan

I find it interesting/surprising that Tesla topped the www.iseecars.com/most-dangero... fatalities per mile ranking from 2018-2022. Fatality rate is strongly negatively correlated with price and weight and Teslas are much more expensive and heavier than average.

A funny side effect of the crackdown on "AI" scraping is that I keep getting ban

A funny side effect of the crackdown on "AI" scraping is that I keep getting banned from sites for browsing too quickly. I barely use reddit anymore and I still managed to get IP banned for scraping (the error message indicated that I should get in touch with them if I want to do bulk accesses).

Steve Ballmer was an underrated CEO danluu.com/ballmer/

Steve Ballmer was an underrated CEO danluu.com/ballmer/

A former Apple engineer discusses Google product culture: > My director wore an

A former Apple engineer discusses Google product culture: > My director wore an Apple Watch and had an iPhone ... my VP too. Nobody was expected to eat the dog food and so few did. This was crazy to me coming from Apple ....

On the 2011-2012 FTC antitrust investigation of Google: danluu.com/ftc-google-a

On the 2011-2012 FTC antitrust investigation of Google: danluu.com/ftc-google-a...

Every once in a while, I think about going to work in the game industry.

Every once in a while, I think about going to work in the game industry.

Interesting comment about SGI leadership knowing about the problems they were fa

Interesting comment about SGI leadership knowing about the problems they were facing and still being unable to come up with a way to handle them.

Great! I'm looking forward to reading the blog post! I've noticed that you some

Great! I'm looking forward to reading the blog post! I've noticed that you sometimes turn your social media comments into blog post, which is something I should probably do more of.

Thanks for the comments. As someone who doesn't really work on this stuff, I fin

Thanks for the comments. As someone who doesn't really work on this stuff, I find this super interesting! As an outsider, it seems like wasm might become mainstream whether or not it delivers benefits to end users, just like heavy SPAs became mainstream regardless of the benefits.

I recently tried Blazor and the performance is incredibly bad (like, 5s to 10s i

I recently tried Blazor and the performance is incredibly bad (like, 5s to 10s initial load time for very simple apps, which you can push down to maybe 3s or something via various config options). From news.ycombinator.com/item?id=3836..., I guess people still like it because it's nice for devs.

The effort to do this kind of work ended up getting defunded after a while even

The effort to do this kind of work ended up getting defunded after a while even though the gains were measurable and very large, so even showing huge gains here wasn't sufficient. On Ember, I didn't know it was such a performance problem. I think that's interesting.

We did see much larger impacts in the long-term holdback than in the initial tes

We did see much larger impacts in the long-term holdback than in the initial test for the reason you mentioned — if someone thinks the app takes 60s to open, they probably won't open it very often, and you've already lost a huge fraction of users who've previously used the app and found it too slow

On a slow device, this decreased time from opening the app to seeing a tweet fro

On a slow device, this decreased time from opening the app to seeing a tweet from something like 60s to 48s. It's incredible that people in that range would even use the app, but apparently some did, and it got more people into the range where they'd use the app or got them to use the app more.

on mobile, changes that made the app go from extremely slow to only very slow ha

on mobile, changes that made the app go from extremely slow to only very slow had large, measurable, impacts on retention/engagement/revenue. I forget the exact numbers, but a change that reduced the loading time of feature flags ended up increasing revenue something like 0.7%.

It's interesting that the impact wasn't easily observable on mobile. For Twitter

It's interesting that the impact wasn't easily observable on mobile. For Twitter, there was an experiment where (I forget the exact number) 500ms or 1s of delay was added on web and the impact was huge, and it was clear we could easily reduce latency by that much (but never did), and

Yeah, I've seen the Discourse people say that Discourse is super fast after the

Yeah, I've seen the Discourse people say that Discourse is super fast after the initial load because it's an SPA, but it's not, even my laptop, which is a faster machine than almost anyone has who isn't on a real workstation. They seem to genuinely believe they have good performance.

How web bloat impacts users with slow devices: danluu.com/slow-device/

How web bloat impacts users with slow devices: danluu.com/slow-device/

We're coming up on a decade since danluu.com/cpu-bugs/. Not only was the predict

We're coming up on a decade since danluu.com/cpu-bugs/. Not only was the prediction valid for a decade, it looks like the next decade will be worse. Today's bug of the day: a significant fraction of high-end consumer CPUs crash if you run CPU-intensive workloads: www.radgametools.com/oodleintel.htm

Diseconomies of scale in fraud, spam, support, and moderation: danluu.com/disec

Diseconomies of scale in fraud, spam, support, and moderation: danluu.com/diseconomies...