Saving 100 terabytes of memory by optimizing 1.1.1.1's DNS cache

> Using twice as much ram per cache entry makes the cache half as large, assuming your cache is bounded by ram, unless the queried, unexpired result set is less than the ram budget (which I would tend to doubt... lots of randomized queries out there; maybe I'm wrong if the cache size dropped).

This is true. I'm arguing that its unlikely this was ever bound by available RAM. Cloudflare is a DDoS protection company that absorbs attacks. They have a lot of available capacity at any moment. When you're building a service in a sitaution where you have more capacity than you'll likely need.

The savings were 100 TB across >300 data centers. The savings were on the order of 50%. So prior to this reduction the service was using something less than 2/3 of TB per datacenter. The service ram usage was about 10GB per instance according to the graph in post. IDK how cloudflare divides thier stuff between machines, but assuming they don't run less than 64 GB per server that's less than 12 servers per datacenter of ram for a flagship product, and they likely run it spread across 65 of the machines in the datacenter that are also doing other stuff. The per-instance RAM likely isn't the the concerning limit.

Overall RAM usage is proabably a bigger concern. Thats why I would think about dedup between instances and distributed caching strategy first. I could focus on redudcing the ram needed per service instance and get a 50% reduction per machine. Or I could focus on deduping 1/n (where n > 2) reduction in total memory usage across all instances. Personally if I was worried about reducing RAM I'd put more energy into growing N.

However all this is a red herring. The assumption people are making is that the cache was always read-only, and it's obvious that Box was the best decision because in a RO cache smaller entries hold more things.

The 1.1.1.1 service advertises improved DNS performace. That's its value add. The biggest performance gain you can have from a cache is not having a cache miss, and in DNS a cache miss means a very expensive recursive lookup. So there's concerns about how to minimize those lookups. If one instance has does a lookup, it makes sense to share that result to the other instances that may need to do a lookup [1]. I don't know off the top of my head if it makes sense to get those updates and modify the existing record or just replace it in the local cache. That comes down to locking strategies and reading patterns in the specific code and service traffic patterns. Until i have hard evidence one way or another I'd like my cache to be able to do both and keep the data structs modifiable until that's nailed down. If per-isntance ram ever becomes the issue, there's easy wins there to buy me time to find better large scale solutions to the problem.

No one is disagreeing that the larger datastructures are larger. No one is disagreeing that they take more RAM, and or even if RAM was the the problem reducing it would be good.

The thing people are pointing out is that this isn't a homework problem about an optimal cache structure in a vacuum. We're pointing our that engineering real large scale solutions has a lot more to consider than a homework problem, and that the thing you're harping about likely didn't have any real budgetary or noticable performance impact on bulding that system. The reduction in ram is just a smallish improvement in operating costs after all the more expensive stuff was figured out.

Put another way 100TB of RAM is ~$350K. Thats one engineer year for a mid-level engineer.[2] Would you rather spend that money to save an equivalent amount of money somewhere, or... would you spend that money putting the engineer on something that saved $700K elsewhere (alternately that generated $700K)?

[1] I talked a lot about dedup and the simple gotcha is "hahah then its not deduped so you need smaller objects". But on a service that is running on a few dozen instances having a few redundant copies to deal with loss of a machine and/or load can still result in 1/(n>2) savings in total ram.

[2] I'm not saying someone worked on this for a year btw, a couple people likely spent a couple months on the code, validation and testing of it. A manager spent time overseeing it. Operations people spent time understaning any effects it had on running systems. Costs add up and it wouldn't suprise me if this didn't end up being roughly break-even for the year.

> And commented how strange it is if they had spent a day at the start of the project whiteboarding out the design against hardware specs at the start of the project - and avoided months of wasted effort - that would be called "premature optimization".

Oof that hit hard. The last project I worked on suffered from a very similar disease, and it has really taken a toll on me psychologically. To work day in and day out on something that you can prove cannot work is unbelievably demoralizing. From an organizational standpoint, it makes a lot of sense to have an "internal consultant" who can deliver bad news like this. I tried to do it from the "inside" which was a huge mistake--got a negative performance review saying I had a "communication problem" because nobody wants to hear "negativity". I can come off online as kind of an asshole, so this may not seem credible, but I did actually deliver this news in a professional, measured manner. It's just that organizations are allergic to it, and their antibody response kicks in. You need someone who is not affected by the organizational hierarchy (or at least not that branch of the tree) to step in and deliver the bad news without fear of retaliation.

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Might be of interest:

- TigerBeetle: A database without dynamic memory allocation, https://news.ycombinator.com/item?id=33192288 (2022).

- Succinct Data Structures: Cramming 80,000 words into a Javascript file, https://news.ycombinator.com/item?id=2348619 (2011).

It could be a new keyword rather than counting on the compiler to prove certain access patterns don't exist. That's a bit of a messy tradeoff. Maybe something like 'unordered struct' or 'packed struct' works, but it would be a nonstandard extension for some time.