4 ms·

I don't imagine it was ivan_gammel's intention, but his mention of the original CCC/tomcat issue I think makes the point quite clear: have you ever had a hashma

I don't imagine it was ivan_gammel's intention, but his mention of the original CCC/tomcat issue I think makes the point quite clear: have you ever had a hashmap of HTTP header to values? I hope it used one of those external libraries, because otherwise that's now a vector for DoS'ing any app that has one. User influenced/controlled input happens way more than we expect, I think the more sensible approach would be for the map to be safe by default, and to reach for a high performance external map for those times when that particular data structure is the bottleneck.

ivan_gammel 1y ago
>I think makes the point quite clear: have you ever had a hashmap of HTTP header to values? Number of headers is limited by default to a relatively small value (10k), which in the worst case of hash collisions results in relatively fast lookup time due to tree-like implementation of HashMap.
- tialaramex 1y ago
  > due to tree-like implementation of HashMap. Exactly, this data structure has to provide counter-measures to cope with the fallout from this other poor choice. The fast data structures for this work don't need a tree here, they're open addressed instead, but this would mean they can't cope if your attacker can arbitrarily collide input, so, too bad.
  - ivan_gammel 1y ago
    This data structure has tree-like fallback for hash collisions in general, which do happen regardless of hash algorithm by definition of what is hash. It is not a coping mechanism for a poor choice, it’s elegant design allowing better performance for Comparable keys. By coincidence it mitigates the DoS attack too.
    - tialaramex 1y ago
      All the closed addressing strategies mean more allocator burden. In this case, each of these trees needs to grow separately. But with open addressing we can avoid that. Suppose we have 10'000 key->value pairs to store. A Swiss Table (a popular open addressed hash table design) will need space for 16384 contiguous pairs plus metadata†. So it'll do that allocation once, but with the closed addressing and trees you're paying to grow trees during insertion, you can't know which trees will grow and which are unused. You're correct that the Swiss Table will see collisions, but an attacker can't choose them, so they're rare. Each collision costs us a search step, but because the Java design incurs a pointer chase (to find the tree) for every lookup that's actually the same price [one memory fetch] as a single collision for the Swiss Table, yet most of the time the Swiss Table sees less than 1.0 collisions per lookup. † It's ensuring no more than 87.5% of the storage is used and then rounding up to a power of two because that means less work for each look-up step. 11429 would be enough space, but 16384 is the next largest power of two.
      - ivan_gammel 1y ago
        > But with open addressing we can avoid that. In risk management there exist several different approaches: accept, mitigate, transfer, avoid. What makes you think that this design must avoid certain risks? The developers of standard libraries like Java usually base their decisions on industry feedback, IIRC, Oracle team did have a lot of telemetry data and they are good engineers. Do you have any insights proving that their approach is wrong?
        tialaramex 1y ago
        > IIRC, Oracle team did have a lot of telemetry data and they are good engineers. Do you have any insights proving that their approach is wrong? Time's arrow Java is a Sun (Microsystems) language, Oracle just bought the entire company much later. So one problem here is that you're imagining this like it's a decision somebody made this century but it isn't, it's a decision from the early 1990s. Computer Science is a young discipline. In the early 1990s the Open Addressed hash tables do exist but it's very usual to write the simple chained hash table with Closed Addressing and as I understand it that's exactly what is provided in Java 1.0. This data structure is today known to be a bad idea, and I've criticised the choice in say C++ to standardize the exact same data structure (as std::unordered_map) in 2011. But that's twenty years later. At some point (I believe this century?) Java engineers went back and improved their hash table to use trees not chains, but they were constrained by the baked in hash, so they could not have picked radically modern data structures which require a different hash strategy without significant disruption. When I got a CS degree the Introspective sort was so new I'd have to have read bleeding edge research papers to know about it. Today it's very silly if you claim you provide a standard library sort and it's not at least as good as Introsort. The Swiss Table is from the 2010s, it's new enough that there's a Youtube video of them introducing this data structure, not because they did that specifically on purpose but because they're giving a talk and obviously all talks are filmed and uploaded to Youtube these days. Hopskotch hashing - another popular Open Addressing strategy - is from 2008. So, the question isn't "Is this the best idea today?" but only "Given what they knew 30+ years ago, was this reasonable back then?" and yeah, of course it was reasonable back then but that doesn't magically mean it's the best choice now. Edited: Added paragraph about the later choice to use trees.

ivan_gammel 1y ago

>I think makes the point quite clear: have you ever had a hashmap of HTTP header to values? Number of headers is limited by default to a relatively small value (10k), which in the worst case of hash collisions results in relatively fast lookup time due to tree-like implementation of HashMap.

tialaramex 1y ago
> due to tree-like implementation of HashMap. Exactly, this data structure has to provide counter-measures to cope with the fallout from this other poor choice. The fast data structures for this work don't need a tree here, they're open addressed instead, but this would mean they can't cope if your attacker can arbitrarily collide input, so, too bad.
- ivan_gammel 1y ago
  This data structure has tree-like fallback for hash collisions in general, which do happen regardless of hash algorithm by definition of what is hash. It is not a coping mechanism for a poor choice, it’s elegant design allowing better performance for Comparable keys. By coincidence it mitigates the DoS attack too.
  - tialaramex 1y ago
    All the closed addressing strategies mean more allocator burden. In this case, each of these trees needs to grow separately. But with open addressing we can avoid that. Suppose we have 10'000 key->value pairs to store. A Swiss Table (a popular open addressed hash table design) will need space for 16384 contiguous pairs plus metadata†. So it'll do that allocation once, but with the closed addressing and trees you're paying to grow trees during insertion, you can't know which trees will grow and which are unused. You're correct that the Swiss Table will see collisions, but an attacker can't choose them, so they're rare. Each collision costs us a search step, but because the Java design incurs a pointer chase (to find the tree) for every lookup that's actually the same price [one memory fetch] as a single collision for the Swiss Table, yet most of the time the Swiss Table sees less than 1.0 collisions per lookup. † It's ensuring no more than 87.5% of the storage is used and then rounding up to a power of two because that means less work for each look-up step. 11429 would be enough space, but 16384 is the next largest power of two.
    - ivan_gammel 1y ago
      > But with open addressing we can avoid that. In risk management there exist several different approaches: accept, mitigate, transfer, avoid. What makes you think that this design must avoid certain risks? The developers of standard libraries like Java usually base their decisions on industry feedback, IIRC, Oracle team did have a lot of telemetry data and they are good engineers. Do you have any insights proving that their approach is wrong?
      - tialaramex 1y ago
        > IIRC, Oracle team did have a lot of telemetry data and they are good engineers. Do you have any insights proving that their approach is wrong? Time's arrow Java is a Sun (Microsystems) language, Oracle just bought the entire company much later. So one problem here is that you're imagining this like it's a decision somebody made this century but it isn't, it's a decision from the early 1990s. Computer Science is a young discipline. In the early 1990s the Open Addressed hash tables do exist but it's very usual to write the simple chained hash table with Closed Addressing and as I understand it that's exactly what is provided in Java 1.0. This data structure is today known to be a bad idea, and I've criticised the choice in say C++ to standardize the exact same data structure (as std::unordered_map) in 2011. But that's twenty years later. At some point (I believe this century?) Java engineers went back and improved their hash table to use trees not chains, but they were constrained by the baked in hash, so they could not have picked radically modern data structures which require a different hash strategy without significant disruption. When I got a CS degree the Introspective sort was so new I'd have to have read bleeding edge research papers to know about it. Today it's very silly if you claim you provide a standard library sort and it's not at least as good as Introsort. The Swiss Table is from the 2010s, it's new enough that there's a Youtube video of them introducing this data structure, not because they did that specifically on purpose but because they're giving a talk and obviously all talks are filmed and uploaded to Youtube these days. Hopskotch hashing - another popular Open Addressing strategy - is from 2008. So, the question isn't "Is this the best idea today?" but only "Given what they knew 30+ years ago, was this reasonable back then?" and yeah, of course it was reasonable back then but that doesn't magically mean it's the best choice now. Edited: Added paragraph about the later choice to use trees.