4 ms·

Can someone explain the reasoning for using a HAMT here? I have played around with hashtables quite a bit, but altough HAMT's are cool and a lot more memory eff

Can someone explain the reasoning for using a HAMT here? I have played around with hashtables quite a bit, but altough HAMT's are cool and a lot more memory efficient, they are likely not faster and will at some point suffer from more severe memory fragmentation than hashtables. Is the concern speed or memory? Because speed would kind of seem strange, since after changing to the new architecture, they should see a strong decrease in messages being exchanged anyways, right? Or are go's maps inefficient? Genuinely curious

fsloth 6y ago
HAMT is excellent for a persistent dictionary. Not persistent as in write-to-disk, but as in immutable-yet-small.
- riwsky 6y ago
  It is not clear from the article that the author needed a persistent data structure. It’s also, frankly, not clear they‘d benefit from using an HAMT at all: the only measurements they present are of their problem domain, not of any implementations—and many of the asymptotics they cite are also true of hashmaps. If their initial hashmap approach did have O(N) inserts because they copied the whole map for “immutability” every time, then sure, HAMTs will beat the pants off of it—but did it?
- benibela 6y ago
  Btw, can it be used to build a persistent dictionary that keeps the key in insertion-order?
  - masklinn 6y ago
    Sure. Map the HAMT to an index into a persistent vector or something e.g. Scala's VectorMap and TreeSeqMap. You could also reuse the old "linked list map" idea, but go through the keys every time e.g. whenever you add a value to the map, add a triple of `(Some(last_key), value, None)` and update the last keys value such that the third item becomes the key of the new item.
    - benibela 6y ago
      >Sure. Map the HAMT to an index into a persistent vector or something e.g. Scala's VectorMap and TreeSeqMap I do not have a persistent vector, only a HAMT. Perhaps the HAMT becomes a standard trie if one sets hashcode(i) = i, and then it can be enumerated in ascending key order? But the double-lookup in two data structures is unnecessary slow. The first HAMT could store (Index, Value) pairs and only use the "vector" for enumeration. But I was hoping for a single data structure that can do lookups and ordered enumerations. >. You could also reuse the old "linked list map" idea, but go through the keys every time e.g. whenever you add a value to the map, add a triple of `(Some(last_key), value, None)` and update the last keys value such that the third item becomes the key of the new item. I have been using a linked list as sole storage so far. The lookup was O(n), which was fast enough with my data, but I wanted to improve it to be more scalable. But enumerating the list is in reverse, is it not? Going from the newest key to the oldest key. I want it to go from the oldest to the newest key. In any case it should be better to store pointers to HAMT nodes rather than keys.
      - masklinn 6y ago
        > I do not have a persistent vector, only a HAMT. Why not get a persistent vector? > But the double-lookup in two data structures is unnecessary slow. Not necessarily. You'd have 2xlog(n) and because you wouldn't be storing the data in the HAMT it could be denser / wider and thus better fit in cache. At least that's what happened with Raymond Hettinger's (non-persistent) naturally ordered dict: rather than a sparse array of buckets, the hashmap is a sparse array of indices and a dense array of buckets leading to much better memory density (even more so with an adaptive sparse array where the stored indices can be u8, u16 or usize depending on the number of items: most maps will only need a sparse array of u8 leading to even better memory behaviour for the most problematic part). > But enumerating the list is in reverse, is it not? Going from the newest key to the oldest key. I want it to go from the oldest to the newest key. Not if you have a doubly linked list, but that increases your costs in storage and update complexity > In any case it should be better to store pointers to HAMT nodes rather than keys. If you're implementing the entire datastructure then yes, I was rather assuming you were on the outside with an existing HAMT.
jsdavo 6y ago
> they are likely not faster If they are cache friendly then they can be very fast. The papers by Phil Bagwell (may he rest in peace) are very rich. Lots of implementation detail. Check them out!
masklinn 6y ago
HAMT are efficient for persistent data structures, compared to more usual trees. They’re slower and less efficient than a mutable hashmap, but way more so if you need to keep the “old revisions” around.

fsloth 6y ago

HAMT is excellent for a persistent dictionary. Not persistent as in write-to-disk, but as in immutable-yet-small.

riwsky 6y ago
It is not clear from the article that the author needed a persistent data structure. It’s also, frankly, not clear they‘d benefit from using an HAMT at all: the only measurements they present are of their problem domain, not of any implementations—and many of the asymptotics they cite are also true of hashmaps. If their initial hashmap approach did have O(N) inserts because they copied the whole map for “immutability” every time, then sure, HAMTs will beat the pants off of it—but did it?
benibela 6y ago
Btw, can it be used to build a persistent dictionary that keeps the key in insertion-order?
- masklinn 6y ago
  Sure. Map the HAMT to an index into a persistent vector or something e.g. Scala's VectorMap and TreeSeqMap. You could also reuse the old "linked list map" idea, but go through the keys every time e.g. whenever you add a value to the map, add a triple of `(Some(last_key), value, None)` and update the last keys value such that the third item becomes the key of the new item.
  - benibela 6y ago
    >Sure. Map the HAMT to an index into a persistent vector or something e.g. Scala's VectorMap and TreeSeqMap I do not have a persistent vector, only a HAMT. Perhaps the HAMT becomes a standard trie if one sets hashcode(i) = i, and then it can be enumerated in ascending key order? But the double-lookup in two data structures is unnecessary slow. The first HAMT could store (Index, Value) pairs and only use the "vector" for enumeration. But I was hoping for a single data structure that can do lookups and ordered enumerations. >. You could also reuse the old "linked list map" idea, but go through the keys every time e.g. whenever you add a value to the map, add a triple of `(Some(last_key), value, None)` and update the last keys value such that the third item becomes the key of the new item. I have been using a linked list as sole storage so far. The lookup was O(n), which was fast enough with my data, but I wanted to improve it to be more scalable. But enumerating the list is in reverse, is it not? Going from the newest key to the oldest key. I want it to go from the oldest to the newest key. In any case it should be better to store pointers to HAMT nodes rather than keys.
    - masklinn 6y ago
      > I do not have a persistent vector, only a HAMT. Why not get a persistent vector? > But the double-lookup in two data structures is unnecessary slow. Not necessarily. You'd have 2xlog(n) and because you wouldn't be storing the data in the HAMT it could be denser / wider and thus better fit in cache. At least that's what happened with Raymond Hettinger's (non-persistent) naturally ordered dict: rather than a sparse array of buckets, the hashmap is a sparse array of indices and a dense array of buckets leading to much better memory density (even more so with an adaptive sparse array where the stored indices can be u8, u16 or usize depending on the number of items: most maps will only need a sparse array of u8 leading to even better memory behaviour for the most problematic part). > But enumerating the list is in reverse, is it not? Going from the newest key to the oldest key. I want it to go from the oldest to the newest key. Not if you have a doubly linked list, but that increases your costs in storage and update complexity > In any case it should be better to store pointers to HAMT nodes rather than keys. If you're implementing the entire datastructure then yes, I was rather assuming you were on the outside with an existing HAMT.

jsdavo 6y ago

> they are likely not faster If they are cache friendly then they can be very fast. The papers by Phil Bagwell (may he rest in peace) are very rich. Lots of implementation detail. Check them out!

masklinn 6y ago

HAMT are efficient for persistent data structures, compared to more usual trees. They’re slower and less efficient than a mutable hashmap, but way more so if you need to keep the “old revisions” around.