Tiny Pointers | Hacker News Reader

Tiny Pointers

arxiv.org

163 pointslevzettelin35 comments

I looked a bit into this a few years back and found it quite interesting. Despite them calling them "Tiny Pointers" I would say it's closer to a open addressing hash map. You have a specific key, and then you can "allocate" an entry in the hash map. This gives you back a "pointer". You can then later use the original key and the pointer together to determine the index of the entry. There's also a slight chance that the allocation will fail (similar to a hash collision in a hash map). The "trick" here is that two different keys can end up having the exact same pointer (because we're always dereferencing with the key). This makes them more compact.

I was struggling a bit to come up with good use cases for it. Their examples are all around combining them with existing data structures and they show that the space complexity is smaller, but it wasn't completely clear to me how feasible this would actually be in practice.

I read the paper a few years ago and I agree that for such an incredible algorithmic improvement, it’s not trivial to find a use case, as you still need to maintain a separate (albeit algorithmically insignificant) lookup table. When I read the paper I (mistakenly) hoped it could be used for indexing on-disk structures without ever hitting the disk for things like B tree internal nodes. To get its sweet sweet algorithmic complexity, up its sleeve is once again (as I recall) the age old trick of rebuilding the structure when the size doubles, which makes it much less efficient than it sounds for most practical use cases. I suppose one good use case for this might be compressing database indexes, where you need to maintain a separate structure anyway and the space savings can be worth it.

I didn't invest a lot of time in it because when these papers show no practical applications I always assume that the constant factors are too high. Especially when there's also no specific values of the constant factors presented. It's still somewhere back in my mind to implement it one day to figure out the nitty gritty details.

Back of the envelope calculations from the abstract: An 8-bit tiny pointer would be sufficient to reference an array of size 2^2^2^3 ≈ 1e77 (≈atoms in the visible universe) with fullness 31/32 ≈ 97%. Or size 65536 and fullness 63/64. For 16-bit tiny pointers, there's no point in going bigger than the universe; fullness goes to 99.98%. Like you, I'd love to see such an example worked out.

It isn't hard to make a datastructure that indexes into itself. BDDs, for example, are often coded in this way. I did an admittedly poor job of one at https://taeric.github.io/trading-with-bdds.html, but I think it is enough to see the idea well enough.

This guy used that paper to create a new hash table which is much faster at high load that current implementations:

https://www.quantamagazine.org/undergraduate-upends-a-40-yea...

That paper was front page 2 days ago. It's the reason this paper is front page today. And while it is fantastic work, it has no obvious break-through application, as was discussed: https://news.ycombinator.com/item?id=43004955

Yeah, I posted about that algorithm on Nim Lang forum. Someone used deepseek to create a Nim version [1]. It seems the constant factor overhead is quite large. Well presuming deepseek implemented it correctly. There's obvious easy perf tweaks, but still seems to have high overhead.

1: https://forum.nim-lang.org/t/12679#78170

That's just called a hash map.

Didn't Python's compact dictionary implementation do this a decade ago?

"The dict type now uses a “compact” representation based on a proposal by Raymond Hettinger which was first implemented by PyPy. The memory usage of the new dict() is between 20% and 25% smaller compared to Python 3.5." -- https://docs.python.org/3.6/whatsnew/3.6.html#whatsnew36-com...

"Note, the sizeof(index) can be as small as a single byte for small dicts, two bytes for bigger dicts and up to sizeof(Py_ssize_t) for huge dict." -- https://mail.python.org/pipermail/python-dev/2012-December/1...

The "tiny pointers" are in the _make_index method in the proof of concept code. -- https://code.activestate.com/recipes/578375-proof-of-concept...

      @staticmethod
      def _make_index(n):
          'New sequence of indices using the smallest possible datatype'
          if n <= 2**7: return array.array('b', [FREE]) * n       # signed char
          if n <= 2**15: return array.array('h', [FREE]) * n      # signed short
          if n <= 2**31: return array.array('l', [FREE]) * n      # signed long
          return [FREE] * n

The logic is still present today in CPython. -- https://raw.githubusercontent.com/python/cpython/3e222e3a159...

  dk_indices is actual hashtable.  It holds index in entries, or DKIX_EMPTY(-1)
  or DKIX_DUMMY(-2).
  Size of indices is dk_size.  Type of each index in indices varies with dk_size:

  * int8  for          dk_size <= 128
  * int16 for 256   <= dk_size <= 2**15
  * int32 for 2**16 <= dk_size <= 2**31
  * int64 for 2**32 <= dk_size

No, although it's not completely unrelated.

These are the "traditional log n-bit pointers" that tiny pointers improve on.

The "compact dictionary" representation you're talking about not always using 64-bit numbers, but rather using log(n)-bit values when you have n entries (i.e. using 8-bit values when there's only <= 128 entries). The paper linked here talks about pointers which uses less than log(n)-bits for n entries.

Note that this is not the paper by Krapivin that https://news.ycombinator.com/item?id=43002511 is about.

It’s the paper mentioned in the same article and one of the author’s is his professors.

Yes.

> How large do the pointers need to be? The natural answer is that each pointer uses log nbits. However, the fact that each pointer has a distinct owner makes it possible to compress the pointers to o(log n) bits.

What if you have to debug the whole situation, such that you don't always know who is the owner of a pointer you are looking at?

> A user k can call Allocate(k) in order to get a tiny pointer p; they can dereference the tiny pointer pby computing a function Dereference(k,p) whose value depends only on k, p, and random bits; and they can free a tiny pointer p by calling a function Free(k,p).

That is tantamount ot saying that the pointer is not actually p but the tuple <k, p>, and so its size consists of the number of bits in k, the number of bits in p plus an indication of where the division between these bits lie: where k ends and p begins.

We can abbreviate <k, p> to p in contexts where k can be implicitly understood.

Here's a somewhat related post where I argue that logic that _can_ have small pointers has lower entropy and is more optimal as bayesian model of the domain: https://benoitessiambre.com/entropy.html

Can someone ELI5?

Most likely related with this post: https://news.ycombinator.com/item?id=43002511

Correct, this is about Yao's conjecture and the implications for other hash implementations.

I could be wrong, but I think the easiest way to think of this is to consider how much extra memory programs took when compiled with 64 bit pointers over 32 bit ones. Suddenly every pointer takes double the memory. Which, sure, isn't a huge deal if you don't have a lot of memory allocations. But, if you do, it can add up.

Places it would likely impact more than you'd realize is in higher level language arrays where each item in the array is a pointer. For similar reasons, many datastructures can be coded such that each "pointer" is an array index instead.

So, extending all of that, what if you could make your pointers even smaller than 32 bits? If you know the addressable need for where the pointer is used, there is no reason you can't go smaller.

Yet these aren't offsets from an address, that is, array indexes?

But the idea is still the same? You can make "smart pointers" that are attached to arenas or whatever you want to call them. On those, the addressable size of the pointer is confined to how large the arena is.

Eh, you underestimate my naivety. To rephrase: are these just array indexes, then?

I mean I presume it's something cleverer than just "let's bit-pack our indexes to save space".

..."and make it dynamic" (arenas)

Oh wait, variable-size tiny pointers. Yes. Now it's getting satisfyingly complicated. Maybe like how UTF-8 works?

I mean, in a way, a pointer is just an array index into heap space? Such that, I think this is accurate? I'd drop the "just", though. They can be thought of that way, but the idea is how to keep the programmer from having to do that manually.

To rephrase, this is how to mechanically bit-pack indexes to save space. Doing it in a pointer means you are likely wanting to share these pointers with other parts of the code, so you also need to have them be self contained. Naive way would be for each pointer to be a tuple identifying arena and having its offset. But there is still a lot of effort that has to be done for that to work well. This paper looks at that effort and gives a pass at how worthwhile it is.

> To rephrase: are these just array indexes, then? I mean I presume it's something cleverer than just "let's bit-pack our indexes to save space".

The pointers can be dereferenced into an array index (when given the key), but they are smaller in size than a "bit-packed index", although I'm not quite sure what you mean by this. If you want to to represent an index into an array of size `n` there's no representation where all indexes take less than log(n). You can make some of them smaller, but then you make other bigger (like in UTF-8). The Tiny Pointers are all smaller than log(n) (and some of them would be exactly equal, despite representing different indexes!), but the only way of determining the actual index is to dereference it together with original key that was used to create it.

How is this novel?

This is like using a fixed sharding scheme plus delta-compression for indices relative to shard addresses plus varint encoding the deltas.

After reading more, it seems the novelty is in the two specific schemes that use these techniques to store data with high probability into tables of extremely high load factors with bounded pointer sizes, which are more complex than simple array indices due to having to resolve which fallback table is storing the key-value pair.

One scheme proves tiny pointer size bounds for fixed length tiny pointers. The other proves bounds for variable length pointers.

Zoomers invented indexes.

Programmer made program. What's new! When will they make magic orbs?!

Every author on this paper is a millennial or gen x.

Is there a peer reviewed version of this or does Hacker News exclusively post non peer reviewed works?

https://dl.acm.org/doi/10.1145/3700594

Why wasn't this one originally linked?

Reply on news.ycombinator.com