Optimize byte pair merge for really big tokens (40x faster for a 2500 token word) #239

l0rinc · 2024-01-15T21:27:07Z

Continuing the optimizations started in #237 and #234, migrated from knuddelsgmbh/jtokkit#75, knuddelsgmbh/jtokkit#76, knuddelsgmbh/jtokkit#77.

This commit is mainly meant to address the issue of really big tokens spiraling out of control, see: #195

The original byte pair merge algorithm diverges quickly for longer character sequences in a superlinear way - e.g. a 20_000 character word (having 2500 tokens) can take several seconds to be tokenized.

Or on https://platform.openai.com/tokenizer:

The new algorithm scales so well that it could theoretically process the whole text in a single byte-pair-merge loop without any regex splitting (though it would need a different token set to be optimal since it produces slightly different results, mostly whitespaces, though - and it also consumes a lot more memory and is slower that the current one):

The new algorithm does the minimum search logarithmically and duplicates in constant time, but has a higher setup cost, so we're only using it for extreme cases (if the piece given by the regex is > 500 characters):

The benchmarking was done step-by-step in the Java clone and here retested in the way described in #234

110 multilingual books + some source codes + some big token files:

Before:

num_threads: 1, num_bytes: 98379144
tiktoken 	7,891,071 bytes / s
tiktoken 	7,927,160 bytes / s
tiktoken 	7,936,709 bytes / s
tiktoken 	7,912,032 bytes / s
tiktoken 	7,928,872 bytes / s

After:

num_threads: 1, num_bytes: 98379144
tiktoken 	9,494,719 bytes / s
tiktoken 	9,547,619 bytes / s
tiktoken 	9,525,891 bytes / s
tiktoken 	9,506,282 bytes / s
tiktoken 	9,563,429 bytes / s

From which only the big token files (the purpose of this PR):

Before:

num_threads: 1, num_bytes: 392784
tiktoken 	195,554 bytes / s
tiktoken 	195,998 bytes / s
tiktoken 	196,051 bytes / s
tiktoken 	194,724 bytes / s
tiktoken 	195,748 bytes / s

After:

num_threads: 1, num_bytes: 392784
tiktoken 	8,604,360 bytes / s
tiktoken 	8,628,191 bytes / s
tiktoken 	8,561,823 bytes / s
tiktoken 	8,675,756 bytes / s
tiktoken 	8,618,370 bytes / s

i.e. 40x faster for 20k character words.

And if we combine this with the previous regex optimizations, we're getting the following for the 110 books + sources + big tokens case:

num_threads: 1, num_bytes: 98379144
tiktoken 	12,043,907 bytes / s
tiktoken 	12,153,199 bytes / s
tiktoken 	12,173,271 bytes / s
tiktoken 	12,085,368 bytes / s
tiktoken 	12,147,123 bytes / s

i.e. 50% faster on average after all optimizations.

I recommend reviewing commit-by-commit:

l0rinc · 2024-01-15T21:27:35Z

tests/test_encoding.py

@@ -61,13 +60,16 @@ def test_simple_regex():
 def test_basic_encode():
    enc = tiktoken.get_encoding("r50k_base")
    assert enc.encode("hello world") == [31373, 995]
+    assert enc.encode("a" * 1000) == [24794] * 250


to cover the big encoder as well

src/lib.rs

l0rinc · 2024-01-15T21:33:38Z

src/lib.rs

@@ -15,9 +16,22 @@ use rustc_hash::FxHashMap as HashMap;

 type Rank = u32;

+const LARGE_ENCODER_CHARACTER_LIMIT: usize = 500;


in the Java version this was controlled by an environmental variable, which enabled us to run all tests against both implementations - should I do it here as well?

l0rinc · 2024-01-15T21:36:12Z

src/lib.rs

+            .unwrap_or(&Rank::MAX)
+    };
+
+    let mut rank_indexes = BTreeMap::<Rank, BTreeSet<usize>>::new();


grouped by rank, the values ordered by index (basically a LinkedHashSet inside)

l0rinc · 2024-01-15T21:36:51Z

src/lib.rs

+    };
+
+    let mut rank_indexes = BTreeMap::<Rank, BTreeSet<usize>>::new();
+    let mut index_rank = vec![Rank::MAX; piece.len() + 1];


mutations seemed easier this way, compared to creating a struct with index/rank/prev/next - especially in Rust

l0rinc · 2024-01-15T21:38:05Z

src/lib.rs

+    let mut token_count = piece.len();
+    while token_count > 2 && rank_indexes.len() > 1 {
+        let mut skip_next = false;
+        if let Some((_, nodes)) = rank_indexes.pop_first() {


next min is popped off in logarithmic time instead of linearly

l0rinc · 2024-01-15T21:38:25Z

src/lib.rs

+    while token_count > 2 && rank_indexes.len() > 1 {
+        let mut skip_next = false;
+        if let Some((_, nodes)) = rank_indexes.pop_first() {
+            for &min_node in &nodes {


duplicates are processed in bulk (since the next min is strictly greater than equal), no need to remove them one-by-one

l0rinc · 2024-01-15T21:39:01Z

src/lib.rs

+        if let Some((_, nodes)) = rank_indexes.pop_first() {
+            for &min_node in &nodes {
+                if skip_next {
+                    skip_next = false;


when merging neighboring elements with the same ranks

l0rinc · 2024-01-26T14:03:10Z

src/lib.rs

-        }
+) -> Vec<(usize, Rank)> {
+    if piece.len() < LARGE_ENCODER_CHARACTER_LIMIT {
+        _byte_pair_merge_small(ranks, piece) // Quadratic, but lightweight


quadtratic algo is usually faster for very small words - which is always the case for natural language, but e.g. DNA sequences or a DOS attack can be avoided by switching to the linearithmic algo

src/lib.rs

l0rinc · 2024-01-30T12:16:43Z

src/lib.rs

+                let prev_node = index_prev[min_node];
+                let next_node = index_next[min_node];
+                let next_next_node = index_next[next_node];
+                let next_next_next_node = index_next[next_next_node];


We're keeping track of the order of characters inside the rank-balanced tree, providing logarithmic access to the minimum rank and constant access to the previous/next

hauntsaninja · 2024-02-09T07:43:57Z

This is great!

Thanks for keeping a simple to follow history. Most of the commits here are straightforward, I've separated them into different PRs (I've preserved authorship information, but let me know if you'd prefer to re-open them yourself)

Store tokens in u32 instead of usize #251 (I got a little excited and merged this one before I realised I should ask you, hope that's alright)
Avoid calling byte_pair_encode for existing tokens #252
Inline custom mapping function in _byte_pair_merge #253

I'm looking at 8f5dd7d and d24b67b now...

l0rinc · 2024-02-09T09:08:30Z

Thanks a lot for the thorough review, Shantanu.
Let me know if you need any help in speeding up the process! :)

After merging you may want to update the benchmark results in the readme.

Based on suggestion in #239 (specifically 8f5dd7d) Like that commit, this: - Does the init in a single loop and saves a loop if there are no merges - Simplifies get_rank and no longer uses it in init (so you don't need multiple skip values) Unlike that commit: - We drop optimisations enabled by ignoring single tokens. These didn't show any benefit on benchmarks for me (this makes sense given typical piece sizes, but let me know if that's unexpected!). Given this, I opted for the simpler version. - I preserve some of the comments from the original that I think are still useful Co-authored-by: @paplorinc --------- Co-authored-by: Lőrinc Pap <1841944+paplorinc@users.noreply.github.com>

l0rinc · 2024-02-11T13:46:24Z

src/lib.rs

+        }
+    }
+
+    successors(Some(0), |&n| index_next.get(n).filter(|&&x| x != usize::MAX).copied())


iterate until there's a valid rank

l0rinc · 2024-02-11T13:49:58Z

@hauntsaninja, I've rebased this PR, removing the merged commits and adjusting the result a bit based on your previous preferences.
Hope it helps. Feel free to push on top of this PR or open a different one, whichever's easier.

l0rinc · 2024-02-11T13:53:13Z

src/lib.rs

+    let mut index_next = vec![usize::MAX; piece.len() + 1];
+
+    let get_rank = |start_idx: usize, end_idx: usize| -> Rank {
+        *piece.get(start_idx..end_idx)


when end_idx is out of bounds we're defaulting to Rank::MAX

We're storing the ranks in a sorted tree of sorted (or linked) trees. Getting the minimum rank is logarithmic and each subsequent occurrence is constant time. To know the previous and next indexes (and the corresponding ranks), we're storing them in arrays (the keys are the indexes). We're updating each after finding the minimum via the tree. We're iterating duplicates without removing them one-by-one, but if they are neighbors, we're skipping them manually.

l0rinc · 2024-02-11T16:05:21Z

src/lib.rs

+                let min_rank = index_rank[min_node];
+
+                let prev_node = index_prev[min_node];
+                let next_node = index_next[min_node];


getting next and previous requires lookups now

vvolhejn · 2024-04-04T14:18:27Z

Hi! Any updates on this PR? It'd be great to have this 🙏 @hauntsaninja

aldanor · 2024-08-28T13:05:56Z

src/lib.rs

+
+                let prev_node = index_prev[min_node];
+                let next_node = index_next[min_node];
+                let next_next_node = index_next[next_node];


@l0rinc I think these lines would panic (out-of-range) if your min_node is close to an end, how do you know you have 3 nodes to the right of it? Your last node's next will be a usize::MAX.

Nevermind, I'm misreading this. The only one that can be "none" is the next_next_next_node; but then it's usize::MAX and the .get() in get_rank() will cover this and return Rank::MAX.

Based on suggestion in openai#239 (specifically openai/tiktoken@8f5dd7d) Like that commit, this: - Does the init in a single loop and saves a loop if there are no merges - Simplifies get_rank and no longer uses it in init (so you don't need multiple skip values) Unlike that commit: - We drop optimisations enabled by ignoring single tokens. These didn't show any benefit on benchmarks for me (this makes sense given typical piece sizes, but let me know if that's unexpected!). Given this, I opted for the simpler version. - I preserve some of the comments from the original that I think are still useful Co-authored-by: @paplorinc --------- Co-authored-by: Lőrinc Pap <1841944+paplorinc@users.noreply.github.com>

l0rinc commented Jan 15, 2024

View reviewed changes