Feature #20415
closedPrecompute literal String hash code during compilation
Description
I worked on a proof of concept with @etienne (Étienne Barrié) which I think has some potential, but I'm looking for feedback on what would be the best implementation.
The proof of concept is here: https://github.com/Shopify/ruby/pull/553
Idea¶
Most string literals are relatively short, hence embedded, and have some wasted bytes at the end of their slot. We could use that wasted space to store the string hash.
The goal being to make looking up a literal String key in a hash, as fast as a Symbol key. The goal isn't to memoize the hash code of all strings, but to only selectively precompute the hash code of literal strings
in the compiler. The compiler could even selectively do this when we literal string is used to lookup a hash (opt_aref
).
Here's the benchmark we used:
hash = 10.times.to_h do |i|
[i, i]
end
dyn_sym = "dynamic_symbol".to_sym
hash[:some_symbol] = 1
hash[dyn_sym] = 1
hash["small"] = 2
hash["frozen_string_literal"] = 2
Benchmark.ips do |x|
x.report("symbol") { hash[:some_symbol] }
x.report("dyn_symbol") { hash[:some_symbol] }
x.report("small_lit") { hash["small"] }
x.report("frozen_lit") { hash["frozen_string_literal"] }
x.compare!(order: :baseline)
end
On Ruby 3.3.0, looking up a String key is a bit slower based on the key size:
Calculating -------------------------------------
symbol 24.175M (± 1.7%) i/s - 122.002M in 5.048306s
dyn_symbol 24.345M (± 1.6%) i/s - 122.019M in 5.013400s
small_lit 21.252M (± 2.1%) i/s - 107.744M in 5.072042s
frozen_lit 20.095M (± 1.3%) i/s - 100.489M in 5.001681s
Comparison:
symbol: 24174848.1 i/s
dyn_symbol: 24345476.9 i/s - same-ish: difference falls within error
small_lit: 21252403.2 i/s - 1.14x slower
frozen_lit: 20094766.0 i/s - 1.20x slower
With the proof of concept performance is pretty much identical:
Calculating -------------------------------------
symbol 23.528M (± 6.9%) i/s - 117.584M in 5.033231s
dyn_symbol 23.777M (± 4.7%) i/s - 120.231M in 5.071734s
small_lit 23.066M (± 2.9%) i/s - 115.376M in 5.006947s
frozen_lit 22.729M (± 1.1%) i/s - 115.693M in 5.090700s
Comparison:
symbol: 23527823.6 i/s
dyn_symbol: 23776757.8 i/s - same-ish: difference falls within error
small_lit: 23065535.3 i/s - same-ish: difference falls within error
frozen_lit: 22729351.6 i/s - same-ish: difference falls within error
Possible implementation¶
The reason I'm opening this issue early is to get feedback on which would be the best implementation.
Store hashcode after the string terminator¶
Right now the proof of concept simply stores the st_index_t
after the string null terminator, and only when the string is embedded and as enough left over space.
Strings with a precomputed hash are marked with an user flag.
Pros:
- Very simple implementation, no need to change a lot of code, and very easy to strip out if we want to.
- Doesn't use any extra memory. If the string doesn't have enough left over bytes, the optimization simply isn't applied.
- The worst case overhead is a single
FL_TEST_RAW
inrb_str_hash
.
Cons:
- The optimization won't apply to certain string sizes. e.g. strings between
17
and23
bytes won't have a precomputed hash code. - Extracting the hash code requires some not so nice pointer arithmetic.
Create another RString union¶
Another possibility would be to add another entry in the RString
struct union, such as we'd have:
struct RString {
struct RBasic basic;
long len;
union {
// ... existing members
struct {
st_index_t hash;
char ary[1];
} embded_literal;
} as;
};
Pros:
- The optimization can now be applied to all string sizes.
- The hashcode is always at the same offset and properly aligned.
Cons:
- Some strings would be bumped by one slot size, so would use marginally more memory.
- Complexify the code base more, need to modify a lot more string related code (e.g.
RSTRING_PTR
and many others) - When compiling such string, if an equal string already exists in the
fstring
table, we'd need to replace it, we can't just mutate it in place to add the hashcode.
Prior art¶
[Feature #15331] is somewhat similar in its idea, but it does it lazily for all strings. Here it's much simpler because limited to string literals, which are the ones likely to be used as Hash keys, and the overhead is on compilation, not runtime (aside from a single flag check). So I think most of the caveats of that original implementation don't apply here.
Updated by Eregon (Benoit Daloze) 7 months ago · Edited
FWIW TruffleRuby already does this, since frozen string literals need to be deduplicated, the hash needs to be computed, so might as well save it while doing so (the only downside being footprint).
truffleruby 24.0.0, like ruby 3.2.2, Oracle GraalVM Native [x86_64-linux]
Calculating -------------------------------------
symbol 107.376M (± 0.7%) i/s (9.31 ns/i) - 541.038M in 5.038971s
dyn_symbol 106.989M (± 0.7%) i/s (9.35 ns/i) - 543.771M in 5.082698s
small_lit 88.014M (± 0.6%) i/s (11.36 ns/i) - 442.996M in 5.033433s
frozen_lit 88.174M (± 0.3%) i/s (11.34 ns/i) - 444.293M in 5.038895s
Comparison:
symbol: 107376115.9 i/s
dyn_symbol: 106989494.3 i/s - same-ish: difference falls within error
frozen_lit: 88173794.6 i/s - 1.22x slower
small_lit: 88013579.7 i/s - 1.22x slower
---
ruby 3.3.0 (2023-12-25 revision 5124f9ac75) [x86_64-linux]
Calculating -------------------------------------
symbol 17.387M (± 0.9%) i/s (57.51 ns/i) - 88.291M in 5.078305s
dyn_symbol 17.416M (± 1.0%) i/s (57.42 ns/i) - 87.555M in 5.027943s
small_lit 14.024M (± 2.6%) i/s (71.30 ns/i) - 70.153M in 5.005519s
frozen_lit 13.891M (± 2.1%) i/s (71.99 ns/i) - 70.870M in 5.104221s
Comparison:
symbol: 17387348.9 i/s
dyn_symbol: 17415643.8 i/s - same-ish: difference falls within error
small_lit: 14024442.1 i/s - 1.24x slower
frozen_lit: 13890537.2 i/s - 1.25x slower
Strings are still slower than Symbol keys, I suspect because eql?
is quite a bit more expensive for Strings.
Even if two Strings are interned it's not correct to compare them by identity, because they could still be eql?
with the same bytes but different encodings. That case does not exist for Symbols.
Updated by byroot (Jean Boussier) 7 months ago
if two Strings are interned it's not correct to compare them by identity, because they could still be eql? with the same bytes but different encodings.
I'm not sure I follow. Surely x.eql?(x)
can use an identity check as a shortcut. If both refs are equal, you can immediately return true. Of course if refs aren't equal, then yes you need to do a full string comparison.
So I'm a bit surprised to see TruffleRuby doesn't have the same performance on that benchmark.
Updated by Eregon (Benoit Daloze) 7 months ago
byroot (Jean Boussier) wrote in #note-2:
I'm not sure I follow. Surely
x.eql?(x)
can use an identity check as a shortcut. If both refs are equal, you can immediately return true. Of course if refs aren't equal, then yes you need to do a full string comparison.
Yes, if they are the same object of course they are eql?, but if they are not the same object they can still be eql?, even if both are interned/fstring/frozen string literals.
So I'm a bit surprised to see TruffleRuby doesn't have the same performance on that benchmark.
For this benchmark, whether eql?
is called only with equal keys depends on the Hash representation, and whether they are not two keys falling in the same bucket.
For this benchmark eql?
should only be called with equal keys on TruffleRuby (due to using a buckets representation), and not the case on CRuby (due to using an array instead of buckets until 8 pairs IIRC).
I'll try to take a look at some compiler graphs to have more definite insights on it.
Updated by byroot (Jean Boussier) 7 months ago
Yes, if they are the same object of course they are eql?, but if they are not the same object they can still be eql?, even if both are interned/fstring/frozen string literals.
That was my understanding.
due to using an array instead of buckets until 8 pairs IIRC
Yes, that is why my benchmark use a larger Hash.
Updated by etienne (Étienne Barrié) 7 months ago
We pushed a cleaned-up PR at https://github.com/ruby/ruby/pull/10596.
We settled on storing the hash code after the terminator as it prevents having to add yet another union in RString that would have a general performance impact and complexify the entire code base. And we decided against storing it at the end of the object slot to avoid having to access the slot size which is slower.
compare-ruby: ruby 3.4.0dev (2024-04-22T06:32:21Z main f77618c1fa) [arm64-darwin23]
built-ruby: ruby 3.4.0dev (2024-04-22T10:13:03Z interned-string-ha.. 8a1a32331b) [arm64-darwin23]
last_commit=Precompute embedded string literals hash code
| |compare-ruby|built-ruby|
|:-----------|-----------:|---------:|
|symbol | 39.275M| 39.753M|
| | -| 1.01x|
|dyn_symbol | 37.348M| 37.704M|
| | -| 1.01x|
|small_lit | 29.514M| 33.948M|
| | -| 1.15x|
|frozen_lit | 27.180M| 33.056M|
| | -| 1.22x|
|iseq_lit | 27.391M| 32.242M|
| | -| 1.18x|
Updated by shyouhei (Shyouhei Urabe) 6 months ago
The benchmark seems great. But I'm not yet sure if this is worth the hustle. Is using a string literal as a hash key very common? It would be much convincing to me if there are any non-micro benchmarks.
Updated by byroot (Jean Boussier) 6 months ago
Is using a string literal as a hash key very common?
It's quite common in a few places, e.g. Active Record attribute accessors generate code such as:
# frozen_string_literal: true
def title
_read_attribute("title")
end
(NB: this is in part historical because symbols used to be immortal, this could eventually migrate to symbols in the future).
Similarly, rack applications use a lot of string literal keys to look into the env
hash.
Then you have all the code that consume JSON APIs of various sort, etc.
It would be much convincing to me if there are any non-micro benchmarks.
Yes, we're working on it, @etienne (Étienne Barrié) has some yjit-bench
results, I'll ask him if they are ready.
Overall the difference on larger benchmark isn't huge (but visible IIRC), but I'd argue the patch is self-contained enough that it's worth merging. If the patch was very invasive I'd agree with you, but sometime to get to a 5% improvement you need 10 small 0.5%
improvements combined together, and I think this patch make sense in that context.
Updated by byroot (Jean Boussier) 6 months ago
So there's pretty much only two yjit-bench
benchmarks on which it makes a visible difference rack
and liquid-c
, both use string literal keys:
(ran 3 times to account for variance)
head: ruby 3.4.0dev (2024-05-09T10:37:25Z master 74c911dfa9) +YJIT [arm64-darwin23]
head-str-hashcode: ruby 3.4.0dev (2024-05-09T12:58:37Z interned-string-ha.. 44bc1f4c66) +YJIT [arm64-darwin23]
-------- --------- ---------- ---------------------- ---------- ------------------------- ----------------------
bench head (ms) stddev (%) head-str-hashcode (ms) stddev (%) head-str-hashcode 1st itr head/head-str-hashcode
liquid-c 19.4 4.1 18.8 1.6 1.11 1.03
rack 12.5 1.2 11.8 1.9 1.05 1.05
-------- --------- ---------- ---------------------- ---------- ------------------------- ----------------------
------
-------- --------- ---------- ---------------------- ---------- ------------------------- ----------------------
bench head (ms) stddev (%) head-str-hashcode (ms) stddev (%) head-str-hashcode 1st itr head/head-str-hashcode
liquid-c 19.4 1.0 19.0 1.7 0.98 1.02
rack 12.6 1.2 11.7 1.4 1.04 1.08
-------- --------- ---------- ---------------------- ---------- ------------------------- ----------------------
-------
-------- --------- ---------- ---------------------- ---------- ------------------------- ----------------------
bench head (ms) stddev (%) head-str-hashcode (ms) stddev (%) head-str-hashcode 1st itr head/head-str-hashcode
liquid-c 19.4 1.0 19.0 1.2 1.00 1.02
rack 12.7 13.4 12.0 1.4 1.06 1.05
-------- --------- ---------- ---------------------- ---------- ------------------------- ----------------------
On other benchmarks the difference isn't higher than standard deviation, so not conclusive.
Updated by byroot (Jean Boussier) 5 months ago
- Status changed from Open to Closed
This was discussed and accepted at the last developer meeting, so I merged it as 9e9f1d9301b05604d475573ddd18d6bf5185466c
.