[ruby-core:91468] [Ruby trunk Feature#15331] [PATCH] Faster hashing for short string literals

Issue #15331 has been updated by matz (Yukihiro Matsumoto).

Status changed from Open to Closed

@nobu benchmarked the result from the patch, and it does not improve the performance as we expected. Probably when the hash value is memoized by this patch, the string is short by definition, so the cost of hash value calculation is not big at all. So (even though it doesn't harm the performance at all), I don't think it's worth adding.

Matz.


----------------------------------------
Feature #15331: [PATCH] Faster hashing for short string literals
https://bugs.ruby-lang.org/issues/15331#change-76729

* Author: alanwu (Alan Wu)
* Status: Closed
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
## Background

It's common for applications to use string literals as hash keys, especially
for applications that work with YAML or JSON:

```
paylod['name']
```

At the moment (r65895), hash lookups using a string key is about 30% slower
than using a symbol key.

## Proposal

We memoize the hash code for short fstrings. There is extra, currently unused
space at the end of the RString struct we can use to store the hash code.
The unique nature of fstrings makes it so that every user of each fstring benefit
from the memoized hash.

## Evaluation

The included benchmark hash_aref_fstr.rb is about 20% faster with this optimization.
hash_aref_long_str.rb shows that for long strings which we cannot memoize, there is
no visible performance penalty.

vm2_freezestring.yml is also not visibly slower after this optimization.

I have also attached a bechmark (string_key_vs_symbol_key.rb) that compares hash
lookups with symbols against hash lookups with strings. With this optimization, the
gap in performance is smaller. (10% slower post patch vs 30% slower on trunk)

---Files--------------------------------
string_key_vs_symbol_key.rb (594 Bytes)
0001-Hash-code-memoization-for-short-fstrings.patch (3.62 KB)
0001-Hash-code-memoization-for-short-fstrings-v2.patch (4.29 KB)
0001-Hash-code-memoization-for-short-fstrings-v3.patch (4.36 KB)
0001-Hash-code-memoization-for-short-fstrings-v4.patch (4.31 KB)
0001-Hash-code-memoization-for-short-fstrings-v5.patch (12.9 KB)
0001-Add-st_update_with_hash.patch (3.09 KB)
0002-Hash-code-memoization-for-short-fstrings.patch (10 KB)


-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>