1657559 - Add a cache to fix perf cliffs when atomizing the same string(s) repeatedly

Status: RESOLVED FIXED

(Core :: JavaScript Engine, task, P3)

Description

[Jan de Mooij [:jandem]]

Looking at the Reddit issue in bug 1654087, I noticed we were repeatedly atomizing some strings with length 1897. This ends up hashing the same characters over and over and that's pretty slow.

To fix this I have a patch that adds a StringToAtom cache (for strings with length >= 30) that's purged on GC. The cache is also used to de-duplicate nursery strings with the corresponding atom when tenuring, saving some memory and future atomization costs. (The string-deduplication code we have for nursery GC has a length limit, but that doesn't apply to this because we already did the expensive hashing anyway.)

I tested this on some popular websites and the cache has more hits than misses. For the micro-benchmark below, I get:

before: 2480 ms
after:    30 ms

function f() {
  var keys = [];
  var obj = {};
  for (var i = 0; i < 1000; i++) {
    var str = String.fromCharCode(i % 256).repeat(1897);
    keys.push(str);
    obj[str] = i;
  }
  var t = dateNow();
  var res = 0;
  for (var i = 0; i < 1000; i++) {
    for (var j = 0; j < keys.length; j++) {
      res = obj[keys[j]];
    }
  }
  print(dateNow() - t);
  print(res);
}
f();

Comment 1

[Jan de Mooij [:jandem]]

Attachment: Bug 1657559 - Add StringToAtomCache to fix perf cliffs when atomizing strings repeatedly. r?sfink!,jonco!

This improves performance a lot when a long string is atomized more than once. This
is happening on Reddit (strings with length 1897).

The cache is also used to de-duplicate strings during nursery GC before it's purged.
This handles some cases the existing string deduplication doesn't handle and if a
string is in the StringToAtomCache it should be faster.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/4acbbbe1f341
Add StringToAtomCache to fix perf cliffs when atomizing strings repeatedly. r=sfink,jonco

Comment 3

[Atila Butkovits]

https://hg.mozilla.org/mozilla-central/rev/4acbbbe1f341