Awesome!
What are the pros/cons to mozilla::Hash{Map,Set} vs
std::unordered_{map,set}? I'm assuming perf is the main draw? Do we
have a data on this too?
On Mon, Aug 13, 2018 at 10:44 PM, Nicholas Nethercote
<n.netherc...@gmail.com> wrote:
> Hi,
>
> I recently moved Spidermonkey's js::Hash{Map,Set} classes from
> js/public/HashTable.h into mfbt/HashTable.h and renamed them as
> mozilla::Hash{Map,Set}. They can now be used throughout Gecko.
>
> (js/public/HashTable.h still exists in order to provide renamings of
> mozilla::Hash{Map,Set} as js::Hash{Map,Set}.)
>
> Why might you use mozilla::Hash{Map,Set} instead of PLDHashTable (or
> nsTHashtable and other descendants)?
>
> - Better API: these types provide proper HashMap and HashSet instances, and
> (in my opinion) are easier to use.
>
> - Speed: the microbenchmark in xpcom/rust/gtest/bench-collections/Bench.cpp
> shows that mozilla::HashSet is 2x or more faster than PLDHashTable. E.g.
> compare "MozHash" against "PLDHash" in this graph:
>
> https://treeherder.mozilla.org/perf.html#/graphs?timerange=604800&series=mozilla-central,1730159,1,6&series=mozilla-central,1730162,1,6&series=mozilla-central,1730164,1,6&series=mozilla-central,1732092,1,6&series=mozilla-central,1730163,1,6&series=mozilla-central,1730160,1,6
>
> Bug 1477627 converted a hot hash table from PLDHashTable to
> mozilla::HashSet and appears to have sped up cycle collection in some cases
> by 7%. If you know of another PLDHashTable that is hot, it might be worth
> converting it to mozilla::HashTable.
>
> Both mozilla::Hash{Map,Set} and PLDHashTable use the same double-hashing
> algorithm; the speed difference is due to mozilla::HashSet's extensive use
> of templating and inlining. The downside of this is that mozilla::HashSet
> will increase binary size more than PLDHashTable.
>
> There are overview comments at the top of mfbt/HashTable.h, and the classes
> themselves have more detailed comments about every method.
>
> Nick
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