Sunday, April 20, 2014

Immutable Sasa.Collections.Tree vs. System.Collections.Dictionary vs. C5 HashDictionary

I've previously posted about Sasa's hash-array mapped trie, but I never posted any benchmarks. I recently came across this post on Stackoverflow which provided a decent basic benchmark between .NET's default Dictionary<TKey, TValue>, the C5 collection's hash dictionary, F#'s immutable map, and .NET's new immutable collections.

I slightly modified the file to remove the bench against the F# map and the new immutable collections since I'm still using VS 2010, and I added a simple warmup phase to ensure the methods have all been JIT compiled and the GC run to avoid introducing noise:

static void Warmup()
{
    var x = Tree.Make<string, object>();
    var y = new C5.HashDictionary<string, object>();
    var z = new Dictionary<string, object>();
    z.Add("foo", "bar");
    for (var i = 0; i < 100; ++i)
    {
        x = x.Add("foo" + i, "bar");
        y.Add("foo" + i, "bar");
        z.Add("foo" + i, "bar");
        var tmp1 = x["foo" + i];
        var tmp2 = y["foo" + i];
        var tmp3 = z["foo" + i];
    }
    x = default(Tree<string, object>);
    y = null;
    z = null;
    GC.Collect();
}

The results are still somewhat representative. This is a sample of an average output, where "Imm" is Sasa's immutable HAMT:

# - 100
SCGD -          0 MS -         25 Ticks
C5   -          0 MS -        887 Ticks
Imm  -          0 MS -        387 Ticks

# - 1000
SCGD -          0 MS -        257 Ticks
C5   -          0 MS -        294 Ticks
Imm  -          0 MS -        368 Ticks

# - 10000
SCGD -          1 MS -       4084 Ticks
C5   -          1 MS -       5182 Ticks
Imm  -          1 MS -       5436 Ticks

# - 100000
SCGD -         28 MS -      85742 Ticks
C5   -         32 MS -      99280 Ticks
Imm  -         32 MS -      97720 Ticks

Observations:

  1. C5's standard deviation was somewhat wider than both Sasa's HAMT and SCGD, so it's performance seems slightly less predictable
  2. Sasa's immutable HAMT appears to perform within 5% of the mutable C5 collection at all collection sizes
  3. Sasa's immutable HAMT appears to perform within 15% of the mutable SCGD for large collections where the hash table with higher load factors
  4. Small collections requiring a small load factor clearly advantage the mutable SCGD by up to an order of magnitude, an advantage not shared by C5 for some reason (possibly they maintain a higher load factor)
  5. C5's terrible performance on very small collections of 100 items was consistent on every test run, again possibly because they maintain a high load factor before resizing
  6. Sasa's HAMT takes just as much time to load 1000 items as it takes to load 100 items; this was consistent across every test run, and it's not clear why

Finally, while not exactly apples-to-apples, Sasa's HAMT is easily 3-4× faster than F#'s map given the numbers cited in the above Stackoverflow post. F# still has an advantage for very small collections though. Sasa's HAMT also appears to be at least 2× faster than the new immutable collections.

Also keep in mind that this benchmark only tests lookup performance. F#'s map would have an advantage over Sasa's HAMT in load performance because the HAMT does not yet include a "bulk-load" operation, which the F# map does appear to support.

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