Re: [Haskell-cafe] Fast JSON validation - reducing allocations

12 May 2017


      David Turner  writes:
...
On 11 May 2017 at 19:40, Ben Gamari  wrote:
...
Ccing Luke Maurer under the assumption that he will appreciate seeing
the fruits of his labor.
David Turner  writes:
...
Dear Café,
(snip)
...
There's a copy of the relevant code for option 4 at
https://github.com/DaveCTurner/json-validator. I've hacked around with
it a
...
bit since producing the numbers above, so it's now apparently a bit
slower
than Aeson but allocates less too (~65kB).
Could anyone help, e.g. by pointing me at the bit in the Core that is
allocating within the main loop?
While turning this over in my head I realized that this is the sort of
program which may be helped significantly by GHC 8.2's improved join
point handling. Indeed the timings seem to support this hypothesis:
GHC 8.0.2
benchmarking json-validator/Automaton/testEvent
    time                 22.84 μs   (22.76 μs .. 22.94 μs)
                         1.000 R²   (1.000 R² .. 1.000 R²)
    mean                 22.84 μs   (22.76 μs .. 22.94 μs)
    std dev              297.4 ns   (221.8 ns .. 378.8 ns)
Alloc rate     4,015,256,078,038 bytes per MUT second
GHC 8.2
benchmarking json-validator/Automaton/testEvent
    time                 9.221 μs   (9.141 μs .. 9.318 μs)
                         0.998 R²   (0.996 R² .. 1.000 R²)
    mean                 9.163 μs   (9.084 μs .. 9.356 μs)
    std dev              399.8 ns   (193.0 ns .. 745.4 ns)
    variance introduced by outliers: 54% (severely inflated)
Alloc rate           123,141,635 bytes per MUT second
Wow! I suspect your allocations have now vanished.
Ooo, that's more like it.
Could you run again using the following to get Criterion's estimate of the
allocations-per-call?
json-validator-exe --regress allocated:iters +RTS -T
benchmarking json-validator/Automaton/testEvent
Here it is,

GHC 8.0.2

    benchmarking json-validator/Automaton/testEvent
    time                 34.81 μs   (33.48 μs .. 36.10 μs)
                         0.991 R²   (0.988 R² .. 0.994 R²)
    mean                 34.04 μs   (33.28 μs .. 34.99 μs)
    std dev              2.828 μs   (2.423 μs .. 3.247 μs)
    allocated:           1.000 R²   (1.000 R² .. 1.000 R²)
      iters              65924.651  (65895.364 .. 65951.510)
      y                  -4175.284  (-46736.295 .. 39814.235)
    variance introduced by outliers: 78% (severely inflated)


GHC 8.2

    benchmarking json-validator/Automaton/testEvent
    time                 9.021 μs   (8.923 μs .. 9.162 μs)
                         0.998 R²   (0.994 R² .. 1.000 R²)
    mean                 8.970 μs   (8.905 μs .. 9.232 μs)
    std dev              341.4 ns   (94.90 ns .. 711.7 ns)
    allocated:           0.973 R²   (0.956 R² .. 0.984 R²)
      iters              343.033    (332.692 .. 354.606)
      y                  -7326.339  (-57269.638 .. 47222.231)
    variance introduced by outliers: 47% (moderately inflated)

Cheers,

- Ben