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David Golden
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Hash::Ordered::Benchmarks - Ordered hash benchmarking


version 0.012


The Hash::Ordered internals are simple: a hash of data and an array of ordered keys. I thought this would perform well for common tasks and likely outperform more complicated ordered hash implementations, so I decided to do some benchmarking to test it.

Note: since the initial benchmarking, Hash::Ordered gained just-in-time indexing of the keys array to support faster tombstone deletion, which adds some conditional data structures to the internals. It also now supports tie. The revised benchmarks include the tie mode for comparison with other tied hash implementations.


In my review of alternatives to Hash::Ordered, six seemed sufficiently general-purpose to be worth benchmarking. The modules tested are listed in the benchmark output in shorthand:

Note that Tie::Hash::Indexed is written in XS and also may require forced installation as its tests often fail for Perl 5.18+ due to the hash randomization change.

If there are different methods of doing something with a module, the variations are described in each section below.


I conducted benchmarking with the Benchmark module. The test script is in the devel directory of the distribution. Tests were run on Perl 5.20.2 on a Mac Book Pro (darwin-thread-multi-2level). Each benchmark ran for 5 CPU seconds.

Benchmarks were run at several different scales to reveal differences in efficiency as hash size grows. The details are described in each section below.

A seed list of keys and values was generated from random integers using Math::Random::MT::Auto. The same seed list was used for all benchmarks unless otherwise noted.

I did not test advanced features of these modules, as apples-to-apples comparison is difficult. Still, the performance on common, simple measures could suggest how features that combine these operations might perform.

Ordered hash creation

I tested hash creation for 10, 100 and 1000 elements. For some modules there were different options for creating a hash:

  • Array::AsHash takes an array-reference with an option to use it directly or to clone it. In one case I provided the seed list array reference with the clone option to true ("a:ah_cp"). In another case I created a new array reference from the seed list and provided it directly ("a:ah_rf").

  • Hash::Ordered can be initialized either with new ("h:o_oo") or via tie ("h:o_th").

  • Tie::IxHash can be initialized either with new ("t:ix_oo") or via tie ("t:ix_th").

  • Data::XHash can be created with a list ("t:xh_ls") or an array reference ("t:xh_rf").

As expected, when Array::AsHash gets an array reference, it's very fast. Tie::Hash::Indexed does well here, also. Of the non-XS, more hash-like choices, Hash::Ordered does well.

    Results for ordered hash creation for 10 elements
               t:h:i   136030/s
             a:ah_rf   111411/s
              h:o_oo   101293/s  *
              h:o_th    98646/s  *
             t:ix_oo    61853/s
             t:ix_th    61715/s
             a:ah_cp    56375/s
                a:oh    54337/s
               t:llh    33553/s
             d:xh_ls    14068/s
             d:xh_rf    13926/s

    Results for ordered hash creation for 100 elements
               t:h:i    16503/s
             a:ah_rf    15398/s
              h:o_oo    11226/s  *
              h:o_th    10793/s  *
                a:oh     7783/s
             t:ix_th     7570/s
             t:ix_oo     7405/s
             a:ah_cp     7035/s
               t:llh     3533/s
             d:xh_ls     1561/s
             d:xh_rf     1550/s

    Results for ordered hash creation for 1000 elements
               t:h:i     1552/s
             a:ah_rf     1509/s
              h:o_oo     1160/s  *
              h:o_th     1158/s  *
                a:oh      815/s
             t:ix_th      772/s
             t:ix_oo      757/s
             a:ah_cp      684/s
               t:llh      340/s
             d:xh_ls      154/s
             d:xh_rf      152/s

Getting hash elements

I tested retrieving values for 10% of the keys, randomly selected, from hashes of 10, 100 and 1000 elements. The hash was created beforehand so the benchmarks reflect only element access.

Some modules had choices for how to retrieve an value, usually between a method (denoted with "_oo"), tied hash access ("_th") or with a dereference ("_rf").

Generally, method calls turned out faster than other approaches for a given module, demonstrating the inefficiency of tied objects.

    Results for fetching ~10% of 10 elements
              h:o_oo  1844781/s  *
             d:xh_oo  1292883/s
             t:ix_oo  1187104/s
               t:h:i   932793/s
              h:o_th   817346/s  *
             d:xh_rf   703441/s
             t:ix_th   649291/s
                a:oh   560060/s
               t:llh   514911/s
                a:ah   260639/s

    Results for fetching ~10% of 100 elements
              h:o_oo   285983/s  *
             d:xh_oo   183292/s
             t:ix_oo   165100/s
               t:h:i   128713/s
              h:o_th   107213/s  *
             d:xh_rf    87049/s
             t:ix_th    79642/s
                a:oh    66109/s
               t:llh    58741/s
                a:ah    27533/s

    Results for fetching ~10% of 1000 elements
              h:o_oo    30342/s  *
             d:xh_oo    19004/s
             t:ix_oo    17132/s
               t:h:i    13269/s
              h:o_th    11100/s  *
             d:xh_rf     8919/s
             t:ix_th     7844/s
                a:oh     6763/s
               t:llh     5666/s
                a:ah     2772/s

Setting hash elements

I tested changing values for 10% of the keys, randomly selected, from hashes of 10, 100 and 1000 elements. The hash was created beforehand so the benchmarks reflect only element mutation. No new keys were added.

Some modules had choices for how to modify a value, usually between a method (denoted with "_oo"), tied hash access ("_th") or with a dereference ("_rf").

Again, methods outperformed.

    Results for replacing ~10% of 10 elements
              h:o_oo  1378880/s  *
               t:h:i   945403/s
             d:xh_oo   941643/s
             t:ix_oo   887283/s
              h:o_th   652269/s  *
               t:llh   590160/s
             d:xh_rf   537694/s
                a:oh   530787/s
             t:ix_th   508001/s
                a:ah   159258/s

    Results for replacing ~10% of 100 elements
              h:o_oo   192769/s  *
               t:h:i   126284/s
             d:xh_oo   119845/s
             t:ix_oo   113992/s
              h:o_th    81159/s  *
               t:llh    72403/s
             d:xh_rf    64791/s
                a:oh    62666/s
             t:ix_th    59809/s
                a:ah    16405/s

    Results for replacing ~10% of 1000 elements
              h:o_oo    19909/s  *
               t:h:i    13445/s
             d:xh_oo    12487/s
             t:ix_oo    11601/s
              h:o_th     8357/s  *
               t:llh     7503/s
             d:xh_rf     6599/s
                a:oh     6410/s
             t:ix_th     6118/s
                a:ah     1651/s

Adding hash elements

I tested adding 10, 100 and 1000 elements to an empty hash.

Some modules had choices for how to append a value, usually between a method (denoted with "_oo"), tied hash access ("_th") or with a dereference ("_rf").

For Tie::LLHash, I did not use the "lazy" option, but did the equivalent using tied and a method call:

        tied(%tllh)->last( irand(), 42 ) for 1 .. $n;

Generally, it seemed like the differences were smaller than for other benchmarks. Methods still outperformed.

    Results for adding 10 elements to empty hash
              h:o_oo   341022/s  *
               t:h:i   295079/s
             t:ix_oo   258981/s
              h:o_th   245996/s  *
             t:ix_th   211341/s
               t:llh   191298/s
                a:oh   137447/s
                a:ah   112651/s
             d:xh_oo    87215/s
             d:xh_rf    80379/s

    Results for adding 100 elements to empty hash
              h:o_oo    58519/s  *
               t:h:i    55166/s
             t:ix_oo    48658/s
              h:o_th    42066/s  *
             t:ix_th    38632/s
                a:oh    34842/s
               t:llh    28384/s
             d:xh_oo    24841/s
             d:xh_rf    21517/s
                a:ah    13726/s

    Results for adding 1000 elements to empty hash
              h:o_oo     6497/s  *
               t:h:i     6108/s
             t:ix_oo     5528/s
              h:o_th     4650/s  *
             t:ix_th     4329/s
                a:oh     4233/s
             d:xh_oo     3121/s
               t:llh     3011/s
             d:xh_rf     2696/s
                a:ah     1423/s

Deleting hash elements

I tested creating hashes of 10, 100 and 1000 elements and then deleting 10% of the keys, chosen randomly. I would have liked to have isolated creation from deletion, but I couldn't figure out a way to do that given how Benchmark runs the same tests over and over.

Some modules had choices for how to delete a value, usually between a method (denoted with "_oo"), tied hash access ("_th") or with a dereference ("_rf").

The performance changes (or lack thereof) at the three different sizes reveals implementation differences. (Though recall that some of this is the creation performance difference as well as deletion difference.)

For example, Tie::Hash::Indexed XS does very well, which could be its good creation performance, but could also be good deletion.

Hash::Ordered does linear search deleting a key for the 10 element hash, but automatically switches to indexed, tombstone deletion for the larger hashes. When deleting only 10% of keys, garbage collection of tombstoned keys never occurs, so that amortized cost is not included.

Tie::LLHash improves at larger sizes as deleting from a linked list is faster than splicing out an element of an array. Conversely, Array::AsHash just gets worse.

    Results for creating 10 element hash then deleting ~10%
               t:h:i   131578/s
              h:o_oo    94598/s  *
              h:o_th    84018/s  *
                a:ah    67109/s
             t:ix_oo    55477/s
             t:ix_th    52792/s
                a:oh    46938/s
               t:llh    30399/s
             d:xh_oo    13756/s
             d:xh_rf    13499/s

    Results for creating 100 element hash then deleting ~10%
               t:h:i    17420/s
              h:o_oo     9242/s  *
              h:o_th     8438/s  *
                a:oh     5738/s
             t:ix_oo     3922/s
             t:ix_th     3862/s
                a:ah     3286/s
               t:llh     3250/s
             d:xh_oo     1508/s
             d:xh_rf     1499/s

    Results for creating 1000 element hash then deleting ~10%
               t:h:i     1635/s
              h:o_oo      934/s  *
              h:o_th      799/s  *
               t:llh      319/s
                a:oh      204/s
             d:xh_oo      152/s
             d:xh_rf      151/s
             t:ix_oo       78/s
             t:ix_th       78/s
                a:ah       40/s

Extracting the hash as a list

I tested getting an ordered list of pairs from hashes of 10, 100 and 1000 elements. The hash was created beforehand so the benchmarks reflect only conversion to a list.

Oddly, modules that usually have more than one way to do things don't for this. Even Tie::IxHash doesn't really have an OO way to do it, so I did it longhand:

        @list = map { $_ => $tix_oo->FETCH($_) } $tix_oo->Keys;

Because Array::AsHash keeps its internal representation as an ordered list of pairs, it outperforms the rest handily as it merely needs to dereference that data structure.

    Results for listing pairs of 10 element hash
                a:ah   321044/s
              h:o_oo   178288/s  *
             t:ix_oo    89263/s
               t:h:i    79184/s
              h:o_th    56112/s  *
             t:ix_th    48009/s
                a:oh    47433/s
               t:llh    37996/s
                d:xh    37439/s

    Results for listing pairs of 100 element hash
                a:ah    36399/s
              h:o_oo    19537/s  *
             t:ix_oo     9049/s
               t:h:i     7768/s
              h:o_th     6254/s  *
                a:oh     5060/s
             t:ix_th     4907/s
                d:xh     4122/s
               t:llh     3813/s

    Results for listing pairs of 1000 element hash
                a:ah     3784/s
              h:o_oo     1959/s  *
             t:ix_oo      905/s
               t:h:i      773/s
              h:o_th      625/s  *
                a:oh      523/s
             t:ix_th      492/s
                d:xh      427/s
               t:llh      377/s


With the exception of hash creation and element deletion, Hash::Ordered generally outperformed the other ordered hash implementations. Even for creation, it was the fastest of the pure-Perl, hash-based implementations, often by a large margin.

In the original release of Hash::Ordered, deletion got worse as the hash size grew. The new JIT indexing with tombstones now makes deletion far faster than any pure-Perl implementation.

Array::AsHash, with the opposite internal implementation compared to Hash::Ordered, performs best at creation and listing pairs, but is dead last at element access and modification. I believe the poor performance is mostly due to extra indirection (e.g. an extra function call) and logic in the element access methods. For uses that don't require much element access and have lots of creation/serialization, it could still be a useful choice.

Generally, every module that depends on tie for some portion of its implementation pays a substantial performance penalty. Comparing Hash::Ordered benchmarks with and without tie for individual element operations shows how severe this penalty can be. Tie::Hash::Indexed — likely because of its XS implementation — performs decently, but not well enough in my opinion to justify its use.

As the author of Hash::Ordered, I'm clearly biased, but I think these benchmarks make a very good case for it being the "go to" module for pure-Perl, general-purpose ordered hashes.


David Golden <dagolden@cpan.org>


This software is Copyright (c) 2014 by David Golden.

This is free software, licensed under:

  The Apache License, Version 2.0, January 2004