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Reduce memory usage of Hash object 
## Implementation Summary

* Change entry list from segmented list to flat array.
* Change value of hash bucket from pointer to entry to index of entry list,
  and represent it by variable length bits according to capacity of hash
  buckets.
* Store management information about entry list and hash table to `struct
  RHash` as much as possible.

## Benchmark Summary

Only the results of typical situations on 64-bit Word-boxing are present
here. For more detailed information, including consideration, see below
(although most of the body is written in Japanese).

* https://shuujii.github.io/mruby-hash-benchmark

### Memory Usage

Lower value is better.

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |          344B |          256B |   0.74419x |
|        40 |        1,464B |          840B |   0.57377x |
|       200 |        8,056B |        3,784B |   0.46971x |
|       500 |       17,169B |        9,944B |   0.57949x |

### Performance

Higher value is better.

#### `mrb_hash_set`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  1.41847M i/s |  1.36004M i/s |   0.95881x |
|        40 |  0.39224M i/s |  0.31888M i/s |   0.81296x |
|       200 |  0.03780M i/s |  0.04290M i/s |   1.13494x |
|       500 |  0.01225M i/s |  0.01314M i/s |   1.07275x |

#### `mrb_hash_get`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 26.05920M i/s | 30.19543M i/s |   1.15872x |
|        40 | 44.26420M i/s | 32.75781M i/s |   0.74005x |
|       200 | 44.55171M i/s | 31.56926M i/s |   0.70860x |
|       500 | 39.19250M i/s | 29.73806M i/s |   0.75877x |

#### `mrb_hash_each`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 25.11964M i/s | 30.34167M i/s |   1.20789x |
|        40 | 11.74253M i/s | 13.25539M i/s |   1.12884x |
|       200 |  2.01133M i/s |  2.97214M i/s |   1.47770x |
|       500 |  0.87411M i/s |  1.21178M i/s |   1.38631x |

#### `Hash#[]=`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.50095M i/s |  0.56490M i/s |   1.12764x |
|        40 |  0.19132M i/s |  0.18392M i/s |   0.96129x |
|       200 |  0.03624M i/s |  0.03256M i/s |   0.89860x |
|       500 |  0.01527M i/s |  0.01236M i/s |   0.80935x |
#### `Hash#[]`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 11.53211M i/s | 12.78806M i/s |   1.10891x |
|        40 | 15.26920M i/s | 13.37529M i/s |   0.87596x |
|       200 | 15.28550M i/s | 13.36410M i/s |   0.87430x |
|       500 | 14.57695M i/s | 12.75388M i/s |   0.87494x |

#### `Hash#each`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.30462M i/s |  0.27080M i/s |   0.88898x |
|        40 |  0.12912M i/s |  0.11704M i/s |   0.90642x |
|       200 |  0.02638M i/s |  0.02402M i/s |   0.91071x |
|       500 |  0.01066M i/s |  0.00959M i/s |   0.89953x |

#### `Hash#delete`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  7.84167M i/s |  6.96419M i/s |   0.88810x |
|        40 |  6.91292M i/s |  7.41427M i/s |   1.07252x |
|       200 |  3.75952M i/s |  7.32080M i/s |   1.94727x |
|       500 |  2.10754M i/s |  7.05963M i/s |   3.34970x |

#### `Hash#shift`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 | 14.66444M i/s | 13.18876M i/s |   0.89937x |
|        40 | 11.95124M i/s | 11.10420M i/s |   0.92913x |
|       200 |  5.53681M i/s |  7.88155M i/s |   1.42348x |
|       500 |  2.96728M i/s |  5.40405M i/s |   1.82121x |

#### `Hash#dup`

| Hash Size |   Baseline    |      New      |   Factor   |
|----------:|--------------:|--------------:|-----------:|
|        16 |  0.15063M i/s |  5.37889M i/s |  35.71024x |
|        40 |  0.06515M i/s |  3.38196M i/s |  51.91279x |
|       200 |  0.01359M i/s |  1.46538M i/s | 107.84056x |
|       500 |  0.00559M i/s |  0.75411M i/s | 134.88057x |

### Binary Size

Lower value is better.

|    File    |   Baseline    |      New      |  Factor   |
|:-----------|--------------:|--------------:|----------:|
| mruby      |      730,408B |      734,176B |  1.00519x |
| libmruby.a |    1,068,134B |    1,072,846B |  1.00441x |

## Other Fixes

The following issues have also been fixed in the parts where there was some
change this time.

* [Heap use-after-free in `Hash#value?`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-hash-value-md)
* [Heap use-after-free in `ht_hash_equal`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-ht_hash_equal-md)
* [Heap use-after-free in `ht_hash_func`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-ht_hash_func-md)
* [Heap use-after-free in `mrb_hash_merge`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-heap-use-after-free-in-mrb_hash_merge-md)
* [Self-replacement does not work for `Hash#replace`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-self-replacement-does-not-work-for-hash-replace-md)
* [Repeated deletes and inserts increase memory usage of `Hash`](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-repeated-deletes-and-inserts-increase-memory-usage-of-hash-md)
* [`Hash#rehash` does not reindex completely](https://gist.github.com/shuujii/30e4fcd5844a4112a0ecd4a5b3483101#file-hash-rehash-does-not-reindex-completely-md)
* `mrb_hash_delete_key` does not cause an error for frozen object
* `mrb_hash_new_capa` does not allocate required space first
* [`mrb_os_memsize_of_hash_table` result is incorrect](#5032 (comment))
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@shuujii
shuujii committed Nov 10, 2020
1 parent 8455344 commit f2d8db3
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Showing 4 changed files with 1,944 additions and 1,002 deletions.
47 changes: 34 additions & 13 deletions include/mruby/hash.h
View file
Expand Up @@ -14,10 +14,22 @@
*/
MRB_BEGIN_DECL

/* offset of `iv` must be 3 words */
struct RHash {
MRB_OBJECT_HEADER;
#ifdef MRB_64BIT
uint32_t size;
struct iv_tbl *iv;
struct htable *ht;
uint32_t ea_capa;
uint32_t ea_n_used;
#else
struct iv_tbl *iv;
uint32_t size;
#endif
union {
struct hash_entry *ea;
struct hash_table *ht;
};
};

#define mrb_hash_ptr(v) ((struct RHash*)(mrb_ptr(v)))
Expand Down Expand Up @@ -179,8 +191,9 @@ MRB_API mrb_value mrb_hash_clear(mrb_state *mrb, mrb_value hash);
MRB_API mrb_int mrb_hash_size(mrb_state *mrb, mrb_value hash);

/*
* Copies the hash.
*
* Copies the hash. This function does NOT copy the instance variables
* (except for the default value). Use mrb_obj_dup() to copy the instance
* variables as well.
*
* @param mrb The mruby state reference.
* @param hash The target hash.
Expand All @@ -198,16 +211,24 @@ MRB_API mrb_value mrb_hash_dup(mrb_state *mrb, mrb_value hash);
*/
MRB_API void mrb_hash_merge(mrb_state *mrb, mrb_value hash1, mrb_value hash2);

/* RHASH_TBL allocates st_table if not available. */
#define RHASH(obj) ((struct RHash*)(mrb_ptr(obj)))
#define RHASH_TBL(h) (RHASH(h)->ht)
#define RHASH_IFNONE(h) mrb_iv_get(mrb, (h), MRB_SYM(ifnone))
#define RHASH_PROCDEFAULT(h) RHASH_IFNONE(h)

#define MRB_HASH_DEFAULT 1
#define MRB_HASH_PROC_DEFAULT 2
#define MRB_RHASH_DEFAULT_P(h) (RHASH(h)->flags & MRB_HASH_DEFAULT)
#define MRB_RHASH_PROCDEFAULT_P(h) (RHASH(h)->flags & MRB_HASH_PROC_DEFAULT)
#define RHASH(hash) ((struct RHash*)(mrb_ptr(hash)))
#define RHASH_IFNONE(hash) mrb_iv_get(mrb, (hash), MRB_SYM(ifnone))
#define RHASH_PROCDEFAULT(hash) RHASH_IFNONE(hash)

#define MRB_HASH_IB_BIT_BIT 5
#define MRB_HASH_AR_EA_CAPA_BIT 5
#define MRB_HASH_IB_BIT_SHIFT 0
#define MRB_HASH_AR_EA_CAPA_SHIFT 0
#define MRB_HASH_AR_EA_N_USED_SHIFT MRB_HASH_AR_EA_CAPA_BIT
#define MRB_HASH_SIZE_FLAGS_SHIFT (MRB_HASH_AR_EA_CAPA_BIT * 2)
#define MRB_HASH_IB_BIT_MASK ((1 << MRB_HASH_IB_BIT_BIT) - 1)
#define MRB_HASH_AR_EA_CAPA_MASK ((1 << MRB_HASH_AR_EA_CAPA_BIT) - 1)
#define MRB_HASH_AR_EA_N_USED_MASK (MRB_HASH_AR_EA_CAPA_MASK << MRB_HASH_AR_EA_N_USED_SHIFT)
#define MRB_HASH_DEFAULT (1 << (MRB_HASH_SIZE_FLAGS_SHIFT + 0))
#define MRB_HASH_PROC_DEFAULT (1 << (MRB_HASH_SIZE_FLAGS_SHIFT + 1))
#define MRB_HASH_HT (1 << (MRB_HASH_SIZE_FLAGS_SHIFT + 2))
#define MRB_RHASH_DEFAULT_P(hash) (RHASH(hash)->flags & MRB_HASH_DEFAULT)
#define MRB_RHASH_PROCDEFAULT_P(hash) (RHASH(hash)->flags & MRB_HASH_PROC_DEFAULT)

/* GC functions */
void mrb_gc_mark_hash(mrb_state*, struct RHash*);
Expand Down
20 changes: 0 additions & 20 deletions mrblib/hash.rb
View file
Expand Up @@ -144,26 +144,6 @@ def each_value(&block)
self
end

##
# Replaces the contents of <i>hsh</i> with the contents of other hash
#
# ISO 15.2.13.4.23
def replace(hash)
raise TypeError, "Hash required (#{hash.class} given)" unless Hash === hash
self.clear
hash.each_key{|k|
self[k] = hash[k]
}
if hash.default_proc
self.default_proc = hash.default_proc
else
self.default = hash.default
end
self
end
# ISO 15.2.13.4.17
alias initialize_copy replace

##
# Return a hash which contains the content of
# +self+ and +other+. If a block is given
Expand Down

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