Issue #12142 has been updated by Yura Sokolov.


Vladimir Makarov wrote:
> Open addressing removes pointer making a smaller element which increases
> probability to be read from memory by reading the same cache line or/and
> probability to stay in a cache.
> 
> In case of collisions, I believe checking other entry again improves data
> locality in comparison with going by pointers through disperse elements
> (may be allocated originally for different tables).

I didn't say to use pointers. I mean to use indices:

````C
typedef uint32_t st_index_t;
typedef st_index_t st_entry_t;

struct st_table_entry {
    st_index_t hash;
    st_index_t next;
    st_data_t key;
    st_data_t value;
};
````

Then you may use closed addressing with same effect as your open addressing.
But instead of probing next element in `entries` array, you will probe it by `next` index.
Cache locality will remain the same , cause anyway you'll probe "random" entries.
(unless you store hash in st_entry and use quadrating probing, which has better hash locality).

This way `entries` array could be smaller than `elements` entries. And small table may have
no `entries` at all, just chain all elements in a hash smaller than n elements (n=6 for example).



>> st_foreach is broken in case table were rebuilt.
> Sorry, I did not catch what you meant. St_foreach works fine in the proposed implementation
> even if the table rebuilds. Moreover st_foreach in the proposed implementation
> can work forever adding and removing elements meanwhile the current implementation
> will result out of memory in such case.

https://github.com/vnmakarov/ruby/blob/hash_tables_with_open_addressing/st.c#L927-L930

````C
      if (rebuilds_num != tab->rebuilds_num) {
          elements = tab->elements;
          curr_element_ptr = &elements[i];
      }
````
why do you beleive that index `i` will be the same for this entry????

https://github.com/vnmakarov/ruby/blob/hash_tables_with_open_addressing/st.c#L442-L443

You skip deleted elements in a middle, so if there were deleted elements, `i` will be smaller after rebuild.

````
before rebuild:
   |   0   |    1    |   2   |
   | key=x | deleted | key=y |
after rebuild:
   |   0   |   1   |
   | key=x | key=y |
````
So if you position during iteration were on `key=y` (so `i=2`), then after rebuild it points to wrong place.

----------------------------------------
Feature #12142: Hash tables with open addressing
https://bugs.ruby-lang.org/issues/12142#change-57302

* Author: Vladimir Makarov
* Status: Open
* Priority: Normal
* Assignee: 
----------------------------------------
~~~
 Hello, the following patch contains a new implementation of hash
tables (major files st.c and include/ruby/st.h).

  Modern processors have several levels of cache.  Usually,the CPU
reads one or a few lines of the cache from memory (or another level of
cache).  So CPU is much faster at reading data stored close to each
other.  The current implementation of Ruby hash tables does not fit
well to modern processor cache organization, which requires better
data locality for faster program speed.

The new hash table implementation achieves a better data locality
mainly by

  o switching to open addressing hash tables for access by keys.
    Removing hash collision lists lets us avoid *pointer chasing*, a
    common problem that produces bad data locality.  I see a tendency
    to move from chaining hash tables to open addressing hash tables
    due to their better fit to modern CPU memory organizations.
    CPython recently made such switch
    (https://hg.python.org/cpython/file/ff1938d12240/Objects/dictobject.c).
    PHP did this a bit earlier
    https://nikic.github.io/2014/12/22/PHPs-new-hashtable-implementation.html.
    GCC has widely-used such hash tables
    (https://gcc.gnu.org/svn/gcc/trunk/libiberty/hashtab.c) internally
    for more than 15 years.

  o removing doubly linked lists and putting the elements into an array
    for accessing to elements by their inclusion order.  That also
    removes pointer chaising on the doubly linked lists used for
    traversing elements by their inclusion order.

A more detailed description of the proposed implementation can be
found in the top comment of the file st.c.

The new implementation was benchmarked on 21 MRI hash table benchmarks
for two most widely used targets x86-64 (Intel 4.2GHz i7-4790K) and ARM
(Exynos 5410 - 1.6GHz Cortex-A15):

make benchmark-each ITEM=bm_hash OPTS='-r 3 -v' COMPARE_RUBY='<trunk ruby>'

Here the results for x86-64:

hash_aref_dsym       1.094
hash_aref_dsym_long          1.383
hash_aref_fix        1.048
hash_aref_flo        1.860
hash_aref_miss       1.107
hash_aref_str        1.107
hash_aref_sym        1.191
hash_aref_sym_long           1.113
hash_flatten         1.258
hash_ident_flo       1.627
hash_ident_num       1.045
hash_ident_obj       1.143
hash_ident_str       1.127
hash_ident_sym       1.152
hash_keys            2.714
hash_shift           2.209
hash_shift_u16       1.442
hash_shift_u24       1.413
hash_shift_u32       1.396
hash_to_proc         2.831
hash_values          2.701

The average performance improvement is more 50%.  ARM results are
analogous -- no any benchmark performance degradation and about the
same average improvement.

The patch can be seen as

https://github.com/vnmakarov/ruby/compare/trunk...hash_tables_with_open_addressing.patch

or in a less convenient way as pull request changes

https://github.com/ruby/ruby/pull/1264/files


This is my first patch for MRI and may be my proposal and
implementation have pitfalls.  But I am keen to learn and work on
inclusion of this code into MRI.

~~~



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