Issue #12142 has been updated by Yura Sokolov.


Murmur is not used for Hash, cause it is target for hashDoS - it has seed independent collisions.
City64 also has seed independent collisions.
That is why SipHash were born and adopted by community.

http://emboss.github.io/blog/2012/12/14/breaking-murmur-hash-flooding-dos-reloaded/

But SipHash could be relaxed:

- currently it is SipHash24 - 2 rounds for 8byte block and 4 rounds for finalization
- but even SipHash author confirms that for internal hash table (ie when hash sum is not exposed
  to attacker) SipHash13 is just enough.
  https://github.com/rust-lang/rust/issues/29754#issuecomment-156073946

Single call to st_hash in hash.c is just to combine two hashsums (calculated with SipHash) into one.

All usage of st_init_strtable are not performance critical.

> I made the entries array is cyclical to exclude overhead of table
compaction or/and table size change for usage the hash tables as a
queue.

I doubt using hash table as a queue is a useful case. But I could be mistaken.
And cyclic allocation doesn't solve LRU usecase at all :-(

> also changed the specialized hash function (rb_num_hash_start) used for bm_hash_ident tests

I've seen you do it. Great catch!
I also fixed it in other way (cause I don't use perturb I must be ensure all bits
are mixed into lower bits).

> I also tried double probing

It takes me a time to realize that you mean `quadratic probing` :-)
`double probing` may be: test slot, then test neighbor, do long jump and repeat.
It is not wildly used technique.

Do you configure with `--with-jemalloc` ?
`trunk` is much faster with jemalloc linked, and it is hard to beat it by performance.

Unfortunately, Redmine still doesn't work with your branch, so I cann't benchmark it.


----------------------------------------
Feature #12142: Hash tables with open addressing
https://bugs.ruby-lang.org/issues/12142#change-57468

* 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.

~~~

---Files--------------------------------
0001-st.c-use-array-for-storing-st_table_entry.patch (46.7 KB)
0001-st.c-change-st_table-implementation.patch (59.4 KB)


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