Issue #12142 has been updated by Vladimir Makarov.


Eric Wong wrote:
> vmakarov / redhat.com wrote:
>  >   * I also *changed the specialized hash function* (rb_num_hash_start)
>  >     used for bm_hash_ident tests.  It permits to improve collisions for
>  >     these tests, e.g. from 73% to 0.3% for hash_ident_num.
>  
>  Nice.  Do you think it's worth it to split this change out for
>  use with the current st implementation?

Thank you.  Right now it has a little sense especially for small tables.  The current implementation simply ignores a lot of higher bits, e.g. a table with 32-bit entries uses only 5 lower bits of 64-bit hash.  The change I've done is good when you use all 64-bits of the hash and my implementation finally consumes all these 64 bits although several collisions should occur for this.

To be a good hash function when you use only small lower part of the hash, the function should behave well on avalanche test.  It means that when you change just one bit in any part of the key, "avalanche" of changes happens in the result hash (the best when half of hash bits through the all hash changes).  I guess any simple specialized function will be bad on such test.  Major their advantage is high speed.  Murmur, City64, SipHash24 are quite good on such tests but they are much slower.

I think I'll have a few patches when I am done with the hash tables: the hash table itself, hash functions, code for recognizing a denial attack and switching to stronger hash functions.  I am not sure that all (or any) will be finally accepted.


----------------------------------------
Feature #12142: Hash tables with open addressing
https://bugs.ruby-lang.org/issues/12142#change-57504

* 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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