Issue #12142 has been updated by Vladimir Makarov.


Yura Sokolov wrote:
> Good day, everyone.
> 
> I'm adding alternative patch version for st_table.
> It is my compromise with Vladimir and his proposals.

Yura, I am glad that finally you accepted open address tables.  Now
there is no major differences between my original table proposal and
your current implementation.  Although there are differences in details.

I don't like idea adding more and more new things to the patch without
separate evaluation of them.  Therefore I stopped to change my patch
five mounths ago and only did a rebase.  Effect of my 4 major parts of
the patch on performance were evaluated and posted by me to confirm
their inclusion or not.

IMHO, Ruby team should accept some base patch (in case of my patch it
is what was done 5 months ago and currently on
https://github.com/vnmakarov/ruby/tree/hash_tables_with_open_addressing)
and after that additional patches from you or/and me could be considered
and evaluated.

The base patch could be what they already evaluated
or agree to evaluate.  Otherwise, you and me will create new "better" versions
of the patch creating stress to them.  Adding new features separately is also
good for keeping this thread readable (I suspect we made a record for the longest
discussion on this Ruby discussion board).

Your last new features are interesting but some of them is not obvious
to me, for example, using tables with less 2^32 **by default** or
faster hash table grow as you wrote for jemalloc allocation pattern
(As I know MRI Ruby does not use jemalloc yet and I am not sure it
should be used because it uses more memory for some loads and also
because Glibc community is serious to improve their malloc --
https://gcc.gnu.org/wiki/cauldron2016#WholeSysTrace).



----------------------------------------
Feature #12142: Hash tables with open addressing
https://bugs.ruby-lang.org/issues/12142#change-60718

* 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-change-st_table-implementation.patch (59.4 KB)
st-march31.patch (114 KB)
base.patch (93.8 KB)
hash.patch (4.48 KB)
strong_hash.patch (8.08 KB)
city.patch (19.4 KB)
new-hash-table-benchmarks.patch (1.34 KB)
hash_improvements_and_st_implementation_changes.mbox (101 KB)
hash_improvements_and_st_array_with_open_addressing.mbox (108 KB)


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