On ar_table, Do not keep a full-length hash value (FLHV, 8 bytes)
but keep a 1 byte hint from a FLHV (lowest byte of FLHV).
An ar_table only contains at least 8 entries, so hints consumes
8 bytes at most. We can store hints in RHash::ar_hint.
On 32bit CPU, we use 4 entries ar_table.
The advantages:
We don't need to keep FLHV so ar_table only consumes
16 bytes (VALUEs of key and value) * 8 entries = 128 bytes.
We don't need to scan ar_table, but only need to check hints
in many cases. Especially we don't need to access ar_table
if there is no match entries (in many cases).
It will increase memory cache locality.
The disadvantages:
This technique can increase #eql? time because hints can
conflicts (in theory, it conflicts once in 256 times).
It can introduce incompatibility if there is a object x where
x.eql? returns true even if hash values are different.
I believe we don't need to care such irregular case.
We need to re-calculate FLHV if we need to switch from ar_table
to st_table (e.g. exceeds 8 entries).
It also can introduce incompatibility, on mutating key objects.
I believe we don't need to care such irregular case too.
Add new debug counters to measure the performance:
artable_hint_hit - hint is matched and eql?#=>true
artable_hint_miss - hint is not matched but eql?#=>false
artable_hint_notfound - lookup counts
Related issues
Feature #15602: Eliminate recording full-width hash value for small Hash
Use 1 byte hint for ar_table [Feature #15602]
On ar_table, Do not keep a full-length hash value (FLHV, 8 bytes)
but keep a 1 byte hint from a FLHV (lowest byte of FLHV).
An ar_table only contains at least 8 entries, so hints consumes
8 bytes at most. We can store hints in RHash::ar_hint.
On 32bit CPU, we use 4 entries ar_table.
The advantages:
16 bytes (VALUEs of key and value) * 8 entries = 128 bytes.
in many cases. Especially we don't need to access ar_table
if there is no match entries (in many cases).
It will increase memory cache locality.
The disadvantages:
#eql?
time because hints canconflicts (in theory, it conflicts once in 256 times).
It can introduce incompatibility if there is a object x where
x.eql? returns true even if hash values are different.
I believe we don't need to care such irregular case.
to st_table (e.g. exceeds 8 entries).
It also can introduce incompatibility, on mutating key objects.
I believe we don't need to care such irregular case too.
Add new debug counters to measure the performance: