Use array instead of linked list for rbs location's child

[pocke]

This PR reduces memory usage by replacing the linked list with an array for RBS::Location.

RBS::Location has child locations. They are implemented as two linked lists, but this is not memory efficient.
Therefore this PR replaces the linked lists with an array.

This PR is the first part of memory usage improvement for RBS::Location. I'll try reducing memory with another approach. See this document for more information. https://hackmd.io/QNkMU6roTqSn_iXb2ENhsA

Data structure

This is the previous data structure.
Comments are byte sizes (on my 64bits local machine).

typedef struct rbs_loc_list {
  ID name; // 8
  range rg; // 32
  struct rbs_loc_list *next; // 8
} rbs_loc_list; // 48

typedef struct {
  VALUE buffer; // 8
  range rg; // 32
  rbs_loc_list *requireds; // 8
  rbs_loc_list *optionals; // 8
} rbs_loc; // 56

By this change, these structs are changed to the following:

typedef struct {
  ID name; // 8
  range rg; // 32
} rbs_loc_entry; // 40

typedef unsigned int rbs_loc_entry_bitmap;

typedef struct {
  unsigned short len;  // 2
  unsigned short cap;  // 2
  rbs_loc_entry_bitmap required_p; // 4
  rbs_loc_entry entries[0]; // 0
} rbs_loc_children; // 8

typedef struct {
  VALUE buffer; // 8
  range rg; // 32
  rbs_loc_children *children; // 8
} rbs_loc; // 48

The byte size is changed like following:

• If rbs_loc does not have children:
  • before: 56 bytes
  • after: 48 bytes
• If rbs_loc has children:
  • before: 56 + (48 * size) bytes
  • after: 48 + 8 + (40 * size) bytes

Benchmarking

Memory Usage

With bundle exec ruby benchmark/memory_new_env.rb and benchmark/memory_new_rails_env.rb

before

$ bundle exec ruby benchmark/memory_new_env.rb
Total allocated: 43027240 bytes (363668 objects)
Total retained:  12315720 bytes (98104 objects)

retained memory by class
-----------------------------------
   4261392  String
   3899952  RBS::Location
   1262864  Array
   1086272  Hash
    257840  RBS::Types::Function

$ bundle exec ruby benchmark/memory_new_rails_env.rb
Total allocated: 224460224 bytes (1994661 objects)
Total retained:  57453656 bytes (553231 objects)

retained memory by class
-----------------------------------
  21650208  RBS::Location
  11885144  String
   7415928  Array
   6143008  Hash
   1481920  RBS::Types::Function

after

$ bundle exec ruby benchmark/memory_new_env.rb
Total allocated: 42677872 bytes (363668 objects)
Total retained:  11966352 bytes (98104 objects)

retained memory by class
-----------------------------------
   4261392  String
   3550584  RBS::Location
   1262864  Array
   1086272  Hash
    257840  RBS::Types::Function

$ bundle exec ruby benchmark/memory_new_rails_env.rb
Total allocated: 222420976 bytes (1994661 objects)
Total retained:  55414424 bytes (553231 objects)

retained memory by class
-----------------------------------
  19610976  RBS::Location
  11885144  String
   7415928  Array
   6143008  Hash
   1481920  RBS::Types::Function

The retained memory is reduced 3% (349kb) on the core env and 4% (2MB) on the Rails env.

Execution time

With bundle exec ruby benchmark/benchmark_new_env.rb.

before

Warming up --------------------------------------
             new_env     1.000 i/100ms
       new_rails_env     1.000 i/100ms
Calculating -------------------------------------
             new_env      6.146 (± 0.0%) i/s -     62.000 in  10.136343s
       new_rails_env      0.966 (± 0.0%) i/s -     10.000 in  10.377986s

after

Warming up --------------------------------------
             new_env     1.000 i/100ms
       new_rails_env     1.000 i/100ms
Calculating -------------------------------------
             new_env      6.399 (±15.6%) i/s -     64.000 in  10.070012s
       new_rails_env      1.013 (± 0.0%) i/s -     11.000 in  10.910335s

It is 1.04x faster on the core env and 1.05x faster on the Rails env.

[soutaro]

👍