Contents ‣ Understand this session completely in 10 seconds ‣ Why ROM and RubyKaigi 2018 wrap up and new our approach ‣ nvgen ‣ Introduction to mruby’s objects in C ‣ Class objects in depth ‣ Fix to work with ROM ‣ Benchmark ‣ Future works
Why do we want to use ROM more ‣RAM is small and precious ‣ROM is large, but not used well in dynamic language ‣because ROM cannot be modified in execution time
mrb_open() ‣Initialize GC and heap memory ‣initialize Symbols and Strings ‣Initialize Standard Classes in C ‣initialize Standard Classes in Ruby ‣initialize mrbgems in C and/or Ruby
mrb_open() ‣Initialize GC and heap memory ‣initialize Symbols and Strings ‣Initialize Standard Classes in C ‣initialize Standard Classes in Ruby ‣initialize mrbgems in C and/or Ruby
mrb_load_state_allocf() ‣ mrb_open() ‣ normal VM initializer ‣mrb_open_allocf(mrb_allocf f, void *ud)
‣use custom allocation function f ‣ mrb_load_state_allocf(mrb_state *mrb, mrb_allocf f, void *ud) ‣ use custom allocation function and initialized VM image
How to put Symbols on ROM ‣ All Symbols should be defined ‣ to use structures in ROM ‣ Access to Symbols should be fast ‣ → We use gperf (perfect hash Generator)
nvgen ‣ execute mrb_open() ‣ generate Objects, strings, symbols as C structures ‣ dump all symbols into presym.key ‣ dump all structures into generated.c mruby nvgen generated.c presym.c presym.key
mruby’s Objects in C ‣mrb_value ‣small objects data represented in 4 bytes ‣ex) Integer, Symbol, true/false/nil ‣struct RBasic* ‣objects data larger than 4 bytes ‣ex) String, Array, Class, user-defined classes
other struct RXXXX ‣ RObject is Object class ‣ RClass is Class class ‣ RClass have super class and method tables ‣ Similar to RArray, RHash, and so on struct RObject { MRB_OBJECT_HEADER; struct iv_tbl *iv; }; struct RClass { MRB_OBJECT_HEADER; struct iv_tbl *iv; struct kh_mt *mt; struct RClass *super; };
mruby VM has class objects Proc String Float Fixnum Symbol Kernel Object Class Array Hash TrueClass FalseClass Module Range NilClass mrb_state (mruby VM)
What’s structures in RAM ‣body of Hash ‣body of instance variables ‣methods defined at executing time ‣context ‣instance_vars and methods ‣current VM (mrb_state)
Function Name Resolution ‣use nm command to save symbol table of nvgen ‣nvgen parses the file and get address of functions ‣This method is used by TOPPERS configurator. $ nm nvgen > nvgen.syms $ ./nvgen nvgen.syms > generated.c
String’s pointer to char* ‣String object has a pointer to C string (char*) ‣But String is not mutable ‣To fix this, String used in mruby/mrbgems should be freezed
Using TLSF malloc ‣We want to know if a structure is in global variable area or in the heap area ‣Global variable should not be free() ‣Use TLSF malloc instead of standard malloc() ‣With TLSF malloc, we can know the object is in heap or not ‣see github.com/yamanekko/mruby-tlsf for detail
GC supporting ROM and RAM ‣ mruby uses tri-color GC (Black, White, Gray) ‣ We use quad-color GC, add Red ‣ Red are never GCed; objects in ROM are Red ‣ At first of GC cycle, all children of Red objects are marked ‣ VM has a list of Red objects
support flags in RAM ‣ obj->flags is used as status of objects ‣ flags should be mutable ‣ So an array of flags for ROM’s objects are located in RAM ‣ add macro to read and write them ‣ obj->flags in ROM are indexes of the array
Finding methods ‣ method tables are in both ROM and RAM ‣ tables for pre-defined methods are in ROM ‣ using iv_tbl to store pointer of pre-defined method tables ‣ key is __mt2__, which is not used as instance variables
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