module GC

The GC module provides an interface to Ruby’s mark and sweep garbage collection mechanism.

Some of the underlying methods are also available via the ObjectSpace module.

You may obtain information about the operation of the GC through GC::Profiler.

Constants

INTERNAL_CONSTANTS

Internal constants in the garbage collector.

OPTS

GC build options

Public Class Methods

add_stress_to_class(class[, ...]) click to toggle source

Raises NoMemoryError when allocating an instance of the given classes.

static VALUE
rb_gcdebug_add_stress_to_class(int argc, VALUE *argv, VALUE self)
{
    rb_objspace_t *objspace = &rb_objspace;

    if (!stress_to_class) {
        set_stress_to_class(rb_ary_hidden_new(argc));
    }
    rb_ary_cat(stress_to_class, argv, argc);
    return self;
}
auto_compact → true or false click to toggle source

Returns whether or not automatic compaction has been enabled.

static VALUE
gc_get_auto_compact(VALUE _)
{
    return RBOOL(ruby_enable_autocompact);
}
auto_compact = flag click to toggle source

Updates automatic compaction mode.

When enabled, the compactor will execute on every major collection.

Enabling compaction will degrade performance on major collections.

static VALUE
gc_set_auto_compact(VALUE _, VALUE v)
{
    GC_ASSERT(GC_COMPACTION_SUPPORTED);

    ruby_enable_autocompact = RTEST(v);

#if RGENGC_CHECK_MODE
    ruby_autocompact_compare_func = NULL;

    if (SYMBOL_P(v)) {
        ID id = RB_SYM2ID(v);
        if (id == rb_intern("empty")) {
            ruby_autocompact_compare_func = compare_free_slots;
        }
    }
#endif

    return v;
}
compact → hash click to toggle source

This function compacts objects together in Ruby’s heap. It eliminates unused space (or fragmentation) in the heap by moving objects in to that unused space.

The returned hash contains statistics about the objects that were moved; see GC.latest_compact_info.

This method is only expected to work on CRuby.

To test whether GC compaction is supported, use the idiom:

GC.respond_to?(:compact)
static VALUE
gc_compact(VALUE self)
{
    /* Run GC with compaction enabled */
    gc_start_internal(NULL, self, Qtrue, Qtrue, Qtrue, Qtrue);

    return gc_compact_stats(self);
}
count → Integer click to toggle source

The number of times GC occurred.

It returns the number of times GC occurred since the process started.

# File gc.rb, line 105
def self.count
  Primitive.gc_count
end
disable → true or false click to toggle source

Disables garbage collection, returning true if garbage collection was already disabled.

GC.disable   #=> false
GC.disable   #=> true
# File gc.rb, line 69
def self.disable
  Primitive.gc_disable
end
enable → true or false click to toggle source

Enables garbage collection, returning true if garbage collection was previously disabled.

GC.disable   #=> false
GC.enable    #=> true
GC.enable    #=> false
# File gc.rb, line 57
def self.enable
  Primitive.gc_enable
end
latest_compact_info → hash click to toggle source

Returns information about object moved in the most recent GC compaction.

The returned hash has the following keys:

  • :considered: a hash containing the type of the object as the key and the number of objects of that type that were considered for movement.

  • :moved: a hash containing the type of the object as the key and the number of objects of that type that were actually moved.

  • :moved_up: a hash containing the type of the object as the key and the number of objects of that type that were increased in size.

  • :moved_down: a hash containing the type of the object as the key and the number of objects of that type that were decreased in size.

Some objects can’t be moved (due to pinning) so these numbers can be used to calculate compaction efficiency.

static VALUE
gc_compact_stats(VALUE self)
{
    size_t i;
    rb_objspace_t *objspace = &rb_objspace;
    VALUE h = rb_hash_new();
    VALUE considered = rb_hash_new();
    VALUE moved = rb_hash_new();
    VALUE moved_up = rb_hash_new();
    VALUE moved_down = rb_hash_new();

    for (i=0; i<T_MASK; i++) {
        if (objspace->rcompactor.considered_count_table[i]) {
            rb_hash_aset(considered, type_sym(i), SIZET2NUM(objspace->rcompactor.considered_count_table[i]));
        }

        if (objspace->rcompactor.moved_count_table[i]) {
            rb_hash_aset(moved, type_sym(i), SIZET2NUM(objspace->rcompactor.moved_count_table[i]));
        }

        if (objspace->rcompactor.moved_up_count_table[i]) {
            rb_hash_aset(moved_up, type_sym(i), SIZET2NUM(objspace->rcompactor.moved_up_count_table[i]));
        }

        if (objspace->rcompactor.moved_down_count_table[i]) {
            rb_hash_aset(moved_down, type_sym(i), SIZET2NUM(objspace->rcompactor.moved_down_count_table[i]));
        }
    }

    rb_hash_aset(h, ID2SYM(rb_intern("considered")), considered);
    rb_hash_aset(h, ID2SYM(rb_intern("moved")), moved);
    rb_hash_aset(h, ID2SYM(rb_intern("moved_up")), moved_up);
    rb_hash_aset(h, ID2SYM(rb_intern("moved_down")), moved_down);

    return h;
}
latest_gc_info → hash click to toggle source
latest_gc_info(hash) → hash
latest_gc_info(:major_by) → :malloc

Returns information about the most recent garbage collection.

If the optional argument, hash, is given, it is overwritten and returned. This is intended to avoid probe effect.

# File gc.rb, line 266
def self.latest_gc_info hash_or_key = nil
  Primitive.gc_latest_gc_info hash_or_key
end
malloc_allocated_size → Integer click to toggle source

Returns the size of memory allocated by malloc().

Only available if ruby was built with CALC_EXACT_MALLOC_SIZE.

static VALUE
gc_malloc_allocated_size(VALUE self)
{
    return UINT2NUM(rb_objspace.malloc_params.allocated_size);
}
malloc_allocations → Integer click to toggle source

Returns the number of malloc() allocations.

Only available if ruby was built with CALC_EXACT_MALLOC_SIZE.

static VALUE
gc_malloc_allocations(VALUE self)
{
    return UINT2NUM(rb_objspace.malloc_params.allocations);
}
measure_total_time → true/false click to toggle source

Return measure_total_time flag (default: true). Note that measurement can affect the application performance.

# File gc.rb, line 308
def self.measure_total_time
  Primitive.cexpr! %{
    RBOOL(rb_objspace.flags.measure_gc)
  }
end
measure_total_time = true/false click to toggle source

Enable to measure GC time. You can get the result with GC.stat(:time). Note that GC time measurement can cause some performance overhead.

# File gc.rb, line 296
def self.measure_total_time=(flag)
  Primitive.cstmt! %{
    rb_objspace.flags.measure_gc = RTEST(flag) ? TRUE : FALSE;
    return flag;
  }
end
remove_stress_to_class(class[, ...]) click to toggle source

No longer raises NoMemoryError when allocating an instance of the given classes.

static VALUE
rb_gcdebug_remove_stress_to_class(int argc, VALUE *argv, VALUE self)
{
    rb_objspace_t *objspace = &rb_objspace;
    int i;

    if (stress_to_class) {
        for (i = 0; i < argc; ++i) {
            rb_ary_delete_same(stress_to_class, argv[i]);
        }
        if (RARRAY_LEN(stress_to_class) == 0) {
            set_stress_to_class(0);
        }
    }
    return Qnil;
}
start(full_mark: true, immediate_mark: true, immediate_sweep: true) click to toggle source

Initiates garbage collection, even if manually disabled.

The full_mark keyword argument determines whether or not to perform a major garbage collection cycle. When set to true, a major garbage collection cycle is ran, meaning all objects are marked. When set to false, a minor garbage collection cycle is ran, meaning only young objects are marked.

The immediate_mark keyword argument determines whether or not to perform incremental marking. When set to true, marking is completed during the call to this method. When set to false, marking is performed in steps that is interleaved with future Ruby code execution, so marking might not be completed during this method call. Note that if full_mark is false then marking will always be immediate, regardless of the value of immediate_mark.

The immedate_sweep keyword argument determines whether or not to defer sweeping (using lazy sweep). When set to true, sweeping is performed in steps that is interleaved with future Ruby code execution, so sweeping might not be completed during this method call. When set to false, sweeping is completed during the call to this method.

Note: These keyword arguments are implementation and version dependent. They are not guaranteed to be future-compatible, and may be ignored if the underlying implementation does not support them.

# File gc.rb, line 38
def self.start full_mark: true, immediate_mark: true, immediate_sweep: true
  Primitive.gc_start_internal full_mark, immediate_mark, immediate_sweep, false
end
stat → Hash click to toggle source
stat(hash) → Hash
stat(:key) → Numeric

Returns a Hash containing information about the GC.

The contents of the hash are implementation specific and may change in the future without notice.

The hash includes information about internal statistics about GC such as:

count

The total number of garbage collections ran since application start (count includes both minor and major garbage collections)

time

The total time spent in garbage collections (in milliseconds)

heap_allocated_pages

The total number of :heap_eden_pages + :heap_tomb_pages

heap_sorted_length

The number of pages that can fit into the buffer that holds references to all pages

heap_allocatable_pages

The total number of pages the application could allocate without additional GC

heap_available_slots

The total number of slots in all :heap_allocated_pages

heap_live_slots

The total number of slots which contain live objects

heap_free_slots

The total number of slots which do not contain live objects

heap_final_slots

The total number of slots with pending finalizers to be run

heap_marked_slots

The total number of objects marked in the last GC

heap_eden_pages

The total number of pages which contain at least one live slot

heap_tomb_pages

The total number of pages which do not contain any live slots

total_allocated_pages

The cumulative number of pages allocated since application start

total_freed_pages

The cumulative number of pages freed since application start

total_allocated_objects

The cumulative number of objects allocated since application start

total_freed_objects

The cumulative number of objects freed since application start

malloc_increase_bytes

Amount of memory allocated on the heap for objects. Decreased by any GC

malloc_increase_bytes_limit

When :malloc_increase_bytes crosses this limit, GC is triggered

minor_gc_count

The total number of minor garbage collections run since process start

major_gc_count

The total number of major garbage collections run since process start

compact_count

The total number of compactions run since process start

read_barrier_faults

The total number of times the read barrier was triggered during compaction

total_moved_objects

The total number of objects compaction has moved

remembered_wb_unprotected_objects

The total number of objects without write barriers

remembered_wb_unprotected_objects_limit

When :remembered_wb_unprotected_objects crosses this limit, major GC is triggered

old_objects

Number of live, old objects which have survived at least 3 garbage collections

old_objects_limit

When :old_objects crosses this limit, major GC is triggered

oldmalloc_increase_bytes

Amount of memory allocated on the heap for objects. Decreased by major GC

oldmalloc_increase_bytes_limit

When :old_malloc_increase_bytes crosses this limit, major GC is triggered

If the optional argument, hash, is given, it is overwritten and returned. This is intended to avoid probe effect.

This method is only expected to work on CRuby.

# File gc.rb, line 189
def self.stat hash_or_key = nil
  Primitive.gc_stat hash_or_key
end
stat_heap → Hash click to toggle source
stat_heap(nil, hash) → Hash
stat_heap(heap_name) → Hash
stat_heap(heap_name, hash) → Hash
stat_heap(heap_name, :key) → Numeric

Returns information for heaps in the GC.

If the first optional argument, heap_name, is passed in and not nil, it returns a Hash containing information about the particular heap. Otherwise, it will return a Hash with heap names as keys and a Hash containing information about the heap as values.

If the second optional argument, hash_or_key, is given as Hash, it will be overwritten and returned. This is intended to avoid the probe effect.

If both optional arguments are passed in and the second optional argument is a symbol, it will return a Numeric of the value for the particular heap.

On CRuby, heap_name is of the type Integer but may be of type String on other implementations.

The contents of the hash are implementation specific and may change in the future without notice.

If the optional argument, hash, is given, it is overwritten and returned.

This method is only expected to work on CRuby.

The hash includes the following keys about the internal information in the GC:

slot_size

The slot size of the heap in bytes.

heap_allocatable_pages

The number of pages that can be allocated without triggering a new garbage collection cycle.

heap_eden_pages

The number of pages in the eden heap.

heap_eden_slots

The total number of slots in all of the pages in the eden heap.

heap_tomb_pages

The number of pages in the tomb heap. The tomb heap only contains pages that do not have any live objects.

heap_tomb_slots

The total number of slots in all of the pages in the tomb heap.

total_allocated_pages

The total number of pages that have been allocated in the heap.

total_freed_pages

The total number of pages that have been freed and released back to the system in the heap.

force_major_gc_count

The number of times major garbage collection cycles this heap has forced to start due to running out of free slots.

force_incremental_marking_finish_count

The number of times this heap has forced incremental marking to complete due to running out of pooled slots.

# File gc.rb, line 252
def self.stat_heap heap_name = nil, hash_or_key = nil
  Primitive.gc_stat_heap heap_name, hash_or_key
end
stress → integer, true or false click to toggle source

Returns current status of GC stress mode.

# File gc.rb, line 77
def self.stress
  Primitive.gc_stress_get
end
stress = flag → flag click to toggle source

Updates the GC stress mode.

When stress mode is enabled, the GC is invoked at every GC opportunity: all memory and object allocations.

Enabling stress mode will degrade performance, it is only for debugging.

flag can be true, false, or an integer bit-ORed following flags.

0x01:: no major GC
0x02:: no immediate sweep
0x04:: full mark after malloc/calloc/realloc
# File gc.rb, line 95
def self.stress=(flag)
  Primitive.gc_stress_set_m flag
end
total_time → int click to toggle source

Return measured GC total time in nano seconds.

# File gc.rb, line 318
def self.total_time
  Primitive.cexpr! %{
    ULL2NUM(rb_objspace.profile.marking_time_ns + rb_objspace.profile.sweeping_time_ns)
  }
end
verify_compaction_references(toward: nil, double_heap: false) → hash click to toggle source

Verify compaction reference consistency.

This method is implementation specific. During compaction, objects that were moved are replaced with T_MOVED objects. No object should have a reference to a T_MOVED object after compaction.

This function expands the heap to ensure room to move all objects, compacts the heap to make sure everything moves, updates all references, then performs a full GC. If any object contains a reference to a T_MOVED object, that object should be pushed on the mark stack, and will make a SEGV.

# File gc.rb, line 285
def self.verify_compaction_references(toward: nil, double_heap: false, expand_heap: false)
  Primitive.gc_verify_compaction_references(double_heap, expand_heap, toward == :empty)
end
verify_internal_consistency → nil click to toggle source

Verify internal consistency.

This method is implementation specific. Now this method checks generational consistency if RGenGC is supported.

static VALUE
gc_verify_internal_consistency_m(VALUE dummy)
{
    gc_verify_internal_consistency(&rb_objspace);
    return Qnil;
}

Public Instance Methods

garbage_collect(full_mark: true, immediate_mark: true, immediate_sweep: true) click to toggle source

Alias of GC.start

# File gc.rb, line 43
def garbage_collect full_mark: true, immediate_mark: true, immediate_sweep: true
  Primitive.gc_start_internal full_mark, immediate_mark, immediate_sweep, false
end