堆内存分布
ART中的堆内存主要通过 Space 相关类进行管理，因此了解Heap的内存布局，首先需要了解有哪些Space。
如下是Heap中所有Space的继承关系。
Space枚举有下面几种类型：
c++
# /art/runtime/gc/space/space.h
69 enum SpaceType {
70 kSpaceTypeImageSpace,
71 kSpaceTypeMallocSpace,
72 kSpaceTypeZygoteSpace,
73 kSpaceTypeBumpPointerSpace,
74 kSpaceTypeLargeObjectSpace,
75 kSpaceTypeRegionSpace,
76 };
58 enum GcRetentionPolicy {
59 // Objects are retained forever with this policy for a space.
60 kGcRetentionPolicyNeverCollect,
61 // Every GC cycle will attempt to collect objects in this space.
62 kGcRetentionPolicyAlwaysCollect,
63 // Objects will be considered for collection only in "full" GC cycles, ie faster partial
64 // collections won't scan these areas such as the Zygote.
65 kGcRetentionPolicyFullCollect,
66 };- kGcRetentionPolicyNeverCollect 表示Space空间不会进行GC
- kGcRetentionPolicyAlwaysCollect 表示每次GC触发时都尝试进行垃圾回收
- kGcRetentionPolicyFullCollect 表示只在FullGC 的情况下进行垃圾回收
每种类型的Space的功能可以查看：juejin.cn/post/713874...
ART中的Heap中使用了哪些呢？
c++
# /art/runtime/gc/heap.h
1336 // All-known continuous spaces, where objects lie within fixed bounds.
1337 std::vector<space::ContinuousSpace*> continuous_spaces_ GUARDED_BY(Locks::mutator_lock_);
1338
1339 // All-known discontinuous spaces, where objects may be placed throughout virtual memory.
1340 std::vector<space::DiscontinuousSpace*> discontinuous_spaces_ GUARDED_BY(Locks::mutator_lock_);
1341
1342 // All-known alloc spaces, where objects may be or have been allocated.
1343 std::vector<space::AllocSpace*> alloc_spaces_;
1344
1345 // A space where non-movable objects are allocated, when compaction is enabled it contains
1346 // Classes, ArtMethods, ArtFields, and non moving objects.
1347 space::MallocSpace* non_moving_space_;
1348
1349 // Space which we use for the kAllocatorTypeROSAlloc.
1350 space::RosAllocSpace* rosalloc_space_;
1351
1352 // Space which we use for the kAllocatorTypeDlMalloc.
1353 space::DlMallocSpace* dlmalloc_space_;
1354
1355 // The main space is the space which the GC copies to and from on process state updates. This
1356 // space is typically either the dlmalloc_space_ or the rosalloc_space_.
1357 space::MallocSpace* main_space_;
1358
1359 // The large object space we are currently allocating into.
1360 space::LargeObjectSpace* large_object_space_;具体可查看：juejin.cn/post/739464...
App进程中的Heap由5个Space构成：
-
Image space：用于将boot相关的
.art、.oat、.vdex文件加载到内存中，其中的对象不可移动、不可回收。 -
Zygote space：zygote在第一次fork前会将main space和non-moving space中使用的对象规整到一起，成为zygote space，其中的对象不可移动、不可回收。
-
Non-moving space：DirectByteBuffer和早期的Bitmap，这类对象需要保证自己的内存不被移动，因为它的地址可能会被传递到native层使用。因此这类对象所处的空间称为non-moving space，其中的对象不可移动但可以被回收。
-
Large object space：它用于管理≥12KB的基本类型数组（譬如int\[\]）和字符串对象（java.lang.String）。其中的对象不会引用其他对象，因此是引用关系链的末端。作为末端的节点，它们在三色标记中不会出现灰色的状态，因此可以省去一些中间辅助的数据和环节。其中的对象不可移动但可以被回收。
-
Main space：App堆内存分配的主要场所，其中的对象既可以被移动，也可以被回收。
堆内存分配
Allocator分配器枚举类型如下：
c++
# /art/runtime/gc/allocator_type.h
27 // Different types of allocators.
28 // Those marked with * have fast path entrypoints callable from generated code.
29 enum AllocatorType : char {
30 // BumpPointer spaces are currently only used for ZygoteSpace construction.
31 kAllocatorTypeBumpPointer, // Use global CAS-based BumpPointer allocator. (*)
32 kAllocatorTypeTLAB, // Use TLAB allocator within BumpPointer space. (*)
33 kAllocatorTypeRosAlloc, // Use RosAlloc (segregated size, free list) allocator. (*)
34 kAllocatorTypeDlMalloc, // Use dlmalloc (well-known C malloc) allocator. (*)
35 kAllocatorTypeNonMoving, // Special allocator for non moving objects.
36 kAllocatorTypeLOS, // Large object space.
37 // The following differ from the BumpPointer allocators primarily in that memory is
38 // allocated from multiple regions, instead of a single contiguous space.
39 kAllocatorTypeRegion, // Use CAS-based contiguous bump-pointer allocation within a region. (*)
40 kAllocatorTypeRegionTLAB, // Use region pieces as TLABs. Default for most small objects. (*)
41 };根据分配器类型不同，以不同的方式为对象分配内存，分配过程详细可查看：juejin.cn/post/713874...