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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Memory allocator, based on tcmalloc.
// The main allocator works in runs of pages.
// Small allocation sizes (up to and including 32 kB) are
// rounded to one of about 100 size classes, each of which
// has its own free list of objects of exactly that size.
// Any free page of memory can be split into a set of objects
// of one size class, which are then managed using free list
// allocators.
// The allocator's data structures are:
// FixAlloc: a free-list allocator for fixed-size objects,
// used to manage storage used by the allocator.
// MHeap: the malloc heap, managed at page (4096-byte) granularity.
// MSpan: a run of pages managed by the MHeap.
// MCentral: a shared free list for a given size class.
// MCache: a per-thread (in Go, per-M) cache for small objects.
// MStats: allocation statistics.
// Allocating a small object proceeds up a hierarchy of caches:
// 1. Round the size up to one of the small size classes
// and look in the corresponding MCache free list.
// If the list is not empty, allocate an object from it.
// This can all be done without acquiring a lock.
// 2. If the MCache free list is empty, replenish it by
// taking a bunch of objects from the MCentral free list.
// Moving a bunch amortizes the cost of acquiring the MCentral lock.
// 3. If the MCentral free list is empty, replenish it by
// allocating a run of pages from the MHeap and then
// chopping that memory into a objects of the given size.
// Allocating many objects amortizes the cost of locking
// the heap.
// 4. If the MHeap is empty or has no page runs large enough,
// allocate a new group of pages (at least 1MB) from the
// operating system. Allocating a large run of pages
// amortizes the cost of talking to the operating system.
// Freeing a small object proceeds up the same hierarchy:
// 1. Look up the size class for the object and add it to
// the MCache free list.
// 2. If the MCache free list is too long or the MCache has
// too much memory, return some to the MCentral free lists.
// 3. If all the objects in a given span have returned to
// the MCentral list, return that span to the page heap.
// 4. If the heap has too much memory, return some to the
// operating system.
// TODO(rsc): Step 4 is not implemented.
// Allocating and freeing a large object uses the page heap
// directly, bypassing the MCache and MCentral free lists.
// The small objects on the MCache and MCentral free lists
// may or may not be zeroed. They are zeroed if and only if
// the second word of the object is zero. The spans in the
// page heap are always zeroed. When a span full of objects
// is returned to the page heap, the objects that need to be
// are zeroed first. There are two main benefits to delaying the
// zeroing this way:
// 1. stack frames allocated from the small object lists
// can avoid zeroing altogether.
// 2. the cost of zeroing when reusing a small object is
// charged to the mutator, not the garbage collector.
// This C code was written with an eye toward translating to Go
// in the future. Methods have the form Type_Method(Type *t, ...).
typedef struct FixAlloc FixAlloc;
typedef struct MCentral MCentral;
typedef struct MHeap MHeap;
typedef struct MSpan MSpan;
typedef struct MStats MStats;
typedef struct MLink MLink;
PageShift = 12,
PageSize = 1<<PageShift,
PageMask = PageSize - 1,
typedef uintptr PageID; // address >> PageShift
// Computed constant. The definition of MaxSmallSize and the
// algorithm in msize.c produce some number of different allocation
// size classes. NumSizeClasses is that number. It's needed here
// because there are static arrays of this length; when msize runs its
// size choosing algorithm it double-checks that NumSizeClasses agrees.
NumSizeClasses = 61,
// Tunable constants.
MaxSmallSize = 32<<10,
FixAllocChunk = 128<<10, // Chunk size for FixAlloc
MaxMCacheListLen = 256, // Maximum objects on MCacheList
MaxMCacheSize = 2<<20, // Maximum bytes in one MCache
MaxMHeapList = 1<<(20 - PageShift), // Maximum page length for fixed-size list in MHeap.
HeapAllocChunk = 1<<20, // Chunk size for heap growth
// Number of bits in page to span calculations (4k pages).
// On 64-bit, we limit the arena to 16G, so 22 bits suffices.
// On 32-bit, we don't bother limiting anything: 20 bits for 4G.
#ifdef _64BIT
MHeapMap_Bits = 22,
MHeapMap_Bits = 20,
// A generic linked list of blocks. (Typically the block is bigger than sizeof(MLink).)
struct MLink
MLink *next;
// SysAlloc obtains a large chunk of zeroed memory from the
// operating system, typically on the order of a hundred kilobytes
// or a megabyte. If the pointer argument is non-nil, the caller
// wants a mapping there or nowhere.
// SysUnused notifies the operating system that the contents
// of the memory region are no longer needed and can be reused
// for other purposes. The program reserves the right to start
// accessing those pages in the future.
// SysFree returns it unconditionally; this is only used if
// an out-of-memory error has been detected midway through
// an allocation. It is okay if SysFree is a no-op.
// SysReserve reserves address space without allocating memory.
// If the pointer passed to it is non-nil, the caller wants the
// reservation there, but SysReserve can still choose another
// location if that one is unavailable.
// SysMap maps previously reserved address space for use.
void* runtime·SysAlloc(uintptr nbytes);
void runtime·SysFree(void *v, uintptr nbytes);
void runtime·SysUnused(void *v, uintptr nbytes);
void runtime·SysMap(void *v, uintptr nbytes);
void* runtime·SysReserve(void *v, uintptr nbytes);
// FixAlloc is a simple free-list allocator for fixed size objects.
// Malloc uses a FixAlloc wrapped around SysAlloc to manages its
// MCache and MSpan objects.
// Memory returned by FixAlloc_Alloc is not zeroed.
// The caller is responsible for locking around FixAlloc calls.
// Callers can keep state in the object but the first word is
// smashed by freeing and reallocating.
struct FixAlloc
uintptr size;
void *(*alloc)(uintptr);
void (*first)(void *arg, byte *p); // called first time p is returned
void *arg;
MLink *list;
byte *chunk;
uint32 nchunk;
uintptr inuse; // in-use bytes now
uintptr sys; // bytes obtained from system
void runtime·FixAlloc_Init(FixAlloc *f, uintptr size, void *(*alloc)(uintptr), void (*first)(void*, byte*), void *arg);
void* runtime·FixAlloc_Alloc(FixAlloc *f);
void runtime·FixAlloc_Free(FixAlloc *f, void *p);
// Statistics.
// Shared with Go: if you edit this structure, also edit extern.go.
struct MStats
// General statistics. No locking; approximate.
uint64 alloc; // bytes allocated and still in use
uint64 total_alloc; // bytes allocated (even if freed)
uint64 sys; // bytes obtained from system (should be sum of xxx_sys below)
uint64 nlookup; // number of pointer lookups
uint64 nmalloc; // number of mallocs
uint64 nfree; // number of frees
// Statistics about malloc heap.
// protected by mheap.Lock
uint64 heap_alloc; // bytes allocated and still in use
uint64 heap_sys; // bytes obtained from system
uint64 heap_idle; // bytes in idle spans
uint64 heap_inuse; // bytes in non-idle spans
uint64 heap_objects; // total number of allocated objects
// Statistics about allocation of low-level fixed-size structures.
// Protected by FixAlloc locks.
uint64 stacks_inuse; // bootstrap stacks
uint64 stacks_sys;
uint64 mspan_inuse; // MSpan structures
uint64 mspan_sys;
uint64 mcache_inuse; // MCache structures
uint64 mcache_sys;
uint64 buckhash_sys; // profiling bucket hash table
// Statistics about garbage collector.
// Protected by stopping the world during GC.
uint64 next_gc; // next GC (in heap_alloc time)
uint64 pause_total_ns;
uint64 pause_ns[256];
uint32 numgc;
bool enablegc;
bool debuggc;
// Statistics about allocation size classes.
// No locking; approximate.
struct {
uint32 size;
uint64 nmalloc;
uint64 nfree;
} by_size[NumSizeClasses];
#define mstats runtime·MemStats /* name shared with Go */
extern MStats mstats;
// Size classes. Computed and initialized by InitSizes.
// SizeToClass(0 <= n <= MaxSmallSize) returns the size class,
// 1 <= sizeclass < NumSizeClasses, for n.
// Size class 0 is reserved to mean "not small".
// class_to_size[i] = largest size in class i
// class_to_allocnpages[i] = number of pages to allocate when
// making new objects in class i
// class_to_transfercount[i] = number of objects to move when
// taking a bunch of objects out of the central lists
// and putting them in the thread free list.
int32 runtime·SizeToClass(int32);
extern int32 runtime·class_to_size[NumSizeClasses];
extern int32 runtime·class_to_allocnpages[NumSizeClasses];
extern int32 runtime·class_to_transfercount[NumSizeClasses];
extern void runtime·InitSizes(void);
// Per-thread (in Go, per-M) cache for small objects.
// No locking needed because it is per-thread (per-M).
typedef struct MCacheList MCacheList;
struct MCacheList
MLink *list;
uint32 nlist;
uint32 nlistmin;
struct MCache
MCacheList list[NumSizeClasses];
uint64 size;
int64 local_alloc; // bytes allocated (or freed) since last lock of heap
int64 local_objects; // objects allocated (or freed) since last lock of heap
int32 next_sample; // trigger heap sample after allocating this many bytes
void* runtime·MCache_Alloc(MCache *c, int32 sizeclass, uintptr size, int32 zeroed);
void runtime·MCache_Free(MCache *c, void *p, int32 sizeclass, uintptr size);
void runtime·MCache_ReleaseAll(MCache *c);
// An MSpan is a run of pages.
MSpanInUse = 0,
struct MSpan
MSpan *next; // in a span linked list
MSpan *prev; // in a span linked list
MSpan *allnext; // in the list of all spans
PageID start; // starting page number
uintptr npages; // number of pages in span
MLink *freelist; // list of free objects
uint32 ref; // number of allocated objects in this span
uint32 sizeclass; // size class
uint32 state; // MSpanInUse etc
byte *limit; // end of data in span
void runtime·MSpan_Init(MSpan *span, PageID start, uintptr npages);
// Every MSpan is in one doubly-linked list,
// either one of the MHeap's free lists or one of the
// MCentral's span lists. We use empty MSpan structures as list heads.
void runtime·MSpanList_Init(MSpan *list);
bool runtime·MSpanList_IsEmpty(MSpan *list);
void runtime·MSpanList_Insert(MSpan *list, MSpan *span);
void runtime·MSpanList_Remove(MSpan *span); // from whatever list it is in
// Central list of free objects of a given size.
struct MCentral
int32 sizeclass;
MSpan nonempty;
MSpan empty;
int32 nfree;
void runtime·MCentral_Init(MCentral *c, int32 sizeclass);
int32 runtime·MCentral_AllocList(MCentral *c, int32 n, MLink **first);
void runtime·MCentral_FreeList(MCentral *c, int32 n, MLink *first);
// Main malloc heap.
// The heap itself is the "free[]" and "large" arrays,
// but all the other global data is here too.
struct MHeap
MSpan free[MaxMHeapList]; // free lists of given length
MSpan large; // free lists length >= MaxMHeapList
MSpan *allspans;
// span lookup
MSpan *map[1<<MHeapMap_Bits];
// range of addresses we might see in the heap
byte *bitmap;
uintptr bitmap_mapped;
byte *arena_start;
byte *arena_used;
byte *arena_end;
// central free lists for small size classes.
// the union makes sure that the MCentrals are
// spaced 64 bytes apart, so that each MCentral.Lock
// gets its own cache line.
union {
byte pad[64];
} central[NumSizeClasses];
FixAlloc spanalloc; // allocator for Span*
FixAlloc cachealloc; // allocator for MCache*
extern MHeap runtime·mheap;
void runtime·MHeap_Init(MHeap *h, void *(*allocator)(uintptr));
MSpan* runtime·MHeap_Alloc(MHeap *h, uintptr npage, int32 sizeclass, int32 acct);
void runtime·MHeap_Free(MHeap *h, MSpan *s, int32 acct);
MSpan* runtime·MHeap_Lookup(MHeap *h, void *v);
MSpan* runtime·MHeap_LookupMaybe(MHeap *h, void *v);
void runtime·MGetSizeClassInfo(int32 sizeclass, uintptr *size, int32 *npages, int32 *nobj);
void* runtime·MHeap_SysAlloc(MHeap *h, uintptr n);
void runtime·MHeap_MapBits(MHeap *h);
void* runtime·mallocgc(uintptr size, uint32 flag, int32 dogc, int32 zeroed);
int32 runtime·mlookup(void *v, byte **base, uintptr *size, MSpan **s);
void runtime·gc(int32 force);
void runtime·markallocated(void *v, uintptr n, bool noptr);
void runtime·checkallocated(void *v, uintptr n);
void runtime·markfreed(void *v, uintptr n);
void runtime·checkfreed(void *v, uintptr n);
int32 runtime·checking;
void runtime·markspan(void *v, uintptr size, uintptr n, bool leftover);
void runtime·unmarkspan(void *v, uintptr size);
bool runtime·blockspecial(void*);
void runtime·setblockspecial(void*);
// flags to malloc
FlagNoPointers = 1<<0, // no pointers here
FlagNoProfiling = 1<<1, // must not profile
FlagNoGC = 1<<2, // must not free or scan for pointers
void runtime·MProf_Malloc(void*, uintptr);
void runtime·MProf_Free(void*, uintptr);
// Malloc profiling settings.
// Must match definition in extern.go.
enum {
MProf_None = 0,
MProf_Sample = 1,
MProf_All = 2,
extern int32 runtime·malloc_profile;
typedef struct Finalizer Finalizer;
struct Finalizer
Finalizer *next; // for use by caller of getfinalizer
void (*fn)(void*);
void *arg;
int32 nret;
Finalizer* runtime·getfinalizer(void*, bool);