1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Memory allocator, based on tcmalloc.
6 // http://goog-perftools.sourceforge.net/doc/tcmalloc.html
8 // The main allocator works in runs of pages.
9 // Small allocation sizes (up to and including 32 kB) are
10 // rounded to one of about 100 size classes, each of which
11 // has its own free list of objects of exactly that size.
12 // Any free page of memory can be split into a set of objects
13 // of one size class, which are then managed using free list
16 // The allocator's data structures are:
18 // FixAlloc: a free-list allocator for fixed-size objects,
19 // used to manage storage used by the allocator.
20 // MHeap: the malloc heap, managed at page (4096-byte) granularity.
21 // MSpan: a run of pages managed by the MHeap.
22 // MCentral: a shared free list for a given size class.
23 // MCache: a per-thread (in Go, per-M) cache for small objects.
24 // MStats: allocation statistics.
26 // Allocating a small object proceeds up a hierarchy of caches:
28 // 1. Round the size up to one of the small size classes
29 // and look in the corresponding MCache free list.
30 // If the list is not empty, allocate an object from it.
31 // This can all be done without acquiring a lock.
33 // 2. If the MCache free list is empty, replenish it by
34 // taking a bunch of objects from the MCentral free list.
35 // Moving a bunch amortizes the cost of acquiring the MCentral lock.
37 // 3. If the MCentral free list is empty, replenish it by
38 // allocating a run of pages from the MHeap and then
39 // chopping that memory into a objects of the given size.
40 // Allocating many objects amortizes the cost of locking
43 // 4. If the MHeap is empty or has no page runs large enough,
44 // allocate a new group of pages (at least 1MB) from the
45 // operating system. Allocating a large run of pages
46 // amortizes the cost of talking to the operating system.
48 // Freeing a small object proceeds up the same hierarchy:
50 // 1. Look up the size class for the object and add it to
51 // the MCache free list.
53 // 2. If the MCache free list is too long or the MCache has
54 // too much memory, return some to the MCentral free lists.
56 // 3. If all the objects in a given span have returned to
57 // the MCentral list, return that span to the page heap.
59 // 4. If the heap has too much memory, return some to the
62 // TODO(rsc): Step 4 is not implemented.
64 // Allocating and freeing a large object uses the page heap
65 // directly, bypassing the MCache and MCentral free lists.
67 // The small objects on the MCache and MCentral free lists
68 // may or may not be zeroed. They are zeroed if and only if
69 // the second word of the object is zero. The spans in the
70 // page heap are always zeroed. When a span full of objects
71 // is returned to the page heap, the objects that need to be
72 // are zeroed first. There are two main benefits to delaying the
75 // 1. stack frames allocated from the small object lists
76 // can avoid zeroing altogether.
77 // 2. the cost of zeroing when reusing a small object is
78 // charged to the mutator, not the garbage collector.
80 // This C code was written with an eye toward translating to Go
81 // in the future. Methods have the form Type_Method(Type *t, ...).
83 typedef struct MCentral MCentral
;
84 typedef struct MHeap MHeap
;
85 typedef struct MSpan MSpan
;
86 typedef struct MStats MStats
;
87 typedef struct MLink MLink
;
88 typedef struct MTypes MTypes
;
93 PageSize
= 1<<PageShift
,
94 PageMask
= PageSize
- 1,
96 typedef uintptr PageID
; // address >> PageShift
100 // Computed constant. The definition of MaxSmallSize and the
101 // algorithm in msize.c produce some number of different allocation
102 // size classes. NumSizeClasses is that number. It's needed here
103 // because there are static arrays of this length; when msize runs its
104 // size choosing algorithm it double-checks that NumSizeClasses agrees.
107 // Tunable constants.
108 MaxSmallSize
= 32<<10,
110 FixAllocChunk
= 128<<10, // Chunk size for FixAlloc
111 MaxMCacheListLen
= 256, // Maximum objects on MCacheList
112 MaxMCacheSize
= 2<<20, // Maximum bytes in one MCache
113 MaxMHeapList
= 1<<(20 - PageShift
), // Maximum page length for fixed-size list in MHeap.
114 HeapAllocChunk
= 1<<20, // Chunk size for heap growth
116 // Number of bits in page to span calculations (4k pages).
117 // On 64-bit, we limit the arena to 128GB, or 37 bits.
118 // On 32-bit, we don't bother limiting anything, so we use the full 32-bit address.
119 #if __SIZEOF_POINTER__ == 8
120 MHeapMap_Bits
= 37 - PageShift
,
122 MHeapMap_Bits
= 32 - PageShift
,
125 // Max number of threads to run garbage collection.
126 // 2, 3, and 4 are all plausible maximums depending
127 // on the hardware details of the machine. The garbage
128 // collector scales well to 8 cpus.
132 // Maximum memory allocation size, a hint for callers.
133 // This must be a #define instead of an enum because it
135 #if __SIZEOF_POINTER__ == 8
136 #define MaxMem (1ULL<<(MHeapMap_Bits+PageShift)) /* 128 GB */
138 #define MaxMem ((uintptr)-1)
141 // A generic linked list of blocks. (Typically the block is bigger than sizeof(MLink).)
147 // SysAlloc obtains a large chunk of zeroed memory from the
148 // operating system, typically on the order of a hundred kilobytes
149 // or a megabyte. If the pointer argument is non-nil, the caller
150 // wants a mapping there or nowhere.
152 // SysUnused notifies the operating system that the contents
153 // of the memory region are no longer needed and can be reused
154 // for other purposes. The program reserves the right to start
155 // accessing those pages in the future.
157 // SysFree returns it unconditionally; this is only used if
158 // an out-of-memory error has been detected midway through
159 // an allocation. It is okay if SysFree is a no-op.
161 // SysReserve reserves address space without allocating memory.
162 // If the pointer passed to it is non-nil, the caller wants the
163 // reservation there, but SysReserve can still choose another
164 // location if that one is unavailable.
166 // SysMap maps previously reserved address space for use.
168 void* runtime_SysAlloc(uintptr nbytes
);
169 void runtime_SysFree(void *v
, uintptr nbytes
);
170 void runtime_SysUnused(void *v
, uintptr nbytes
);
171 void runtime_SysMap(void *v
, uintptr nbytes
);
172 void* runtime_SysReserve(void *v
, uintptr nbytes
);
174 // FixAlloc is a simple free-list allocator for fixed size objects.
175 // Malloc uses a FixAlloc wrapped around SysAlloc to manages its
176 // MCache and MSpan objects.
178 // Memory returned by FixAlloc_Alloc is not zeroed.
179 // The caller is responsible for locking around FixAlloc calls.
180 // Callers can keep state in the object but the first word is
181 // smashed by freeing and reallocating.
185 void *(*alloc
)(uintptr
);
186 void (*first
)(void *arg
, byte
*p
); // called first time p is returned
191 uintptr inuse
; // in-use bytes now
192 uintptr sys
; // bytes obtained from system
195 void runtime_FixAlloc_Init(FixAlloc
*f
, uintptr size
, void *(*alloc
)(uintptr
), void (*first
)(void*, byte
*), void *arg
);
196 void* runtime_FixAlloc_Alloc(FixAlloc
*f
);
197 void runtime_FixAlloc_Free(FixAlloc
*f
, void *p
);
201 // Shared with Go: if you edit this structure, also edit type MemStats in mem.go.
204 // General statistics.
205 uint64 alloc
; // bytes allocated and still in use
206 uint64 total_alloc
; // bytes allocated (even if freed)
207 uint64 sys
; // bytes obtained from system (should be sum of xxx_sys below, no locking, approximate)
208 uint64 nlookup
; // number of pointer lookups
209 uint64 nmalloc
; // number of mallocs
210 uint64 nfree
; // number of frees
212 // Statistics about malloc heap.
213 // protected by mheap.Lock
214 uint64 heap_alloc
; // bytes allocated and still in use
215 uint64 heap_sys
; // bytes obtained from system
216 uint64 heap_idle
; // bytes in idle spans
217 uint64 heap_inuse
; // bytes in non-idle spans
218 uint64 heap_released
; // bytes released to the OS
219 uint64 heap_objects
; // total number of allocated objects
221 // Statistics about allocation of low-level fixed-size structures.
222 // Protected by FixAlloc locks.
223 uint64 stacks_inuse
; // bootstrap stacks
225 uint64 mspan_inuse
; // MSpan structures
227 uint64 mcache_inuse
; // MCache structures
229 uint64 buckhash_sys
; // profiling bucket hash table
231 // Statistics about garbage collector.
232 // Protected by stopping the world during GC.
233 uint64 next_gc
; // next GC (in heap_alloc time)
234 uint64 last_gc
; // last GC (in absolute time)
235 uint64 pause_total_ns
;
236 uint64 pause_ns
[256];
241 // Statistics about allocation size classes.
246 } by_size
[NumSizeClasses
];
250 __asm__ (GOSYM_PREFIX
"runtime.VmemStats");
253 // Size classes. Computed and initialized by InitSizes.
255 // SizeToClass(0 <= n <= MaxSmallSize) returns the size class,
256 // 1 <= sizeclass < NumSizeClasses, for n.
257 // Size class 0 is reserved to mean "not small".
259 // class_to_size[i] = largest size in class i
260 // class_to_allocnpages[i] = number of pages to allocate when
261 // making new objects in class i
262 // class_to_transfercount[i] = number of objects to move when
263 // taking a bunch of objects out of the central lists
264 // and putting them in the thread free list.
266 int32
runtime_SizeToClass(int32
);
267 extern int32 runtime_class_to_size
[NumSizeClasses
];
268 extern int32 runtime_class_to_allocnpages
[NumSizeClasses
];
269 extern int32 runtime_class_to_transfercount
[NumSizeClasses
];
270 extern void runtime_InitSizes(void);
273 // Per-thread (in Go, per-M) cache for small objects.
274 // No locking needed because it is per-thread (per-M).
275 typedef struct MCacheList MCacheList
;
285 MCacheList list
[NumSizeClasses
];
287 intptr local_cachealloc
; // bytes allocated (or freed) from cache since last lock of heap
288 intptr local_objects
; // objects allocated (or freed) from cache since last lock of heap
289 intptr local_alloc
; // bytes allocated (or freed) since last lock of heap
290 uintptr local_total_alloc
; // bytes allocated (even if freed) since last lock of heap
291 uintptr local_nmalloc
; // number of mallocs since last lock of heap
292 uintptr local_nfree
; // number of frees since last lock of heap
293 uintptr local_nlookup
; // number of pointer lookups since last lock of heap
294 int32 next_sample
; // trigger heap sample after allocating this many bytes
295 // Statistics about allocation size classes since last lock of heap
299 } local_by_size
[NumSizeClasses
];
303 void* runtime_MCache_Alloc(MCache
*c
, int32 sizeclass
, uintptr size
, int32 zeroed
);
304 void runtime_MCache_Free(MCache
*c
, void *p
, int32 sizeclass
, uintptr size
);
305 void runtime_MCache_ReleaseAll(MCache
*c
);
307 // MTypes describes the types of blocks allocated within a span.
308 // The compression field describes the layout of the data.
311 // All blocks are free, or no type information is available for
313 // The data field has no meaning.
315 // The span contains just one block.
316 // The data field holds the type information.
317 // The sysalloc field has no meaning.
319 // The span contains multiple blocks.
320 // The data field points to an array of type [NumBlocks]uintptr,
321 // and each element of the array holds the type of the corresponding
324 // The span contains at most seven different types of blocks.
325 // The data field points to the following structure:
327 // type [8]uintptr // type[0] is always 0
328 // index [NumBlocks]byte
330 // The type of the i-th block is: data.type[data.index[i]]
340 byte compression
; // one of MTypes_*
341 bool sysalloc
; // whether (void*)data is from runtime_SysAlloc
345 // An MSpan is a run of pages.
355 MSpan
*next
; // in a span linked list
356 MSpan
*prev
; // in a span linked list
357 PageID start
; // starting page number
358 uintptr npages
; // number of pages in span
359 MLink
*freelist
; // list of free objects
360 uint32 ref
; // number of allocated objects in this span
361 int32 sizeclass
; // size class
362 uintptr elemsize
; // computed from sizeclass or from npages
363 uint32 state
; // MSpanInUse etc
364 int64 unusedsince
; // First time spotted by GC in MSpanFree state
365 uintptr npreleased
; // number of pages released to the OS
366 byte
*limit
; // end of data in span
367 MTypes types
; // types of allocated objects in this span
370 void runtime_MSpan_Init(MSpan
*span
, PageID start
, uintptr npages
);
372 // Every MSpan is in one doubly-linked list,
373 // either one of the MHeap's free lists or one of the
374 // MCentral's span lists. We use empty MSpan structures as list heads.
375 void runtime_MSpanList_Init(MSpan
*list
);
376 bool runtime_MSpanList_IsEmpty(MSpan
*list
);
377 void runtime_MSpanList_Insert(MSpan
*list
, MSpan
*span
);
378 void runtime_MSpanList_Remove(MSpan
*span
); // from whatever list it is in
381 // Central list of free objects of a given size.
391 void runtime_MCentral_Init(MCentral
*c
, int32 sizeclass
);
392 int32
runtime_MCentral_AllocList(MCentral
*c
, int32 n
, MLink
**first
);
393 void runtime_MCentral_FreeList(MCentral
*c
, int32 n
, MLink
*first
);
394 void runtime_MCentral_FreeSpan(MCentral
*c
, MSpan
*s
, int32 n
, MLink
*start
, MLink
*end
);
397 // The heap itself is the "free[]" and "large" arrays,
398 // but all the other global data is here too.
402 MSpan free
[MaxMHeapList
]; // free lists of given length
403 MSpan large
; // free lists length >= MaxMHeapList
409 MSpan
*map
[1<<MHeapMap_Bits
];
411 // range of addresses we might see in the heap
413 uintptr bitmap_mapped
;
418 // central free lists for small size classes.
419 // the union makes sure that the MCentrals are
420 // spaced CacheLineSize bytes apart, so that each MCentral.Lock
421 // gets its own cache line.
424 byte pad
[CacheLineSize
];
425 } central
[NumSizeClasses
];
427 FixAlloc spanalloc
; // allocator for Span*
428 FixAlloc cachealloc
; // allocator for MCache*
430 extern MHeap runtime_mheap
;
432 void runtime_MHeap_Init(MHeap
*h
, void *(*allocator
)(uintptr
));
433 MSpan
* runtime_MHeap_Alloc(MHeap
*h
, uintptr npage
, int32 sizeclass
, int32 acct
, int32 zeroed
);
434 void runtime_MHeap_Free(MHeap
*h
, MSpan
*s
, int32 acct
);
435 MSpan
* runtime_MHeap_Lookup(MHeap
*h
, void *v
);
436 MSpan
* runtime_MHeap_LookupMaybe(MHeap
*h
, void *v
);
437 void runtime_MGetSizeClassInfo(int32 sizeclass
, uintptr
*size
, int32
*npages
, int32
*nobj
);
438 void* runtime_MHeap_SysAlloc(MHeap
*h
, uintptr n
);
439 void runtime_MHeap_MapBits(MHeap
*h
);
440 void runtime_MHeap_Scavenger(void*);
442 void* runtime_mallocgc(uintptr size
, uint32 flag
, int32 dogc
, int32 zeroed
);
443 int32
runtime_mlookup(void *v
, byte
**base
, uintptr
*size
, MSpan
**s
);
444 void runtime_gc(int32 force
);
445 void runtime_markallocated(void *v
, uintptr n
, bool noptr
);
446 void runtime_checkallocated(void *v
, uintptr n
);
447 void runtime_markfreed(void *v
, uintptr n
);
448 void runtime_checkfreed(void *v
, uintptr n
);
449 extern int32 runtime_checking
;
450 void runtime_markspan(void *v
, uintptr size
, uintptr n
, bool leftover
);
451 void runtime_unmarkspan(void *v
, uintptr size
);
452 bool runtime_blockspecial(void*);
453 void runtime_setblockspecial(void*, bool);
454 void runtime_purgecachedstats(MCache
*);
455 void* runtime_new(const Type
*);
456 #define runtime_cnew(T) runtime_new(T)
458 void runtime_settype(void*, uintptr
);
459 void runtime_settype_flush(M
*, bool);
460 void runtime_settype_sysfree(MSpan
*);
461 uintptr
runtime_gettype(void*);
466 FlagNoPointers
= 1<<0, // no pointers here
467 FlagNoProfiling
= 1<<1, // must not profile
468 FlagNoGC
= 1<<2, // must not free or scan for pointers
471 typedef struct Obj Obj
;
474 byte
*p
; // data pointer
475 uintptr n
; // size of data in bytes
476 uintptr ti
; // type info
479 void runtime_MProf_Malloc(void*, uintptr
);
480 void runtime_MProf_Free(void*, uintptr
);
481 void runtime_MProf_GC(void);
482 void runtime_MProf_Mark(void (*addroot
)(Obj
));
483 int32
runtime_gcprocs(void);
484 void runtime_helpgc(int32 nproc
);
485 void runtime_gchelper(void);
487 struct __go_func_type
;
488 bool runtime_getfinalizer(void *p
, bool del
, void (**fn
)(void*), const struct __go_func_type
**ft
);
489 void runtime_walkfintab(void (*fn
)(void*), void (*scan
)(Obj
));
493 TypeInfo_SingleObject
= 0,
497 // Enables type information at the end of blocks allocated from heap
498 DebugTypeAtBlockEnd
= 0,
501 // defined in mgc0.go
502 void runtime_gc_m_ptr(Eface
*);
503 void runtime_gc_itab_ptr(Eface
*);
505 void runtime_memorydump(void);
507 void runtime_time_scan(void (*)(Obj
));
508 void runtime_trampoline_scan(void (*)(Obj
));