github.com/4ad/go@v0.0.0-20161219182952-69a12818b605/src/runtime/mcache.go (about)

     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.
     4  
     5  package runtime
     6  
     7  import "unsafe"
     8  
     9  // Per-thread (in Go, per-P) cache for small objects.
    10  // No locking needed because it is per-thread (per-P).
    11  //
    12  // mcaches are allocated from non-GC'd memory, so any heap pointers
    13  // must be specially handled.
    14  type mcache struct {
    15  	// The following members are accessed on every malloc,
    16  	// so they are grouped here for better caching.
    17  	next_sample int32   // trigger heap sample after allocating this many bytes
    18  	local_scan  uintptr // bytes of scannable heap allocated
    19  
    20  	// Allocator cache for tiny objects w/o pointers.
    21  	// See "Tiny allocator" comment in malloc.go.
    22  
    23  	// tiny points to the beginning of the current tiny block, or
    24  	// nil if there is no current tiny block.
    25  	//
    26  	// tiny is a heap pointer. Since mcache is in non-GC'd memory,
    27  	// we handle it by clearing it in releaseAll during mark
    28  	// termination.
    29  	tiny             uintptr
    30  	tinyoffset       uintptr
    31  	local_tinyallocs uintptr // number of tiny allocs not counted in other stats
    32  
    33  	// The rest is not accessed on every malloc.
    34  	alloc [_NumSizeClasses]*mspan // spans to allocate from
    35  
    36  	stackcache [_NumStackOrders]stackfreelist
    37  
    38  	// Local allocator stats, flushed during GC.
    39  	local_nlookup    uintptr                  // number of pointer lookups
    40  	local_largefree  uintptr                  // bytes freed for large objects (>maxsmallsize)
    41  	local_nlargefree uintptr                  // number of frees for large objects (>maxsmallsize)
    42  	local_nsmallfree [_NumSizeClasses]uintptr // number of frees for small objects (<=maxsmallsize)
    43  }
    44  
    45  // A gclink is a node in a linked list of blocks, like mlink,
    46  // but it is opaque to the garbage collector.
    47  // The GC does not trace the pointers during collection,
    48  // and the compiler does not emit write barriers for assignments
    49  // of gclinkptr values. Code should store references to gclinks
    50  // as gclinkptr, not as *gclink.
    51  type gclink struct {
    52  	next gclinkptr
    53  }
    54  
    55  // A gclinkptr is a pointer to a gclink, but it is opaque
    56  // to the garbage collector.
    57  type gclinkptr uintptr
    58  
    59  // ptr returns the *gclink form of p.
    60  // The result should be used for accessing fields, not stored
    61  // in other data structures.
    62  func (p gclinkptr) ptr() *gclink {
    63  	return (*gclink)(unsafe.Pointer(p))
    64  }
    65  
    66  type stackfreelist struct {
    67  	list gclinkptr // linked list of free stacks
    68  	size uintptr   // total size of stacks in list
    69  }
    70  
    71  // dummy MSpan that contains no free objects.
    72  var emptymspan mspan
    73  
    74  func allocmcache() *mcache {
    75  	lock(&mheap_.lock)
    76  	c := (*mcache)(mheap_.cachealloc.alloc())
    77  	unlock(&mheap_.lock)
    78  	memclr(unsafe.Pointer(c), unsafe.Sizeof(*c))
    79  	for i := 0; i < _NumSizeClasses; i++ {
    80  		c.alloc[i] = &emptymspan
    81  	}
    82  	c.next_sample = nextSample()
    83  	return c
    84  }
    85  
    86  func freemcache(c *mcache) {
    87  	systemstack(func() {
    88  		c.releaseAll()
    89  		stackcache_clear(c)
    90  
    91  		// NOTE(rsc,rlh): If gcworkbuffree comes back, we need to coordinate
    92  		// with the stealing of gcworkbufs during garbage collection to avoid
    93  		// a race where the workbuf is double-freed.
    94  		// gcworkbuffree(c.gcworkbuf)
    95  
    96  		lock(&mheap_.lock)
    97  		purgecachedstats(c)
    98  		mheap_.cachealloc.free(unsafe.Pointer(c))
    99  		unlock(&mheap_.lock)
   100  	})
   101  }
   102  
   103  // Gets a span that has a free object in it and assigns it
   104  // to be the cached span for the given sizeclass. Returns this span.
   105  func (c *mcache) refill(sizeclass int32) *mspan {
   106  	_g_ := getg()
   107  
   108  	_g_.m.locks++
   109  	// Return the current cached span to the central lists.
   110  	s := c.alloc[sizeclass]
   111  
   112  	if uintptr(s.allocCount) != s.nelems {
   113  		throw("refill of span with free space remaining")
   114  	}
   115  
   116  	if s != &emptymspan {
   117  		s.incache = false
   118  	}
   119  
   120  	// Get a new cached span from the central lists.
   121  	s = mheap_.central[sizeclass].mcentral.cacheSpan()
   122  	if s == nil {
   123  		throw("out of memory")
   124  	}
   125  
   126  	if uintptr(s.allocCount) == s.nelems {
   127  		throw("span has no free space")
   128  	}
   129  
   130  	c.alloc[sizeclass] = s
   131  	_g_.m.locks--
   132  	return s
   133  }
   134  
   135  func (c *mcache) releaseAll() {
   136  	for i := 0; i < _NumSizeClasses; i++ {
   137  		s := c.alloc[i]
   138  		if s != &emptymspan {
   139  			mheap_.central[i].mcentral.uncacheSpan(s)
   140  			c.alloc[i] = &emptymspan
   141  		}
   142  	}
   143  	// Clear tinyalloc pool.
   144  	c.tiny = 0
   145  	c.tinyoffset = 0
   146  }