4 //////////////////////////////////////////////////////////
6 // A memory pool implements POOLCREATE, POOLALLOC and
7 // POOLFREE to improve memory allocation by reusing records.
9 // This implementation uses a lock-free singly-linked list
10 // to store reusable records. The list is initialized with
11 // one valid record, and the list is considered empty when
12 // it has only one record; this allows the enqueue operation's
13 // CAS to assume tail can always be dereferenced.
15 // poolfree adds newly freed records to the list BACK
17 // poolalloc either takes records from FRONT or mallocs
19 //////////////////////////////////////////////////////////
28 typedef struct MemPoolItem_t {
33 typedef struct MemPool_t {
37 // avoid cache line contention between producer/consumer...
38 char buffer[CACHELINESIZE - sizeof(void*)];
44 // the memory pool must always have at least one
46 static MemPool* poolcreate( int itemSize ) {
47 MemPool* p = calloc( 1, sizeof( MemPool ) );
48 p->itemSize = itemSize;
49 p->head = calloc( 1, itemSize );
57 // in: a ptr, expected old, desired new
60 // Pass in a ptr, what you expect the old value is and
61 // what you want the new value to be.
62 // The CAS returns what the value is actually: if it matches
63 // your proposed old value then you assume the update was successful,
64 // otherwise someone did CAS before you, so try again (the return
65 // value is the old value you will pass next time.)
67 static inline void poolfreeinto( MemPool* p, void* ptr ) {
69 MemPoolItem* tailCurrent;
70 MemPoolItem* tailActual;
72 // set up the now unneeded record to as the tail of the
73 // free list by treating its first bytes as next pointer,
74 MemPoolItem* tailNew = (MemPoolItem*) ptr;
78 // make sure the null happens before the insertion,
79 // also makes sure that we reload tailCurrent, etc..
82 tailCurrent = p->tail;
83 tailActual = (MemPoolItem*)
84 CAS( &(p->tail), // ptr to set
85 (long) tailCurrent, // current tail's next should be NULL
86 (long) tailNew // try set to our new tail
88 if( tailActual == tailCurrent ) {
89 // success, update tail
90 tailCurrent->next = tailNew;
94 // if CAS failed, retry entire operation
100 static inline void* poolalloc( MemPool* p ) {
102 // to protect CAS in poolfree from dereferencing
103 // null, treat the queue as empty when there is
104 // only one item. The dequeue operation is only
105 // executed by the thread that owns the pool, so
106 // it doesn't require an atomic op
107 MemPoolItem* headCurrent = p->head;
108 MemPoolItem* next=headCurrent->next;
111 // only one item, so don't take from pool
112 return (void*) RUNMALLOC( p->itemSize );
117 //////////////////////////////////////////////////////////
120 // static inline void prefetch(void *x)
122 // asm volatile("prefetcht0 %0" :: "m" (*(unsigned long *)x));
126 // but this built-in gcc one seems the most portable:
127 //////////////////////////////////////////////////////////
128 //__builtin_prefetch( &(p->head->next) );
129 asm volatile( "prefetcht0 (%0)" :: "r" (next));
131 return (void*)headCurrent;
135 static void pooldestroy( MemPool* p ) {
136 MemPoolItem* i = p->head;
149 #endif // ___MEMPOOL_H__