1 ================ 2 CIRCULAR BUFFERS 3 ================ 4 5By: David Howells <dhowells@redhat.com> 6 Paul E. McKenney <paulmck@linux.vnet.ibm.com> 7 8 9Linux provides a number of features that can be used to implement circular 10buffering. There are two sets of such features: 11 12 (1) Convenience functions for determining information about power-of-2 sized 13 buffers. 14 15 (2) Memory barriers for when the producer and the consumer of objects in the 16 buffer don't want to share a lock. 17 18To use these facilities, as discussed below, there needs to be just one 19producer and just one consumer. It is possible to handle multiple producers by 20serialising them, and to handle multiple consumers by serialising them. 21 22 23Contents: 24 25 (*) What is a circular buffer? 26 27 (*) Measuring power-of-2 buffers. 28 29 (*) Using memory barriers with circular buffers. 30 - The producer. 31 - The consumer. 32 33 34========================== 35WHAT IS A CIRCULAR BUFFER? 36========================== 37 38First of all, what is a circular buffer? A circular buffer is a buffer of 39fixed, finite size into which there are two indices: 40 41 (1) A 'head' index - the point at which the producer inserts items into the 42 buffer. 43 44 (2) A 'tail' index - the point at which the consumer finds the next item in 45 the buffer. 46 47Typically when the tail pointer is equal to the head pointer, the buffer is 48empty; and the buffer is full when the head pointer is one less than the tail 49pointer. 50 51The head index is incremented when items are added, and the tail index when 52items are removed. The tail index should never jump the head index, and both 53indices should be wrapped to 0 when they reach the end of the buffer, thus 54allowing an infinite amount of data to flow through the buffer. 55 56Typically, items will all be of the same unit size, but this isn't strictly 57required to use the techniques below. The indices can be increased by more 58than 1 if multiple items or variable-sized items are to be included in the 59buffer, provided that neither index overtakes the other. The implementer must 60be careful, however, as a region more than one unit in size may wrap the end of 61the buffer and be broken into two segments. 62 63 64============================ 65MEASURING POWER-OF-2 BUFFERS 66============================ 67 68Calculation of the occupancy or the remaining capacity of an arbitrarily sized 69circular buffer would normally be a slow operation, requiring the use of a 70modulus (divide) instruction. However, if the buffer is of a power-of-2 size, 71then a much quicker bitwise-AND instruction can be used instead. 72 73Linux provides a set of macros for handling power-of-2 circular buffers. These 74can be made use of by: 75 76 #include <linux/circ_buf.h> 77 78The macros are: 79 80 (*) Measure the remaining capacity of a buffer: 81 82 CIRC_SPACE(head_index, tail_index, buffer_size); 83 84 This returns the amount of space left in the buffer[1] into which items 85 can be inserted. 86 87 88 (*) Measure the maximum consecutive immediate space in a buffer: 89 90 CIRC_SPACE_TO_END(head_index, tail_index, buffer_size); 91 92 This returns the amount of consecutive space left in the buffer[1] into 93 which items can be immediately inserted without having to wrap back to the 94 beginning of the buffer. 95 96 97 (*) Measure the occupancy of a buffer: 98 99 CIRC_CNT(head_index, tail_index, buffer_size); 100 101 This returns the number of items currently occupying a buffer[2]. 102 103 104 (*) Measure the non-wrapping occupancy of a buffer: 105 106 CIRC_CNT_TO_END(head_index, tail_index, buffer_size); 107 108 This returns the number of consecutive items[2] that can be extracted from 109 the buffer without having to wrap back to the beginning of the buffer. 110 111 112Each of these macros will nominally return a value between 0 and buffer_size-1, 113however: 114 115 [1] CIRC_SPACE*() are intended to be used in the producer. To the producer 116 they will return a lower bound as the producer controls the head index, 117 but the consumer may still be depleting the buffer on another CPU and 118 moving the tail index. 119 120 To the consumer it will show an upper bound as the producer may be busy 121 depleting the space. 122 123 [2] CIRC_CNT*() are intended to be used in the consumer. To the consumer they 124 will return a lower bound as the consumer controls the tail index, but the 125 producer may still be filling the buffer on another CPU and moving the 126 head index. 127 128 To the producer it will show an upper bound as the consumer may be busy 129 emptying the buffer. 130 131 [3] To a third party, the order in which the writes to the indices by the 132 producer and consumer become visible cannot be guaranteed as they are 133 independent and may be made on different CPUs - so the result in such a 134 situation will merely be a guess, and may even be negative. 135 136 137=========================================== 138USING MEMORY BARRIERS WITH CIRCULAR BUFFERS 139=========================================== 140 141By using memory barriers in conjunction with circular buffers, you can avoid 142the need to: 143 144 (1) use a single lock to govern access to both ends of the buffer, thus 145 allowing the buffer to be filled and emptied at the same time; and 146 147 (2) use atomic counter operations. 148 149There are two sides to this: the producer that fills the buffer, and the 150consumer that empties it. Only one thing should be filling a buffer at any one 151time, and only one thing should be emptying a buffer at any one time, but the 152two sides can operate simultaneously. 153 154 155THE PRODUCER 156------------ 157 158The producer will look something like this: 159 160 spin_lock(&producer_lock); 161 162 unsigned long head = buffer->head; 163 unsigned long tail = ACCESS_ONCE(buffer->tail); 164 165 if (CIRC_SPACE(head, tail, buffer->size) >= 1) { 166 /* insert one item into the buffer */ 167 struct item *item = buffer[head]; 168 169 produce_item(item); 170 171 smp_wmb(); /* commit the item before incrementing the head */ 172 173 buffer->head = (head + 1) & (buffer->size - 1); 174 175 /* wake_up() will make sure that the head is committed before 176 * waking anyone up */ 177 wake_up(consumer); 178 } 179 180 spin_unlock(&producer_lock); 181 182This will instruct the CPU that the contents of the new item must be written 183before the head index makes it available to the consumer and then instructs the 184CPU that the revised head index must be written before the consumer is woken. 185 186Note that wake_up() doesn't have to be the exact mechanism used, but whatever 187is used must guarantee a (write) memory barrier between the update of the head 188index and the change of state of the consumer, if a change of state occurs. 189 190 191THE CONSUMER 192------------ 193 194The consumer will look something like this: 195 196 spin_lock(&consumer_lock); 197 198 unsigned long head = ACCESS_ONCE(buffer->head); 199 unsigned long tail = buffer->tail; 200 201 if (CIRC_CNT(head, tail, buffer->size) >= 1) { 202 /* read index before reading contents at that index */ 203 smp_read_barrier_depends(); 204 205 /* extract one item from the buffer */ 206 struct item *item = buffer[tail]; 207 208 consume_item(item); 209 210 smp_mb(); /* finish reading descriptor before incrementing tail */ 211 212 buffer->tail = (tail + 1) & (buffer->size - 1); 213 } 214 215 spin_unlock(&consumer_lock); 216 217This will instruct the CPU to make sure the index is up to date before reading 218the new item, and then it shall make sure the CPU has finished reading the item 219before it writes the new tail pointer, which will erase the item. 220 221 222Note the use of ACCESS_ONCE() in both algorithms to read the opposition index. 223This prevents the compiler from discarding and reloading its cached value - 224which some compilers will do across smp_read_barrier_depends(). This isn't 225strictly needed if you can be sure that the opposition index will _only_ be 226used the once. 227 228 229=============== 230FURTHER READING 231=============== 232 233See also Documentation/memory-barriers.txt for a description of Linux's memory 234barrier facilities. 235