postgresql 的buffer 的clock sweep 算法

typedef struct sbufdesc
{
BufferTag tag; /* ID of page contained in buffer */
BufFlags flags; /* see bit deFinitions above */
uint16 usage_count; /* usage counter for clock sweep code */
unsigned refcount; /* # of backends holding pins on buffer */
int wait_backend_pid; /* backend PID of pin-count waiter */

slock_t buf_hdr_lock; /* protects the above fields */

int buf_id; /* buffer's index number (from 0) */
int freeNext; /* link in freelist chain */

LWLockId io_in_progress_lock; /* to wait for I/O to complete */
LWLockId content_lock; /* to lock access to buffer contents */
} BufferDesc;

/* nothing on the freelist,so run the "clock sweep" algorithm */
trycounter = NBuffers;
for (;;)
{
buf = &BufferDescriptors[StrategyControl->nextVictimBuffer];

if (++StrategyControl->nextVictimBuffer >= NBuffers)
{
StrategyControl->nextVictimBuffer = 0;
StrategyControl->completePasses++;
}

/* * If the buffer is pinned or has a nonzero usage_count,we cannot use * it; decrement the usage_count (unless pinned) and keep scanning. */ LockBufHdr(buf); if (buf->refcount == 0) { if (buf->usage_count > 0) { buf->usage_count--; trycounter = NBuffers; } else { /* Found a usable buffer */ if (strategy != NULL) AddBufferToRing(strategy,buf); return buf; } } else if (--trycounter == 0) { /* * We've scanned all the buffers without making any state changes,* so all the buffers are pinned (or were when we looked at them). * We Could hope that someone will free one eventually,but it's * probably better to fail than to risk getting stuck in an * infinite loop. */ UnlockBufHdr(buf); elog(ERROR,"no unpinned buffers available"); } UnlockBufHdr(buf);}