Line data Source code
1 : /*-------------------------------------------------------------------------
2 : *
3 : * freelist.c
4 : * routines for managing the buffer pool's replacement strategy.
5 : *
6 : *
7 : * Portions Copyright (c) 1996-2017, PostgreSQL Global Development Group
8 : * Portions Copyright (c) 1994, Regents of the University of California
9 : *
10 : *
11 : * IDENTIFICATION
12 : * src/backend/storage/buffer/freelist.c
13 : *
14 : *-------------------------------------------------------------------------
15 : */
16 : #include "postgres.h"
17 :
18 : #include "port/atomics.h"
19 : #include "storage/buf_internals.h"
20 : #include "storage/bufmgr.h"
21 : #include "storage/proc.h"
22 :
23 : #define INT_ACCESS_ONCE(var) ((int)(*((volatile int *)&(var))))
24 :
25 :
26 : /*
27 : * The shared freelist control information.
28 : */
29 : typedef struct
30 : {
31 : /* Spinlock: protects the values below */
32 : slock_t buffer_strategy_lock;
33 :
34 : /*
35 : * Clock sweep hand: index of next buffer to consider grabbing. Note that
36 : * this isn't a concrete buffer - we only ever increase the value. So, to
37 : * get an actual buffer, it needs to be used modulo NBuffers.
38 : */
39 : pg_atomic_uint32 nextVictimBuffer;
40 :
41 : int firstFreeBuffer; /* Head of list of unused buffers */
42 : int lastFreeBuffer; /* Tail of list of unused buffers */
43 :
44 : /*
45 : * NOTE: lastFreeBuffer is undefined when firstFreeBuffer is -1 (that is,
46 : * when the list is empty)
47 : */
48 :
49 : /*
50 : * Statistics. These counters should be wide enough that they can't
51 : * overflow during a single bgwriter cycle.
52 : */
53 : uint32 completePasses; /* Complete cycles of the clock sweep */
54 : pg_atomic_uint32 numBufferAllocs; /* Buffers allocated since last reset */
55 :
56 : /*
57 : * Bgworker process to be notified upon activity or -1 if none. See
58 : * StrategyNotifyBgWriter.
59 : */
60 : int bgwprocno;
61 : } BufferStrategyControl;
62 :
63 : /* Pointers to shared state */
64 : static BufferStrategyControl *StrategyControl = NULL;
65 :
66 : /*
67 : * Private (non-shared) state for managing a ring of shared buffers to re-use.
68 : * This is currently the only kind of BufferAccessStrategy object, but someday
69 : * we might have more kinds.
70 : */
71 : typedef struct BufferAccessStrategyData
72 : {
73 : /* Overall strategy type */
74 : BufferAccessStrategyType btype;
75 : /* Number of elements in buffers[] array */
76 : int ring_size;
77 :
78 : /*
79 : * Index of the "current" slot in the ring, ie, the one most recently
80 : * returned by GetBufferFromRing.
81 : */
82 : int current;
83 :
84 : /*
85 : * True if the buffer just returned by StrategyGetBuffer had been in the
86 : * ring already.
87 : */
88 : bool current_was_in_ring;
89 :
90 : /*
91 : * Array of buffer numbers. InvalidBuffer (that is, zero) indicates we
92 : * have not yet selected a buffer for this ring slot. For allocation
93 : * simplicity this is palloc'd together with the fixed fields of the
94 : * struct.
95 : */
96 : Buffer buffers[FLEXIBLE_ARRAY_MEMBER];
97 : } BufferAccessStrategyData;
98 :
99 :
100 : /* Prototypes for internal functions */
101 : static BufferDesc *GetBufferFromRing(BufferAccessStrategy strategy,
102 : uint32 *buf_state);
103 : static void AddBufferToRing(BufferAccessStrategy strategy,
104 : BufferDesc *buf);
105 :
106 : /*
107 : * ClockSweepTick - Helper routine for StrategyGetBuffer()
108 : *
109 : * Move the clock hand one buffer ahead of its current position and return the
110 : * id of the buffer now under the hand.
111 : */
112 : static inline uint32
113 0 : ClockSweepTick(void)
114 : {
115 : uint32 victim;
116 :
117 : /*
118 : * Atomically move hand ahead one buffer - if there's several processes
119 : * doing this, this can lead to buffers being returned slightly out of
120 : * apparent order.
121 : */
122 0 : victim =
123 0 : pg_atomic_fetch_add_u32(&StrategyControl->nextVictimBuffer, 1);
124 :
125 0 : if (victim >= NBuffers)
126 : {
127 0 : uint32 originalVictim = victim;
128 :
129 : /* always wrap what we look up in BufferDescriptors */
130 0 : victim = victim % NBuffers;
131 :
132 : /*
133 : * If we're the one that just caused a wraparound, force
134 : * completePasses to be incremented while holding the spinlock. We
135 : * need the spinlock so StrategySyncStart() can return a consistent
136 : * value consisting of nextVictimBuffer and completePasses.
137 : */
138 0 : if (victim == 0)
139 : {
140 : uint32 expected;
141 : uint32 wrapped;
142 0 : bool success = false;
143 :
144 0 : expected = originalVictim + 1;
145 :
146 0 : while (!success)
147 : {
148 : /*
149 : * Acquire the spinlock while increasing completePasses. That
150 : * allows other readers to read nextVictimBuffer and
151 : * completePasses in a consistent manner which is required for
152 : * StrategySyncStart(). In theory delaying the increment
153 : * could lead to an overflow of nextVictimBuffers, but that's
154 : * highly unlikely and wouldn't be particularly harmful.
155 : */
156 0 : SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
157 :
158 0 : wrapped = expected % NBuffers;
159 :
160 0 : success = pg_atomic_compare_exchange_u32(&StrategyControl->nextVictimBuffer,
161 : &expected, wrapped);
162 0 : if (success)
163 0 : StrategyControl->completePasses++;
164 0 : SpinLockRelease(&StrategyControl->buffer_strategy_lock);
165 : }
166 : }
167 : }
168 0 : return victim;
169 : }
170 :
171 : /*
172 : * have_free_buffer -- a lockless check to see if there is a free buffer in
173 : * buffer pool.
174 : *
175 : * If the result is true that will become stale once free buffers are moved out
176 : * by other operations, so the caller who strictly want to use a free buffer
177 : * should not call this.
178 : */
179 : bool
180 0 : have_free_buffer()
181 : {
182 0 : if (StrategyControl->firstFreeBuffer >= 0)
183 0 : return true;
184 : else
185 0 : return false;
186 : }
187 :
188 : /*
189 : * StrategyGetBuffer
190 : *
191 : * Called by the bufmgr to get the next candidate buffer to use in
192 : * BufferAlloc(). The only hard requirement BufferAlloc() has is that
193 : * the selected buffer must not currently be pinned by anyone.
194 : *
195 : * strategy is a BufferAccessStrategy object, or NULL for default strategy.
196 : *
197 : * To ensure that no one else can pin the buffer before we do, we must
198 : * return the buffer with the buffer header spinlock still held.
199 : */
200 : BufferDesc *
201 17205 : StrategyGetBuffer(BufferAccessStrategy strategy, uint32 *buf_state)
202 : {
203 : BufferDesc *buf;
204 : int bgwprocno;
205 : int trycounter;
206 : uint32 local_buf_state; /* to avoid repeated (de-)referencing */
207 :
208 : /*
209 : * If given a strategy object, see whether it can select a buffer. We
210 : * assume strategy objects don't need buffer_strategy_lock.
211 : */
212 17205 : if (strategy != NULL)
213 : {
214 2737 : buf = GetBufferFromRing(strategy, buf_state);
215 2737 : if (buf != NULL)
216 553 : return buf;
217 : }
218 :
219 : /*
220 : * If asked, we need to waken the bgwriter. Since we don't want to rely on
221 : * a spinlock for this we force a read from shared memory once, and then
222 : * set the latch based on that value. We need to go through that length
223 : * because otherwise bgprocno might be reset while/after we check because
224 : * the compiler might just reread from memory.
225 : *
226 : * This can possibly set the latch of the wrong process if the bgwriter
227 : * dies in the wrong moment. But since PGPROC->procLatch is never
228 : * deallocated the worst consequence of that is that we set the latch of
229 : * some arbitrary process.
230 : */
231 16652 : bgwprocno = INT_ACCESS_ONCE(StrategyControl->bgwprocno);
232 16652 : if (bgwprocno != -1)
233 : {
234 : /* reset bgwprocno first, before setting the latch */
235 0 : StrategyControl->bgwprocno = -1;
236 :
237 : /*
238 : * Not acquiring ProcArrayLock here which is slightly icky. It's
239 : * actually fine because procLatch isn't ever freed, so we just can
240 : * potentially set the wrong process' (or no process') latch.
241 : */
242 0 : SetLatch(&ProcGlobal->allProcs[bgwprocno].procLatch);
243 : }
244 :
245 : /*
246 : * We count buffer allocation requests so that the bgwriter can estimate
247 : * the rate of buffer consumption. Note that buffers recycled by a
248 : * strategy object are intentionally not counted here.
249 : */
250 16652 : pg_atomic_fetch_add_u32(&StrategyControl->numBufferAllocs, 1);
251 :
252 : /*
253 : * First check, without acquiring the lock, whether there's buffers in the
254 : * freelist. Since we otherwise don't require the spinlock in every
255 : * StrategyGetBuffer() invocation, it'd be sad to acquire it here -
256 : * uselessly in most cases. That obviously leaves a race where a buffer is
257 : * put on the freelist but we don't see the store yet - but that's pretty
258 : * harmless, it'll just get used during the next buffer acquisition.
259 : *
260 : * If there's buffers on the freelist, acquire the spinlock to pop one
261 : * buffer of the freelist. Then check whether that buffer is usable and
262 : * repeat if not.
263 : *
264 : * Note that the freeNext fields are considered to be protected by the
265 : * buffer_strategy_lock not the individual buffer spinlocks, so it's OK to
266 : * manipulate them without holding the spinlock.
267 : */
268 16652 : if (StrategyControl->firstFreeBuffer >= 0)
269 : {
270 : while (true)
271 : {
272 : /* Acquire the spinlock to remove element from the freelist */
273 16652 : SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
274 :
275 16652 : if (StrategyControl->firstFreeBuffer < 0)
276 : {
277 0 : SpinLockRelease(&StrategyControl->buffer_strategy_lock);
278 0 : break;
279 : }
280 :
281 16652 : buf = GetBufferDescriptor(StrategyControl->firstFreeBuffer);
282 16652 : Assert(buf->freeNext != FREENEXT_NOT_IN_LIST);
283 :
284 : /* Unconditionally remove buffer from freelist */
285 16652 : StrategyControl->firstFreeBuffer = buf->freeNext;
286 16652 : buf->freeNext = FREENEXT_NOT_IN_LIST;
287 :
288 : /*
289 : * Release the lock so someone else can access the freelist while
290 : * we check out this buffer.
291 : */
292 16652 : SpinLockRelease(&StrategyControl->buffer_strategy_lock);
293 :
294 : /*
295 : * If the buffer is pinned or has a nonzero usage_count, we cannot
296 : * use it; discard it and retry. (This can only happen if VACUUM
297 : * put a valid buffer in the freelist and then someone else used
298 : * it before we got to it. It's probably impossible altogether as
299 : * of 8.3, but we'd better check anyway.)
300 : */
301 16652 : local_buf_state = LockBufHdr(buf);
302 16652 : if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0
303 16652 : && BUF_STATE_GET_USAGECOUNT(local_buf_state) == 0)
304 : {
305 16652 : if (strategy != NULL)
306 2184 : AddBufferToRing(strategy, buf);
307 16652 : *buf_state = local_buf_state;
308 16652 : return buf;
309 : }
310 0 : UnlockBufHdr(buf, local_buf_state);
311 :
312 0 : }
313 : }
314 :
315 : /* Nothing on the freelist, so run the "clock sweep" algorithm */
316 0 : trycounter = NBuffers;
317 : for (;;)
318 : {
319 0 : buf = GetBufferDescriptor(ClockSweepTick());
320 :
321 : /*
322 : * If the buffer is pinned or has a nonzero usage_count, we cannot use
323 : * it; decrement the usage_count (unless pinned) and keep scanning.
324 : */
325 0 : local_buf_state = LockBufHdr(buf);
326 :
327 0 : if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0)
328 : {
329 0 : if (BUF_STATE_GET_USAGECOUNT(local_buf_state) != 0)
330 : {
331 0 : local_buf_state -= BUF_USAGECOUNT_ONE;
332 :
333 0 : trycounter = NBuffers;
334 : }
335 : else
336 : {
337 : /* Found a usable buffer */
338 0 : if (strategy != NULL)
339 0 : AddBufferToRing(strategy, buf);
340 0 : *buf_state = local_buf_state;
341 0 : return buf;
342 : }
343 : }
344 0 : else if (--trycounter == 0)
345 : {
346 : /*
347 : * We've scanned all the buffers without making any state changes,
348 : * so all the buffers are pinned (or were when we looked at them).
349 : * We could hope that someone will free one eventually, but it's
350 : * probably better to fail than to risk getting stuck in an
351 : * infinite loop.
352 : */
353 0 : UnlockBufHdr(buf, local_buf_state);
354 0 : elog(ERROR, "no unpinned buffers available");
355 : }
356 0 : UnlockBufHdr(buf, local_buf_state);
357 0 : }
358 : }
359 :
360 : /*
361 : * StrategyFreeBuffer: put a buffer on the freelist
362 : */
363 : void
364 7315 : StrategyFreeBuffer(BufferDesc *buf)
365 : {
366 7315 : SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
367 :
368 : /*
369 : * It is possible that we are told to put something in the freelist that
370 : * is already in it; don't screw up the list if so.
371 : */
372 7315 : if (buf->freeNext == FREENEXT_NOT_IN_LIST)
373 : {
374 7315 : buf->freeNext = StrategyControl->firstFreeBuffer;
375 7315 : if (buf->freeNext < 0)
376 0 : StrategyControl->lastFreeBuffer = buf->buf_id;
377 7315 : StrategyControl->firstFreeBuffer = buf->buf_id;
378 : }
379 :
380 7315 : SpinLockRelease(&StrategyControl->buffer_strategy_lock);
381 7315 : }
382 :
383 : /*
384 : * StrategySyncStart -- tell BufferSync where to start syncing
385 : *
386 : * The result is the buffer index of the best buffer to sync first.
387 : * BufferSync() will proceed circularly around the buffer array from there.
388 : *
389 : * In addition, we return the completed-pass count (which is effectively
390 : * the higher-order bits of nextVictimBuffer) and the count of recent buffer
391 : * allocs if non-NULL pointers are passed. The alloc count is reset after
392 : * being read.
393 : */
394 : int
395 388 : StrategySyncStart(uint32 *complete_passes, uint32 *num_buf_alloc)
396 : {
397 : uint32 nextVictimBuffer;
398 : int result;
399 :
400 388 : SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
401 388 : nextVictimBuffer = pg_atomic_read_u32(&StrategyControl->nextVictimBuffer);
402 388 : result = nextVictimBuffer % NBuffers;
403 :
404 388 : if (complete_passes)
405 : {
406 388 : *complete_passes = StrategyControl->completePasses;
407 :
408 : /*
409 : * Additionally add the number of wraparounds that happened before
410 : * completePasses could be incremented. C.f. ClockSweepTick().
411 : */
412 388 : *complete_passes += nextVictimBuffer / NBuffers;
413 : }
414 :
415 388 : if (num_buf_alloc)
416 : {
417 388 : *num_buf_alloc = pg_atomic_exchange_u32(&StrategyControl->numBufferAllocs, 0);
418 : }
419 388 : SpinLockRelease(&StrategyControl->buffer_strategy_lock);
420 388 : return result;
421 : }
422 :
423 : /*
424 : * StrategyNotifyBgWriter -- set or clear allocation notification latch
425 : *
426 : * If bgwprocno isn't -1, the next invocation of StrategyGetBuffer will
427 : * set that latch. Pass -1 to clear the pending notification before it
428 : * happens. This feature is used by the bgwriter process to wake itself up
429 : * from hibernation, and is not meant for anybody else to use.
430 : */
431 : void
432 0 : StrategyNotifyBgWriter(int bgwprocno)
433 : {
434 : /*
435 : * We acquire buffer_strategy_lock just to ensure that the store appears
436 : * atomic to StrategyGetBuffer. The bgwriter should call this rather
437 : * infrequently, so there's no performance penalty from being safe.
438 : */
439 0 : SpinLockAcquire(&StrategyControl->buffer_strategy_lock);
440 0 : StrategyControl->bgwprocno = bgwprocno;
441 0 : SpinLockRelease(&StrategyControl->buffer_strategy_lock);
442 0 : }
443 :
444 :
445 : /*
446 : * StrategyShmemSize
447 : *
448 : * estimate the size of shared memory used by the freelist-related structures.
449 : *
450 : * Note: for somewhat historical reasons, the buffer lookup hashtable size
451 : * is also determined here.
452 : */
453 : Size
454 5 : StrategyShmemSize(void)
455 : {
456 5 : Size size = 0;
457 :
458 : /* size of lookup hash table ... see comment in StrategyInitialize */
459 5 : size = add_size(size, BufTableShmemSize(NBuffers + NUM_BUFFER_PARTITIONS));
460 :
461 : /* size of the shared replacement strategy control block */
462 5 : size = add_size(size, MAXALIGN(sizeof(BufferStrategyControl)));
463 :
464 5 : return size;
465 : }
466 :
467 : /*
468 : * StrategyInitialize -- initialize the buffer cache replacement
469 : * strategy.
470 : *
471 : * Assumes: All of the buffers are already built into a linked list.
472 : * Only called by postmaster and only during initialization.
473 : */
474 : void
475 5 : StrategyInitialize(bool init)
476 : {
477 : bool found;
478 :
479 : /*
480 : * Initialize the shared buffer lookup hashtable.
481 : *
482 : * Since we can't tolerate running out of lookup table entries, we must be
483 : * sure to specify an adequate table size here. The maximum steady-state
484 : * usage is of course NBuffers entries, but BufferAlloc() tries to insert
485 : * a new entry before deleting the old. In principle this could be
486 : * happening in each partition concurrently, so we could need as many as
487 : * NBuffers + NUM_BUFFER_PARTITIONS entries.
488 : */
489 5 : InitBufTable(NBuffers + NUM_BUFFER_PARTITIONS);
490 :
491 : /*
492 : * Get or create the shared strategy control block
493 : */
494 5 : StrategyControl = (BufferStrategyControl *)
495 5 : ShmemInitStruct("Buffer Strategy Status",
496 : sizeof(BufferStrategyControl),
497 : &found);
498 :
499 5 : if (!found)
500 : {
501 : /*
502 : * Only done once, usually in postmaster
503 : */
504 5 : Assert(init);
505 :
506 5 : SpinLockInit(&StrategyControl->buffer_strategy_lock);
507 :
508 : /*
509 : * Grab the whole linked list of free buffers for our strategy. We
510 : * assume it was previously set up by InitBufferPool().
511 : */
512 5 : StrategyControl->firstFreeBuffer = 0;
513 5 : StrategyControl->lastFreeBuffer = NBuffers - 1;
514 :
515 : /* Initialize the clock sweep pointer */
516 5 : pg_atomic_init_u32(&StrategyControl->nextVictimBuffer, 0);
517 :
518 : /* Clear statistics */
519 5 : StrategyControl->completePasses = 0;
520 5 : pg_atomic_init_u32(&StrategyControl->numBufferAllocs, 0);
521 :
522 : /* No pending notification */
523 5 : StrategyControl->bgwprocno = -1;
524 : }
525 : else
526 0 : Assert(!init);
527 5 : }
528 :
529 :
530 : /* ----------------------------------------------------------------
531 : * Backend-private buffer ring management
532 : * ----------------------------------------------------------------
533 : */
534 :
535 :
536 : /*
537 : * GetAccessStrategy -- create a BufferAccessStrategy object
538 : *
539 : * The object is allocated in the current memory context.
540 : */
541 : BufferAccessStrategy
542 422 : GetAccessStrategy(BufferAccessStrategyType btype)
543 : {
544 : BufferAccessStrategy strategy;
545 : int ring_size;
546 :
547 : /*
548 : * Select ring size to use. See buffer/README for rationales.
549 : *
550 : * Note: if you change the ring size for BAS_BULKREAD, see also
551 : * SYNC_SCAN_REPORT_INTERVAL in access/heap/syncscan.c.
552 : */
553 422 : switch (btype)
554 : {
555 : case BAS_NORMAL:
556 : /* if someone asks for NORMAL, just give 'em a "default" object */
557 0 : return NULL;
558 :
559 : case BAS_BULKREAD:
560 0 : ring_size = 256 * 1024 / BLCKSZ;
561 0 : break;
562 : case BAS_BULKWRITE:
563 292 : ring_size = 16 * 1024 * 1024 / BLCKSZ;
564 292 : break;
565 : case BAS_VACUUM:
566 130 : ring_size = 256 * 1024 / BLCKSZ;
567 130 : break;
568 :
569 : default:
570 0 : elog(ERROR, "unrecognized buffer access strategy: %d",
571 : (int) btype);
572 : return NULL; /* keep compiler quiet */
573 : }
574 :
575 : /* Make sure ring isn't an undue fraction of shared buffers */
576 422 : ring_size = Min(NBuffers / 8, ring_size);
577 :
578 : /* Allocate the object and initialize all elements to zeroes */
579 422 : strategy = (BufferAccessStrategy)
580 422 : palloc0(offsetof(BufferAccessStrategyData, buffers) +
581 : ring_size * sizeof(Buffer));
582 :
583 : /* Set fields that don't start out zero */
584 422 : strategy->btype = btype;
585 422 : strategy->ring_size = ring_size;
586 :
587 422 : return strategy;
588 : }
589 :
590 : /*
591 : * FreeAccessStrategy -- release a BufferAccessStrategy object
592 : *
593 : * A simple pfree would do at the moment, but we would prefer that callers
594 : * don't assume that much about the representation of BufferAccessStrategy.
595 : */
596 : void
597 277 : FreeAccessStrategy(BufferAccessStrategy strategy)
598 : {
599 : /* don't crash if called on a "default" strategy */
600 277 : if (strategy != NULL)
601 277 : pfree(strategy);
602 277 : }
603 :
604 : /*
605 : * GetBufferFromRing -- returns a buffer from the ring, or NULL if the
606 : * ring is empty.
607 : *
608 : * The bufhdr spin lock is held on the returned buffer.
609 : */
610 : static BufferDesc *
611 2737 : GetBufferFromRing(BufferAccessStrategy strategy, uint32 *buf_state)
612 : {
613 : BufferDesc *buf;
614 : Buffer bufnum;
615 : uint32 local_buf_state; /* to avoid repeated (de-)referencing */
616 :
617 :
618 : /* Advance to next ring slot */
619 2737 : if (++strategy->current >= strategy->ring_size)
620 19 : strategy->current = 0;
621 :
622 : /*
623 : * If the slot hasn't been filled yet, tell the caller to allocate a new
624 : * buffer with the normal allocation strategy. He will then fill this
625 : * slot by calling AddBufferToRing with the new buffer.
626 : */
627 2737 : bufnum = strategy->buffers[strategy->current];
628 2737 : if (bufnum == InvalidBuffer)
629 : {
630 2184 : strategy->current_was_in_ring = false;
631 2184 : return NULL;
632 : }
633 :
634 : /*
635 : * If the buffer is pinned we cannot use it under any circumstances.
636 : *
637 : * If usage_count is 0 or 1 then the buffer is fair game (we expect 1,
638 : * since our own previous usage of the ring element would have left it
639 : * there, but it might've been decremented by clock sweep since then). A
640 : * higher usage_count indicates someone else has touched the buffer, so we
641 : * shouldn't re-use it.
642 : */
643 553 : buf = GetBufferDescriptor(bufnum - 1);
644 553 : local_buf_state = LockBufHdr(buf);
645 553 : if (BUF_STATE_GET_REFCOUNT(local_buf_state) == 0
646 553 : && BUF_STATE_GET_USAGECOUNT(local_buf_state) <= 1)
647 : {
648 553 : strategy->current_was_in_ring = true;
649 553 : *buf_state = local_buf_state;
650 553 : return buf;
651 : }
652 0 : UnlockBufHdr(buf, local_buf_state);
653 :
654 : /*
655 : * Tell caller to allocate a new buffer with the normal allocation
656 : * strategy. He'll then replace this ring element via AddBufferToRing.
657 : */
658 0 : strategy->current_was_in_ring = false;
659 0 : return NULL;
660 : }
661 :
662 : /*
663 : * AddBufferToRing -- add a buffer to the buffer ring
664 : *
665 : * Caller must hold the buffer header spinlock on the buffer. Since this
666 : * is called with the spinlock held, it had better be quite cheap.
667 : */
668 : static void
669 2184 : AddBufferToRing(BufferAccessStrategy strategy, BufferDesc *buf)
670 : {
671 2184 : strategy->buffers[strategy->current] = BufferDescriptorGetBuffer(buf);
672 2184 : }
673 :
674 : /*
675 : * StrategyRejectBuffer -- consider rejecting a dirty buffer
676 : *
677 : * When a nondefault strategy is used, the buffer manager calls this function
678 : * when it turns out that the buffer selected by StrategyGetBuffer needs to
679 : * be written out and doing so would require flushing WAL too. This gives us
680 : * a chance to choose a different victim.
681 : *
682 : * Returns true if buffer manager should ask for a new victim, and false
683 : * if this buffer should be written and re-used.
684 : */
685 : bool
686 8 : StrategyRejectBuffer(BufferAccessStrategy strategy, BufferDesc *buf)
687 : {
688 : /* We only do this in bulkread mode */
689 8 : if (strategy->btype != BAS_BULKREAD)
690 8 : return false;
691 :
692 : /* Don't muck with behavior of normal buffer-replacement strategy */
693 0 : if (!strategy->current_was_in_ring ||
694 0 : strategy->buffers[strategy->current] != BufferDescriptorGetBuffer(buf))
695 0 : return false;
696 :
697 : /*
698 : * Remove the dirty buffer from the ring; necessary to prevent infinite
699 : * loop if all ring members are dirty.
700 : */
701 0 : strategy->buffers[strategy->current] = InvalidBuffer;
702 :
703 0 : return true;
704 : }
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