Line data Source code
1 : /*-------------------------------------------------------------------------
2 : *
3 : * read_stream.c
4 : * Mechanism for accessing buffered relation data with look-ahead
5 : *
6 : * Code that needs to access relation data typically pins blocks one at a
7 : * time, often in a predictable order that might be sequential or data-driven.
8 : * Calling the simple ReadBuffer() function for each block is inefficient,
9 : * because blocks that are not yet in the buffer pool require I/O operations
10 : * that are small and might stall waiting for storage. This mechanism looks
11 : * into the future and calls StartReadBuffers() and WaitReadBuffers() to read
12 : * neighboring blocks together and ahead of time, with an adaptive look-ahead
13 : * distance.
14 : *
15 : * A user-provided callback generates a stream of block numbers that is used
16 : * to form reads of up to io_combine_limit, by attempting to merge them with a
17 : * pending read. When that isn't possible, the existing pending read is sent
18 : * to StartReadBuffers() so that a new one can begin to form.
19 : *
20 : * The algorithm for controlling the look-ahead distance is based on recent
21 : * cache hit and miss history. When no I/O is necessary, there is no benefit
22 : * in looking ahead more than one block. This is the default initial
23 : * assumption, but when blocks needing I/O are streamed, the distance is
24 : * increased rapidly to try to benefit from I/O combining and concurrency. It
25 : * is reduced gradually when cached blocks are streamed.
26 : *
27 : * The main data structure is a circular queue of buffers of size
28 : * max_pinned_buffers plus some extra space for technical reasons, ready to be
29 : * returned by read_stream_next_buffer(). Each buffer also has an optional
30 : * variable sized object that is passed from the callback to the consumer of
31 : * buffers.
32 : *
33 : * Parallel to the queue of buffers, there is a circular queue of in-progress
34 : * I/Os that have been started with StartReadBuffers(), and for which
35 : * WaitReadBuffers() must be called before returning the buffer.
36 : *
37 : * For example, if the callback returns block numbers 10, 42, 43, 44, 60 in
38 : * successive calls, then these data structures might appear as follows:
39 : *
40 : * buffers buf/data ios
41 : *
42 : * +----+ +-----+ +--------+
43 : * | | | | +----+ 42..44 | <- oldest_io_index
44 : * +----+ +-----+ | +--------+
45 : * oldest_buffer_index -> | 10 | | ? | | +--+ 60..60 |
46 : * +----+ +-----+ | | +--------+
47 : * | 42 | | ? |<-+ | | | <- next_io_index
48 : * +----+ +-----+ | +--------+
49 : * | 43 | | ? | | | |
50 : * +----+ +-----+ | +--------+
51 : * | 44 | | ? | | | |
52 : * +----+ +-----+ | +--------+
53 : * | 60 | | ? |<---+
54 : * +----+ +-----+
55 : * next_buffer_index -> | | | |
56 : * +----+ +-----+
57 : *
58 : * In the example, 5 buffers are pinned, and the next buffer to be streamed to
59 : * the client is block 10. Block 10 was a hit and has no associated I/O, but
60 : * the range 42..44 requires an I/O wait before its buffers are returned, as
61 : * does block 60.
62 : *
63 : *
64 : * Portions Copyright (c) 2024-2025, PostgreSQL Global Development Group
65 : * Portions Copyright (c) 1994, Regents of the University of California
66 : *
67 : * IDENTIFICATION
68 : * src/backend/storage/aio/read_stream.c
69 : *
70 : *-------------------------------------------------------------------------
71 : */
72 : #include "postgres.h"
73 :
74 : #include "miscadmin.h"
75 : #include "storage/aio.h"
76 : #include "storage/fd.h"
77 : #include "storage/smgr.h"
78 : #include "storage/read_stream.h"
79 : #include "utils/memdebug.h"
80 : #include "utils/rel.h"
81 : #include "utils/spccache.h"
82 :
83 : typedef struct InProgressIO
84 : {
85 : int16 buffer_index;
86 : ReadBuffersOperation op;
87 : } InProgressIO;
88 :
89 : /*
90 : * State for managing a stream of reads.
91 : */
92 : struct ReadStream
93 : {
94 : int16 max_ios;
95 : int16 io_combine_limit;
96 : int16 ios_in_progress;
97 : int16 queue_size;
98 : int16 max_pinned_buffers;
99 : int16 forwarded_buffers;
100 : int16 pinned_buffers;
101 : int16 distance;
102 : int16 initialized_buffers;
103 : int read_buffers_flags;
104 : bool sync_mode; /* using io_method=sync */
105 : bool batch_mode; /* READ_STREAM_USE_BATCHING */
106 : bool advice_enabled;
107 : bool temporary;
108 :
109 : /*
110 : * One-block buffer to support 'ungetting' a block number, to resolve flow
111 : * control problems when I/Os are split.
112 : */
113 : BlockNumber buffered_blocknum;
114 :
115 : /*
116 : * The callback that will tell us which block numbers to read, and an
117 : * opaque pointer that will be pass to it for its own purposes.
118 : */
119 : ReadStreamBlockNumberCB callback;
120 : void *callback_private_data;
121 :
122 : /* Next expected block, for detecting sequential access. */
123 : BlockNumber seq_blocknum;
124 : BlockNumber seq_until_processed;
125 :
126 : /* The read operation we are currently preparing. */
127 : BlockNumber pending_read_blocknum;
128 : int16 pending_read_nblocks;
129 :
130 : /* Space for buffers and optional per-buffer private data. */
131 : size_t per_buffer_data_size;
132 : void *per_buffer_data;
133 :
134 : /* Read operations that have been started but not waited for yet. */
135 : InProgressIO *ios;
136 : int16 oldest_io_index;
137 : int16 next_io_index;
138 :
139 : bool fast_path;
140 :
141 : /* Circular queue of buffers. */
142 : int16 oldest_buffer_index; /* Next pinned buffer to return */
143 : int16 next_buffer_index; /* Index of next buffer to pin */
144 : Buffer buffers[FLEXIBLE_ARRAY_MEMBER];
145 : };
146 :
147 : /*
148 : * Return a pointer to the per-buffer data by index.
149 : */
150 : static inline void *
151 7187034 : get_per_buffer_data(ReadStream *stream, int16 buffer_index)
152 : {
153 14374068 : return (char *) stream->per_buffer_data +
154 7187034 : stream->per_buffer_data_size * buffer_index;
155 : }
156 :
157 : /*
158 : * General-use ReadStreamBlockNumberCB for block range scans. Loops over the
159 : * blocks [current_blocknum, last_exclusive).
160 : */
161 : BlockNumber
162 665708 : block_range_read_stream_cb(ReadStream *stream,
163 : void *callback_private_data,
164 : void *per_buffer_data)
165 : {
166 665708 : BlockRangeReadStreamPrivate *p = callback_private_data;
167 :
168 665708 : if (p->current_blocknum < p->last_exclusive)
169 535542 : return p->current_blocknum++;
170 :
171 130166 : return InvalidBlockNumber;
172 : }
173 :
174 : /*
175 : * Ask the callback which block it would like us to read next, with a one block
176 : * buffer in front to allow read_stream_unget_block() to work.
177 : */
178 : static inline BlockNumber
179 10034798 : read_stream_get_block(ReadStream *stream, void *per_buffer_data)
180 : {
181 : BlockNumber blocknum;
182 :
183 10034798 : blocknum = stream->buffered_blocknum;
184 10034798 : if (blocknum != InvalidBlockNumber)
185 12 : stream->buffered_blocknum = InvalidBlockNumber;
186 : else
187 : {
188 : /*
189 : * Tell Valgrind that the per-buffer data is undefined. That replaces
190 : * the "noaccess" state that was set when the consumer moved past this
191 : * entry last time around the queue, and should also catch callbacks
192 : * that fail to initialize data that the buffer consumer later
193 : * accesses. On the first go around, it is undefined already.
194 : */
195 : VALGRIND_MAKE_MEM_UNDEFINED(per_buffer_data,
196 : stream->per_buffer_data_size);
197 10034786 : blocknum = stream->callback(stream,
198 : stream->callback_private_data,
199 : per_buffer_data);
200 : }
201 :
202 10034798 : return blocknum;
203 : }
204 :
205 : /*
206 : * In order to deal with buffer shortages and I/O limits after short reads, we
207 : * sometimes need to defer handling of a block we've already consumed from the
208 : * registered callback until later.
209 : */
210 : static inline void
211 12 : read_stream_unget_block(ReadStream *stream, BlockNumber blocknum)
212 : {
213 : /* We shouldn't ever unget more than one block. */
214 : Assert(stream->buffered_blocknum == InvalidBlockNumber);
215 : Assert(blocknum != InvalidBlockNumber);
216 12 : stream->buffered_blocknum = blocknum;
217 12 : }
218 :
219 : /*
220 : * Start as much of the current pending read as we can. If we have to split it
221 : * because of the per-backend buffer limit, or the buffer manager decides to
222 : * split it, then the pending read is adjusted to hold the remaining portion.
223 : *
224 : * We can always start a read of at least size one if we have no progress yet.
225 : * Otherwise it's possible that we can't start a read at all because of a lack
226 : * of buffers, and then false is returned. Buffer shortages also reduce the
227 : * distance to a level that prevents look-ahead until buffers are released.
228 : */
229 : static bool
230 3614790 : read_stream_start_pending_read(ReadStream *stream)
231 : {
232 : bool need_wait;
233 : int requested_nblocks;
234 : int nblocks;
235 : int flags;
236 : int forwarded;
237 : int16 io_index;
238 : int16 overflow;
239 : int16 buffer_index;
240 : int buffer_limit;
241 :
242 : /* This should only be called with a pending read. */
243 : Assert(stream->pending_read_nblocks > 0);
244 : Assert(stream->pending_read_nblocks <= stream->io_combine_limit);
245 :
246 : /* We had better not exceed the per-stream buffer limit with this read. */
247 : Assert(stream->pinned_buffers + stream->pending_read_nblocks <=
248 : stream->max_pinned_buffers);
249 :
250 : #ifdef USE_ASSERT_CHECKING
251 : /* We had better not be overwriting an existing pinned buffer. */
252 : if (stream->pinned_buffers > 0)
253 : Assert(stream->next_buffer_index != stream->oldest_buffer_index);
254 : else
255 : Assert(stream->next_buffer_index == stream->oldest_buffer_index);
256 :
257 : /*
258 : * Pinned buffers forwarded by a preceding StartReadBuffers() call that
259 : * had to split the operation should match the leading blocks of this
260 : * following StartReadBuffers() call.
261 : */
262 : Assert(stream->forwarded_buffers <= stream->pending_read_nblocks);
263 : for (int i = 0; i < stream->forwarded_buffers; ++i)
264 : Assert(BufferGetBlockNumber(stream->buffers[stream->next_buffer_index + i]) ==
265 : stream->pending_read_blocknum + i);
266 :
267 : /*
268 : * Check that we've cleared the queue/overflow entries corresponding to
269 : * the rest of the blocks covered by this read, unless it's the first go
270 : * around and we haven't even initialized them yet.
271 : */
272 : for (int i = stream->forwarded_buffers; i < stream->pending_read_nblocks; ++i)
273 : Assert(stream->next_buffer_index + i >= stream->initialized_buffers ||
274 : stream->buffers[stream->next_buffer_index + i] == InvalidBuffer);
275 : #endif
276 :
277 : /* Do we need to issue read-ahead advice? */
278 3614790 : flags = stream->read_buffers_flags;
279 3614790 : if (stream->advice_enabled)
280 : {
281 3290 : if (stream->pending_read_blocknum == stream->seq_blocknum)
282 : {
283 : /*
284 : * Sequential: Issue advice until the preadv() calls have caught
285 : * up with the first advice issued for this sequential region, and
286 : * then stay of the way of the kernel's own read-ahead.
287 : */
288 44 : if (stream->seq_until_processed != InvalidBlockNumber)
289 2 : flags |= READ_BUFFERS_ISSUE_ADVICE;
290 : }
291 : else
292 : {
293 : /*
294 : * Random jump: Note the starting location of a new potential
295 : * sequential region and start issuing advice. Skip it this time
296 : * if the preadv() follows immediately, eg first block in stream.
297 : */
298 3246 : stream->seq_until_processed = stream->pending_read_blocknum;
299 3246 : if (stream->pinned_buffers > 0)
300 48 : flags |= READ_BUFFERS_ISSUE_ADVICE;
301 : }
302 : }
303 :
304 : /*
305 : * How many more buffers is this backend allowed?
306 : *
307 : * Forwarded buffers are already pinned and map to the leading blocks of
308 : * the pending read (the remaining portion of an earlier short read that
309 : * we're about to continue). They are not counted in pinned_buffers, but
310 : * they are counted as pins already held by this backend according to the
311 : * buffer manager, so they must be added to the limit it grants us.
312 : */
313 3614790 : if (stream->temporary)
314 23674 : buffer_limit = Min(GetAdditionalLocalPinLimit(), PG_INT16_MAX);
315 : else
316 3591116 : buffer_limit = Min(GetAdditionalPinLimit(), PG_INT16_MAX);
317 : Assert(stream->forwarded_buffers <= stream->pending_read_nblocks);
318 :
319 3614790 : buffer_limit += stream->forwarded_buffers;
320 3614790 : buffer_limit = Min(buffer_limit, PG_INT16_MAX);
321 :
322 3614790 : if (buffer_limit == 0 && stream->pinned_buffers == 0)
323 1164904 : buffer_limit = 1; /* guarantee progress */
324 :
325 : /* Does the per-backend limit affect this read? */
326 3614790 : nblocks = stream->pending_read_nblocks;
327 3614790 : if (buffer_limit < nblocks)
328 : {
329 : int16 new_distance;
330 :
331 : /* Shrink distance: no more look-ahead until buffers are released. */
332 5002 : new_distance = stream->pinned_buffers + buffer_limit;
333 5002 : if (stream->distance > new_distance)
334 3474 : stream->distance = new_distance;
335 :
336 : /* Unless we have nothing to give the consumer, stop here. */
337 5002 : if (stream->pinned_buffers > 0)
338 2496 : return false;
339 :
340 : /* A short read is required to make progress. */
341 2506 : nblocks = buffer_limit;
342 : }
343 :
344 : /*
345 : * We say how many blocks we want to read, but it may be smaller on return
346 : * if the buffer manager decides to shorten the read. Initialize buffers
347 : * to InvalidBuffer (= not a forwarded buffer) as input on first use only,
348 : * and keep the original nblocks number so we can check for forwarded
349 : * buffers as output, below.
350 : */
351 3612294 : buffer_index = stream->next_buffer_index;
352 3612294 : io_index = stream->next_io_index;
353 5888292 : while (stream->initialized_buffers < buffer_index + nblocks)
354 2275998 : stream->buffers[stream->initialized_buffers++] = InvalidBuffer;
355 3612294 : requested_nblocks = nblocks;
356 3612294 : need_wait = StartReadBuffers(&stream->ios[io_index].op,
357 3612294 : &stream->buffers[buffer_index],
358 : stream->pending_read_blocknum,
359 : &nblocks,
360 : flags);
361 3612282 : stream->pinned_buffers += nblocks;
362 :
363 : /* Remember whether we need to wait before returning this buffer. */
364 3612282 : if (!need_wait)
365 : {
366 : /* Look-ahead distance decays, no I/O necessary. */
367 2558324 : if (stream->distance > 1)
368 30826 : stream->distance--;
369 : }
370 : else
371 : {
372 : /*
373 : * Remember to call WaitReadBuffers() before returning head buffer.
374 : * Look-ahead distance will be adjusted after waiting.
375 : */
376 1053958 : stream->ios[io_index].buffer_index = buffer_index;
377 1053958 : if (++stream->next_io_index == stream->max_ios)
378 42240 : stream->next_io_index = 0;
379 : Assert(stream->ios_in_progress < stream->max_ios);
380 1053958 : stream->ios_in_progress++;
381 1053958 : stream->seq_blocknum = stream->pending_read_blocknum + nblocks;
382 : }
383 :
384 : /*
385 : * How many pins were acquired but forwarded to the next call? These need
386 : * to be passed to the next StartReadBuffers() call by leaving them
387 : * exactly where they are in the queue, or released if the stream ends
388 : * early. We need the number for accounting purposes, since they are not
389 : * counted in stream->pinned_buffers but we already hold them.
390 : */
391 3612282 : forwarded = 0;
392 3615002 : while (nblocks + forwarded < requested_nblocks &&
393 102312 : stream->buffers[buffer_index + nblocks + forwarded] != InvalidBuffer)
394 2720 : forwarded++;
395 3612282 : stream->forwarded_buffers = forwarded;
396 :
397 : /*
398 : * We gave a contiguous range of buffer space to StartReadBuffers(), but
399 : * we want it to wrap around at queue_size. Copy overflowing buffers to
400 : * the front of the array where they'll be consumed, but also leave a copy
401 : * in the overflow zone which the I/O operation has a pointer to (it needs
402 : * a contiguous array). Both copies will be cleared when the buffers are
403 : * handed to the consumer.
404 : */
405 3612282 : overflow = (buffer_index + nblocks + forwarded) - stream->queue_size;
406 3612282 : if (overflow > 0)
407 : {
408 : Assert(overflow < stream->queue_size); /* can't overlap */
409 646 : memcpy(&stream->buffers[0],
410 646 : &stream->buffers[stream->queue_size],
411 : sizeof(stream->buffers[0]) * overflow);
412 : }
413 :
414 : /* Compute location of start of next read, without using % operator. */
415 3612282 : buffer_index += nblocks;
416 3612282 : if (buffer_index >= stream->queue_size)
417 543038 : buffer_index -= stream->queue_size;
418 : Assert(buffer_index >= 0 && buffer_index < stream->queue_size);
419 3612282 : stream->next_buffer_index = buffer_index;
420 :
421 : /* Adjust the pending read to cover the remaining portion, if any. */
422 3612282 : stream->pending_read_blocknum += nblocks;
423 3612282 : stream->pending_read_nblocks -= nblocks;
424 :
425 3612282 : return true;
426 : }
427 :
428 : static void
429 6092668 : read_stream_look_ahead(ReadStream *stream)
430 : {
431 : /*
432 : * Allow amortizing the cost of submitting IO over multiple IOs. This
433 : * requires that we don't do any operations that could lead to a deadlock
434 : * with staged-but-unsubmitted IO. The callback needs to opt-in to being
435 : * careful.
436 : */
437 6092668 : if (stream->batch_mode)
438 4983528 : pgaio_enter_batchmode();
439 :
440 10037040 : while (stream->ios_in_progress < stream->max_ios &&
441 10037032 : stream->pinned_buffers + stream->pending_read_nblocks < stream->distance)
442 : {
443 : BlockNumber blocknum;
444 : int16 buffer_index;
445 : void *per_buffer_data;
446 :
447 5899542 : if (stream->pending_read_nblocks == stream->io_combine_limit)
448 : {
449 7322 : read_stream_start_pending_read(stream);
450 7322 : continue;
451 : }
452 :
453 : /*
454 : * See which block the callback wants next in the stream. We need to
455 : * compute the index of the Nth block of the pending read including
456 : * wrap-around, but we don't want to use the expensive % operator.
457 : */
458 5892220 : buffer_index = stream->next_buffer_index + stream->pending_read_nblocks;
459 5892220 : if (buffer_index >= stream->queue_size)
460 3534 : buffer_index -= stream->queue_size;
461 : Assert(buffer_index >= 0 && buffer_index < stream->queue_size);
462 5892220 : per_buffer_data = get_per_buffer_data(stream, buffer_index);
463 5892220 : blocknum = read_stream_get_block(stream, per_buffer_data);
464 5892220 : if (blocknum == InvalidBlockNumber)
465 : {
466 : /* End of stream. */
467 1955158 : stream->distance = 0;
468 1955158 : break;
469 : }
470 :
471 : /* Can we merge it with the pending read? */
472 3937062 : if (stream->pending_read_nblocks > 0 &&
473 427634 : stream->pending_read_blocknum + stream->pending_read_nblocks == blocknum)
474 : {
475 427556 : stream->pending_read_nblocks++;
476 427556 : continue;
477 : }
478 :
479 : /* We have to start the pending read before we can build another. */
480 3509582 : while (stream->pending_read_nblocks > 0)
481 : {
482 88 : if (!read_stream_start_pending_read(stream) ||
483 76 : stream->ios_in_progress == stream->max_ios)
484 : {
485 : /* We've hit the buffer or I/O limit. Rewind and stop here. */
486 12 : read_stream_unget_block(stream, blocknum);
487 12 : if (stream->batch_mode)
488 12 : pgaio_exit_batchmode();
489 12 : return;
490 : }
491 : }
492 :
493 : /* This is the start of a new pending read. */
494 3509494 : stream->pending_read_blocknum = blocknum;
495 3509494 : stream->pending_read_nblocks = 1;
496 : }
497 :
498 : /*
499 : * We don't start the pending read just because we've hit the distance
500 : * limit, preferring to give it another chance to grow to full
501 : * io_combine_limit size once more buffers have been consumed. However,
502 : * if we've already reached io_combine_limit, or we've reached the
503 : * distance limit and there isn't anything pinned yet, or the callback has
504 : * signaled end-of-stream, we start the read immediately. Note that the
505 : * pending read can exceed the distance goal, if the latter was reduced
506 : * after hitting the per-backend buffer limit.
507 : */
508 6092656 : if (stream->pending_read_nblocks > 0 &&
509 3684846 : (stream->pending_read_nblocks == stream->io_combine_limit ||
510 3669732 : (stream->pending_read_nblocks >= stream->distance &&
511 3592270 : stream->pinned_buffers == 0) ||
512 88928 : stream->distance == 0) &&
513 3607384 : stream->ios_in_progress < stream->max_ios)
514 3607380 : read_stream_start_pending_read(stream);
515 :
516 : /*
517 : * There should always be something pinned when we leave this function,
518 : * whether started by this call or not, unless we've hit the end of the
519 : * stream. In the worst case we can always make progress one buffer at a
520 : * time.
521 : */
522 : Assert(stream->pinned_buffers > 0 || stream->distance == 0);
523 :
524 6092644 : if (stream->batch_mode)
525 4983504 : pgaio_exit_batchmode();
526 : }
527 :
528 : /*
529 : * Create a new read stream object that can be used to perform the equivalent
530 : * of a series of ReadBuffer() calls for one fork of one relation.
531 : * Internally, it generates larger vectored reads where possible by looking
532 : * ahead. The callback should return block numbers or InvalidBlockNumber to
533 : * signal end-of-stream, and if per_buffer_data_size is non-zero, it may also
534 : * write extra data for each block into the space provided to it. It will
535 : * also receive callback_private_data for its own purposes.
536 : */
537 : static ReadStream *
538 1135228 : read_stream_begin_impl(int flags,
539 : BufferAccessStrategy strategy,
540 : Relation rel,
541 : SMgrRelation smgr,
542 : char persistence,
543 : ForkNumber forknum,
544 : ReadStreamBlockNumberCB callback,
545 : void *callback_private_data,
546 : size_t per_buffer_data_size)
547 : {
548 : ReadStream *stream;
549 : size_t size;
550 : int16 queue_size;
551 : int16 queue_overflow;
552 : int max_ios;
553 : int strategy_pin_limit;
554 : uint32 max_pinned_buffers;
555 : uint32 max_possible_buffer_limit;
556 : Oid tablespace_id;
557 :
558 : /*
559 : * Decide how many I/Os we will allow to run at the same time. That
560 : * currently means advice to the kernel to tell it that we will soon read.
561 : * This number also affects how far we look ahead for opportunities to
562 : * start more I/Os.
563 : */
564 1135228 : tablespace_id = smgr->smgr_rlocator.locator.spcOid;
565 1135228 : if (!OidIsValid(MyDatabaseId) ||
566 1298880 : (rel && IsCatalogRelation(rel)) ||
567 304134 : IsCatalogRelationOid(smgr->smgr_rlocator.locator.relNumber))
568 : {
569 : /*
570 : * Avoid circularity while trying to look up tablespace settings or
571 : * before spccache.c is ready.
572 : */
573 943548 : max_ios = effective_io_concurrency;
574 : }
575 191680 : else if (flags & READ_STREAM_MAINTENANCE)
576 32268 : max_ios = get_tablespace_maintenance_io_concurrency(tablespace_id);
577 : else
578 159412 : max_ios = get_tablespace_io_concurrency(tablespace_id);
579 :
580 : /* Cap to INT16_MAX to avoid overflowing below */
581 1135228 : max_ios = Min(max_ios, PG_INT16_MAX);
582 :
583 : /*
584 : * If starting a multi-block I/O near the end of the queue, we might
585 : * temporarily need extra space for overflowing buffers before they are
586 : * moved to regular circular position. This is the maximum extra space we
587 : * could need.
588 : */
589 1135228 : queue_overflow = io_combine_limit - 1;
590 :
591 : /*
592 : * Choose the maximum number of buffers we're prepared to pin. We try to
593 : * pin fewer if we can, though. We add one so that we can make progress
594 : * even if max_ios is set to 0 (see also further down). For max_ios > 0,
595 : * this also allows an extra full I/O's worth of buffers: after an I/O
596 : * finishes we don't want to have to wait for its buffers to be consumed
597 : * before starting a new one.
598 : *
599 : * Be careful not to allow int16 to overflow. That is possible with the
600 : * current GUC range limits, so this is an artificial limit of ~32k
601 : * buffers and we'd need to adjust the types to exceed that. We also have
602 : * to allow for the spare entry and the overflow space.
603 : */
604 1135228 : max_pinned_buffers = (max_ios + 1) * io_combine_limit;
605 1135228 : max_pinned_buffers = Min(max_pinned_buffers,
606 : PG_INT16_MAX - queue_overflow - 1);
607 :
608 : /* Give the strategy a chance to limit the number of buffers we pin. */
609 1135228 : strategy_pin_limit = GetAccessStrategyPinLimit(strategy);
610 1135228 : max_pinned_buffers = Min(strategy_pin_limit, max_pinned_buffers);
611 :
612 : /*
613 : * Also limit our queue to the maximum number of pins we could ever be
614 : * allowed to acquire according to the buffer manager. We may not really
615 : * be able to use them all due to other pins held by this backend, but
616 : * we'll check that later in read_stream_start_pending_read().
617 : */
618 1135228 : if (SmgrIsTemp(smgr))
619 13478 : max_possible_buffer_limit = GetLocalPinLimit();
620 : else
621 1121750 : max_possible_buffer_limit = GetPinLimit();
622 1135228 : max_pinned_buffers = Min(max_pinned_buffers, max_possible_buffer_limit);
623 :
624 : /*
625 : * The limit might be zero on a system configured with too few buffers for
626 : * the number of connections. We need at least one to make progress.
627 : */
628 1135228 : max_pinned_buffers = Max(1, max_pinned_buffers);
629 :
630 : /*
631 : * We need one extra entry for buffers and per-buffer data, because users
632 : * of per-buffer data have access to the object until the next call to
633 : * read_stream_next_buffer(), so we need a gap between the head and tail
634 : * of the queue so that we don't clobber it.
635 : */
636 1135228 : queue_size = max_pinned_buffers + 1;
637 :
638 : /*
639 : * Allocate the object, the buffers, the ios and per_buffer_data space in
640 : * one big chunk. Though we have queue_size buffers, we want to be able
641 : * to assume that all the buffers for a single read are contiguous (i.e.
642 : * don't wrap around halfway through), so we allow temporary overflows of
643 : * up to the maximum possible overflow size.
644 : */
645 1135228 : size = offsetof(ReadStream, buffers);
646 1135228 : size += sizeof(Buffer) * (queue_size + queue_overflow);
647 1135228 : size += sizeof(InProgressIO) * Max(1, max_ios);
648 1135228 : size += per_buffer_data_size * queue_size;
649 1135228 : size += MAXIMUM_ALIGNOF * 2;
650 1135228 : stream = (ReadStream *) palloc(size);
651 1135228 : memset(stream, 0, offsetof(ReadStream, buffers));
652 1135228 : stream->ios = (InProgressIO *)
653 1135228 : MAXALIGN(&stream->buffers[queue_size + queue_overflow]);
654 1135228 : if (per_buffer_data_size > 0)
655 259552 : stream->per_buffer_data = (void *)
656 259552 : MAXALIGN(&stream->ios[Max(1, max_ios)]);
657 :
658 1135228 : stream->sync_mode = io_method == IOMETHOD_SYNC;
659 1135228 : stream->batch_mode = flags & READ_STREAM_USE_BATCHING;
660 :
661 : #ifdef USE_PREFETCH
662 :
663 : /*
664 : * Read-ahead advice simulating asynchronous I/O with synchronous calls.
665 : * Issue advice only if AIO is not used, direct I/O isn't enabled, the
666 : * caller hasn't promised sequential access (overriding our detection
667 : * heuristics), and max_ios hasn't been set to zero.
668 : */
669 1135228 : if (stream->sync_mode &&
670 5774 : (io_direct_flags & IO_DIRECT_DATA) == 0 &&
671 5774 : (flags & READ_STREAM_SEQUENTIAL) == 0 &&
672 : max_ios > 0)
673 1384 : stream->advice_enabled = true;
674 : #endif
675 :
676 : /*
677 : * Setting max_ios to zero disables AIO and advice-based pseudo AIO, but
678 : * we still need to allocate space to combine and run one I/O. Bump it up
679 : * to one, and remember to ask for synchronous I/O only.
680 : */
681 1135228 : if (max_ios == 0)
682 : {
683 14 : max_ios = 1;
684 14 : stream->read_buffers_flags = READ_BUFFERS_SYNCHRONOUSLY;
685 : }
686 :
687 : /*
688 : * Capture stable values for these two GUC-derived numbers for the
689 : * lifetime of this stream, so we don't have to worry about the GUCs
690 : * changing underneath us beyond this point.
691 : */
692 1135228 : stream->max_ios = max_ios;
693 1135228 : stream->io_combine_limit = io_combine_limit;
694 :
695 1135228 : stream->per_buffer_data_size = per_buffer_data_size;
696 1135228 : stream->max_pinned_buffers = max_pinned_buffers;
697 1135228 : stream->queue_size = queue_size;
698 1135228 : stream->callback = callback;
699 1135228 : stream->callback_private_data = callback_private_data;
700 1135228 : stream->buffered_blocknum = InvalidBlockNumber;
701 1135228 : stream->seq_blocknum = InvalidBlockNumber;
702 1135228 : stream->seq_until_processed = InvalidBlockNumber;
703 1135228 : stream->temporary = SmgrIsTemp(smgr);
704 :
705 : /*
706 : * Skip the initial ramp-up phase if the caller says we're going to be
707 : * reading the whole relation. This way we start out assuming we'll be
708 : * doing full io_combine_limit sized reads.
709 : */
710 1135228 : if (flags & READ_STREAM_FULL)
711 131778 : stream->distance = Min(max_pinned_buffers, stream->io_combine_limit);
712 : else
713 1003450 : stream->distance = 1;
714 :
715 : /*
716 : * Since we always access the same relation, we can initialize parts of
717 : * the ReadBuffersOperation objects and leave them that way, to avoid
718 : * wasting CPU cycles writing to them for each read.
719 : */
720 19341710 : for (int i = 0; i < max_ios; ++i)
721 : {
722 18206482 : stream->ios[i].op.rel = rel;
723 18206482 : stream->ios[i].op.smgr = smgr;
724 18206482 : stream->ios[i].op.persistence = persistence;
725 18206482 : stream->ios[i].op.forknum = forknum;
726 18206482 : stream->ios[i].op.strategy = strategy;
727 : }
728 :
729 1135228 : return stream;
730 : }
731 :
732 : /*
733 : * Create a new read stream for reading a relation.
734 : * See read_stream_begin_impl() for the detailed explanation.
735 : */
736 : ReadStream *
737 1014638 : read_stream_begin_relation(int flags,
738 : BufferAccessStrategy strategy,
739 : Relation rel,
740 : ForkNumber forknum,
741 : ReadStreamBlockNumberCB callback,
742 : void *callback_private_data,
743 : size_t per_buffer_data_size)
744 : {
745 1014638 : return read_stream_begin_impl(flags,
746 : strategy,
747 : rel,
748 : RelationGetSmgr(rel),
749 1014638 : rel->rd_rel->relpersistence,
750 : forknum,
751 : callback,
752 : callback_private_data,
753 : per_buffer_data_size);
754 : }
755 :
756 : /*
757 : * Create a new read stream for reading a SMgr relation.
758 : * See read_stream_begin_impl() for the detailed explanation.
759 : */
760 : ReadStream *
761 120590 : read_stream_begin_smgr_relation(int flags,
762 : BufferAccessStrategy strategy,
763 : SMgrRelation smgr,
764 : char smgr_persistence,
765 : ForkNumber forknum,
766 : ReadStreamBlockNumberCB callback,
767 : void *callback_private_data,
768 : size_t per_buffer_data_size)
769 : {
770 120590 : return read_stream_begin_impl(flags,
771 : strategy,
772 : NULL,
773 : smgr,
774 : smgr_persistence,
775 : forknum,
776 : callback,
777 : callback_private_data,
778 : per_buffer_data_size);
779 : }
780 :
781 : /*
782 : * Pull one pinned buffer out of a stream. Each call returns successive
783 : * blocks in the order specified by the callback. If per_buffer_data_size was
784 : * set to a non-zero size, *per_buffer_data receives a pointer to the extra
785 : * per-buffer data that the callback had a chance to populate, which remains
786 : * valid until the next call to read_stream_next_buffer(). When the stream
787 : * runs out of data, InvalidBuffer is returned. The caller may decide to end
788 : * the stream early at any time by calling read_stream_end().
789 : */
790 : Buffer
791 12109440 : read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
792 : {
793 : Buffer buffer;
794 : int16 oldest_buffer_index;
795 :
796 : #ifndef READ_STREAM_DISABLE_FAST_PATH
797 :
798 : /*
799 : * A fast path for all-cached scans. This is the same as the usual
800 : * algorithm, but it is specialized for no I/O and no per-buffer data, so
801 : * we can skip the queue management code, stay in the same buffer slot and
802 : * use singular StartReadBuffer().
803 : */
804 12109440 : if (likely(stream->fast_path))
805 : {
806 : BlockNumber next_blocknum;
807 :
808 : /* Fast path assumptions. */
809 : Assert(stream->ios_in_progress == 0);
810 : Assert(stream->forwarded_buffers == 0);
811 : Assert(stream->pinned_buffers == 1);
812 : Assert(stream->distance == 1);
813 : Assert(stream->pending_read_nblocks == 0);
814 : Assert(stream->per_buffer_data_size == 0);
815 : Assert(stream->initialized_buffers > stream->oldest_buffer_index);
816 :
817 : /* We're going to return the buffer we pinned last time. */
818 4142578 : oldest_buffer_index = stream->oldest_buffer_index;
819 : Assert((oldest_buffer_index + 1) % stream->queue_size ==
820 : stream->next_buffer_index);
821 4142578 : buffer = stream->buffers[oldest_buffer_index];
822 : Assert(buffer != InvalidBuffer);
823 :
824 : /* Choose the next block to pin. */
825 4142578 : next_blocknum = read_stream_get_block(stream, NULL);
826 :
827 4142578 : if (likely(next_blocknum != InvalidBlockNumber))
828 : {
829 3971234 : int flags = stream->read_buffers_flags;
830 :
831 3971234 : if (stream->advice_enabled)
832 1058 : flags |= READ_BUFFERS_ISSUE_ADVICE;
833 :
834 : /*
835 : * Pin a buffer for the next call. Same buffer entry, and
836 : * arbitrary I/O entry (they're all free). We don't have to
837 : * adjust pinned_buffers because we're transferring one to caller
838 : * but pinning one more.
839 : *
840 : * In the fast path we don't need to check the pin limit. We're
841 : * always allowed at least one pin so that progress can be made,
842 : * and that's all we need here. Although two pins are momentarily
843 : * held at the same time, the model used here is that the stream
844 : * holds only one, and the other now belongs to the caller.
845 : */
846 3971234 : if (likely(!StartReadBuffer(&stream->ios[0].op,
847 : &stream->buffers[oldest_buffer_index],
848 : next_blocknum,
849 : flags)))
850 : {
851 : /* Fast return. */
852 3944656 : return buffer;
853 : }
854 :
855 : /* Next call must wait for I/O for the newly pinned buffer. */
856 26578 : stream->oldest_io_index = 0;
857 26578 : stream->next_io_index = stream->max_ios > 1 ? 1 : 0;
858 26578 : stream->ios_in_progress = 1;
859 26578 : stream->ios[0].buffer_index = oldest_buffer_index;
860 26578 : stream->seq_blocknum = next_blocknum + 1;
861 : }
862 : else
863 : {
864 : /* No more blocks, end of stream. */
865 171344 : stream->distance = 0;
866 171344 : stream->oldest_buffer_index = stream->next_buffer_index;
867 171344 : stream->pinned_buffers = 0;
868 171344 : stream->buffers[oldest_buffer_index] = InvalidBuffer;
869 : }
870 :
871 197922 : stream->fast_path = false;
872 197922 : return buffer;
873 : }
874 : #endif
875 :
876 7966862 : if (unlikely(stream->pinned_buffers == 0))
877 : {
878 : Assert(stream->oldest_buffer_index == stream->next_buffer_index);
879 :
880 : /* End of stream reached? */
881 5444524 : if (stream->distance == 0)
882 3115718 : return InvalidBuffer;
883 :
884 : /*
885 : * The usual order of operations is that we look ahead at the bottom
886 : * of this function after potentially finishing an I/O and making
887 : * space for more, but if we're just starting up we'll need to crank
888 : * the handle to get started.
889 : */
890 2328806 : read_stream_look_ahead(stream);
891 :
892 : /* End of stream reached? */
893 2328806 : if (stream->pinned_buffers == 0)
894 : {
895 : Assert(stream->distance == 0);
896 1087240 : return InvalidBuffer;
897 : }
898 : }
899 :
900 : /* Grab the oldest pinned buffer and associated per-buffer data. */
901 : Assert(stream->pinned_buffers > 0);
902 3763904 : oldest_buffer_index = stream->oldest_buffer_index;
903 : Assert(oldest_buffer_index >= 0 &&
904 : oldest_buffer_index < stream->queue_size);
905 3763904 : buffer = stream->buffers[oldest_buffer_index];
906 3763904 : if (per_buffer_data)
907 1294814 : *per_buffer_data = get_per_buffer_data(stream, oldest_buffer_index);
908 :
909 : Assert(BufferIsValid(buffer));
910 :
911 : /* Do we have to wait for an associated I/O first? */
912 3763904 : if (stream->ios_in_progress > 0 &&
913 1247494 : stream->ios[stream->oldest_io_index].buffer_index == oldest_buffer_index)
914 : {
915 1079936 : int16 io_index = stream->oldest_io_index;
916 : int32 distance; /* wider temporary value, clamped below */
917 :
918 : /* Sanity check that we still agree on the buffers. */
919 : Assert(stream->ios[io_index].op.buffers ==
920 : &stream->buffers[oldest_buffer_index]);
921 :
922 1079936 : WaitReadBuffers(&stream->ios[io_index].op);
923 :
924 : Assert(stream->ios_in_progress > 0);
925 1079894 : stream->ios_in_progress--;
926 1079894 : if (++stream->oldest_io_index == stream->max_ios)
927 42240 : stream->oldest_io_index = 0;
928 :
929 : /* Look-ahead distance ramps up rapidly after we do I/O. */
930 1079894 : distance = stream->distance * 2;
931 1079894 : distance = Min(distance, stream->max_pinned_buffers);
932 1079894 : stream->distance = distance;
933 :
934 : /*
935 : * If we've reached the first block of a sequential region we're
936 : * issuing advice for, cancel that until the next jump. The kernel
937 : * will see the sequential preadv() pattern starting here.
938 : */
939 1079894 : if (stream->advice_enabled &&
940 544 : stream->ios[io_index].op.blocknum == stream->seq_until_processed)
941 502 : stream->seq_until_processed = InvalidBlockNumber;
942 : }
943 :
944 : /*
945 : * We must zap this queue entry, or else it would appear as a forwarded
946 : * buffer. If it's potentially in the overflow zone (ie from a
947 : * multi-block I/O that wrapped around the queue), also zap the copy.
948 : */
949 3763862 : stream->buffers[oldest_buffer_index] = InvalidBuffer;
950 3763862 : if (oldest_buffer_index < stream->io_combine_limit - 1)
951 2772814 : stream->buffers[stream->queue_size + oldest_buffer_index] =
952 : InvalidBuffer;
953 :
954 : #if defined(CLOBBER_FREED_MEMORY) || defined(USE_VALGRIND)
955 :
956 : /*
957 : * The caller will get access to the per-buffer data, until the next call.
958 : * We wipe the one before, which is never occupied because queue_size
959 : * allowed one extra element. This will hopefully trip up client code
960 : * that is holding a dangling pointer to it.
961 : */
962 : if (stream->per_buffer_data)
963 : {
964 : void *per_buffer_data;
965 :
966 : per_buffer_data = get_per_buffer_data(stream,
967 : oldest_buffer_index == 0 ?
968 : stream->queue_size - 1 :
969 : oldest_buffer_index - 1);
970 :
971 : #if defined(CLOBBER_FREED_MEMORY)
972 : /* This also tells Valgrind the memory is "noaccess". */
973 : wipe_mem(per_buffer_data, stream->per_buffer_data_size);
974 : #elif defined(USE_VALGRIND)
975 : /* Tell it ourselves. */
976 : VALGRIND_MAKE_MEM_NOACCESS(per_buffer_data,
977 : stream->per_buffer_data_size);
978 : #endif
979 : }
980 : #endif
981 :
982 : /* Pin transferred to caller. */
983 : Assert(stream->pinned_buffers > 0);
984 3763862 : stream->pinned_buffers--;
985 :
986 : /* Advance oldest buffer, with wrap-around. */
987 3763862 : stream->oldest_buffer_index++;
988 3763862 : if (stream->oldest_buffer_index == stream->queue_size)
989 528640 : stream->oldest_buffer_index = 0;
990 :
991 : /* Prepare for the next call. */
992 3763862 : read_stream_look_ahead(stream);
993 :
994 : #ifndef READ_STREAM_DISABLE_FAST_PATH
995 : /* See if we can take the fast path for all-cached scans next time. */
996 3763850 : if (stream->ios_in_progress == 0 &&
997 2683612 : stream->forwarded_buffers == 0 &&
998 2682612 : stream->pinned_buffers == 1 &&
999 1500720 : stream->distance == 1 &&
1000 1364336 : stream->pending_read_nblocks == 0 &&
1001 1361722 : stream->per_buffer_data_size == 0)
1002 : {
1003 : /*
1004 : * The fast path spins on one buffer entry repeatedly instead of
1005 : * rotating through the whole queue and clearing the entries behind
1006 : * it. If the buffer it starts with happened to be forwarded between
1007 : * StartReadBuffers() calls and also wrapped around the circular queue
1008 : * partway through, then a copy also exists in the overflow zone, and
1009 : * it won't clear it out as the regular path would. Do that now, so
1010 : * it doesn't need code for that.
1011 : */
1012 388408 : if (stream->oldest_buffer_index < stream->io_combine_limit - 1)
1013 385942 : stream->buffers[stream->queue_size + stream->oldest_buffer_index] =
1014 : InvalidBuffer;
1015 :
1016 388408 : stream->fast_path = true;
1017 : }
1018 : #endif
1019 :
1020 3763850 : return buffer;
1021 : }
1022 :
1023 : /*
1024 : * Transitional support for code that would like to perform or skip reads
1025 : * itself, without using the stream. Returns, and consumes, the next block
1026 : * number that would be read by the stream's look-ahead algorithm, or
1027 : * InvalidBlockNumber if the end of the stream is reached. Also reports the
1028 : * strategy that would be used to read it.
1029 : */
1030 : BlockNumber
1031 0 : read_stream_next_block(ReadStream *stream, BufferAccessStrategy *strategy)
1032 : {
1033 0 : *strategy = stream->ios[0].op.strategy;
1034 0 : return read_stream_get_block(stream, NULL);
1035 : }
1036 :
1037 : /*
1038 : * Reset a read stream by releasing any queued up buffers, allowing the stream
1039 : * to be used again for different blocks. This can be used to clear an
1040 : * end-of-stream condition and start again, or to throw away blocks that were
1041 : * speculatively read and read some different blocks instead.
1042 : */
1043 : void
1044 2330030 : read_stream_reset(ReadStream *stream)
1045 : {
1046 : int16 index;
1047 : Buffer buffer;
1048 :
1049 : /* Stop looking ahead. */
1050 2330030 : stream->distance = 0;
1051 :
1052 : /* Forget buffered block number and fast path state. */
1053 2330030 : stream->buffered_blocknum = InvalidBlockNumber;
1054 2330030 : stream->fast_path = false;
1055 :
1056 : /* Unpin anything that wasn't consumed. */
1057 2573430 : while ((buffer = read_stream_next_buffer(stream, NULL)) != InvalidBuffer)
1058 243400 : ReleaseBuffer(buffer);
1059 :
1060 : /* Unpin any unused forwarded buffers. */
1061 2330030 : index = stream->next_buffer_index;
1062 2330030 : while (index < stream->initialized_buffers &&
1063 437442 : (buffer = stream->buffers[index]) != InvalidBuffer)
1064 : {
1065 : Assert(stream->forwarded_buffers > 0);
1066 0 : stream->forwarded_buffers--;
1067 0 : ReleaseBuffer(buffer);
1068 :
1069 0 : stream->buffers[index] = InvalidBuffer;
1070 0 : if (index < stream->io_combine_limit - 1)
1071 0 : stream->buffers[stream->queue_size + index] = InvalidBuffer;
1072 :
1073 0 : if (++index == stream->queue_size)
1074 0 : index = 0;
1075 : }
1076 :
1077 : Assert(stream->forwarded_buffers == 0);
1078 : Assert(stream->pinned_buffers == 0);
1079 : Assert(stream->ios_in_progress == 0);
1080 :
1081 : /* Start off assuming data is cached. */
1082 2330030 : stream->distance = 1;
1083 2330030 : }
1084 :
1085 : /*
1086 : * Release and free a read stream.
1087 : */
1088 : void
1089 1130672 : read_stream_end(ReadStream *stream)
1090 : {
1091 1130672 : read_stream_reset(stream);
1092 1130672 : pfree(stream);
1093 1130672 : }
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