Branch data Line data Source code
1 : : /*-------------------------------------------------------------------------
2 : : *
3 : : * syncscan.c
4 : : * scan synchronization support
5 : : *
6 : : * When multiple backends run a sequential scan on the same table, we try
7 : : * to keep them synchronized to reduce the overall I/O needed. The goal is
8 : : * to read each page into shared buffer cache only once, and let all backends
9 : : * that take part in the shared scan process the page before it falls out of
10 : : * the cache.
11 : : *
12 : : * Since the "leader" in a pack of backends doing a seqscan will have to wait
13 : : * for I/O, while the "followers" don't, there is a strong self-synchronizing
14 : : * effect once we can get the backends examining approximately the same part
15 : : * of the table at the same time. Hence all that is really needed is to get
16 : : * a new backend beginning a seqscan to begin it close to where other backends
17 : : * are reading. We can scan the table circularly, from block X up to the
18 : : * end and then from block 0 to X-1, to ensure we visit all rows while still
19 : : * participating in the common scan.
20 : : *
21 : : * To accomplish that, we keep track of the scan position of each table, and
22 : : * start new scans close to where the previous scan(s) are. We don't try to
23 : : * do any extra synchronization to keep the scans together afterwards; some
24 : : * scans might progress much more slowly than others, for example if the
25 : : * results need to be transferred to the client over a slow network, and we
26 : : * don't want such queries to slow down others.
27 : : *
28 : : * There can realistically only be a few large sequential scans on different
29 : : * tables in progress at any time. Therefore we just keep the scan positions
30 : : * in a small LRU list which we scan every time we need to look up or update a
31 : : * scan position. The whole mechanism is only applied for tables exceeding
32 : : * a threshold size (but that is not the concern of this module).
33 : : *
34 : : * INTERFACE ROUTINES
35 : : * ss_get_location - return current scan location of a relation
36 : : * ss_report_location - update current scan location
37 : : *
38 : : *
39 : : * Portions Copyright (c) 1996-2026, PostgreSQL Global Development Group
40 : : * Portions Copyright (c) 1994, Regents of the University of California
41 : : *
42 : : * IDENTIFICATION
43 : : * src/backend/access/common/syncscan.c
44 : : *
45 : : *-------------------------------------------------------------------------
46 : : */
47 : : #include "postgres.h"
48 : :
49 : : #include "access/syncscan.h"
50 : : #include "miscadmin.h"
51 : : #include "storage/lwlock.h"
52 : : #include "storage/shmem.h"
53 : : #include "utils/rel.h"
54 : :
55 : :
56 : : /* GUC variables */
57 : : #ifdef TRACE_SYNCSCAN
58 : : bool trace_syncscan = false;
59 : : #endif
60 : :
61 : :
62 : : /*
63 : : * Size of the LRU list.
64 : : *
65 : : * Note: the code assumes that SYNC_SCAN_NELEM > 1.
66 : : *
67 : : * XXX: What's a good value? It should be large enough to hold the
68 : : * maximum number of large tables scanned simultaneously. But a larger value
69 : : * means more traversing of the LRU list when starting a new scan.
70 : : */
71 : : #define SYNC_SCAN_NELEM 20
72 : :
73 : : /*
74 : : * Interval between reports of the location of the current scan, in pages.
75 : : *
76 : : * Note: This should be smaller than the ring size (see buffer/freelist.c)
77 : : * we use for bulk reads. Otherwise a scan joining other scans might start
78 : : * from a page that's no longer in the buffer cache. This is a bit fuzzy;
79 : : * there's no guarantee that the new scan will read the page before it leaves
80 : : * the buffer cache anyway, and on the other hand the page is most likely
81 : : * still in the OS cache.
82 : : */
83 : : #define SYNC_SCAN_REPORT_INTERVAL (128 * 1024 / BLCKSZ)
84 : :
85 : :
86 : : /*
87 : : * The scan locations structure is essentially a doubly-linked LRU with head
88 : : * and tail pointer, but designed to hold a fixed maximum number of elements in
89 : : * fixed-size shared memory.
90 : : */
91 : : typedef struct ss_scan_location_t
92 : : {
93 : : RelFileLocator relfilelocator; /* identity of a relation */
94 : : BlockNumber location; /* last-reported location in the relation */
95 : : } ss_scan_location_t;
96 : :
97 : : typedef struct ss_lru_item_t
98 : : {
99 : : struct ss_lru_item_t *prev;
100 : : struct ss_lru_item_t *next;
101 : : ss_scan_location_t location;
102 : : } ss_lru_item_t;
103 : :
104 : : typedef struct ss_scan_locations_t
105 : : {
106 : : ss_lru_item_t *head;
107 : : ss_lru_item_t *tail;
108 : : ss_lru_item_t items[FLEXIBLE_ARRAY_MEMBER]; /* SYNC_SCAN_NELEM items */
109 : : } ss_scan_locations_t;
110 : :
111 : : #define SizeOfScanLocations(N) \
112 : : (offsetof(ss_scan_locations_t, items) + (N) * sizeof(ss_lru_item_t))
113 : :
114 : : /* Pointer to struct in shared memory */
115 : : static ss_scan_locations_t *scan_locations;
116 : :
117 : : /* prototypes for internal functions */
118 : : static BlockNumber ss_search(RelFileLocator relfilelocator,
119 : : BlockNumber location, bool set);
120 : :
121 : :
122 : : /*
123 : : * SyncScanShmemSize --- report amount of shared memory space needed
124 : : */
125 : : Size
126 : 9 : SyncScanShmemSize(void)
127 : : {
128 : 9 : return SizeOfScanLocations(SYNC_SCAN_NELEM);
129 : : }
130 : :
131 : : /*
132 : : * SyncScanShmemInit --- initialize this module's shared memory
133 : : */
134 : : void
135 : 6 : SyncScanShmemInit(void)
136 : : {
137 : 6 : int i;
138 : 6 : bool found;
139 : :
140 : 6 : scan_locations = (ss_scan_locations_t *)
141 : 6 : ShmemInitStruct("Sync Scan Locations List",
142 : : SizeOfScanLocations(SYNC_SCAN_NELEM),
143 : : &found);
144 : :
145 [ + - ]: 6 : if (!IsUnderPostmaster)
146 : : {
147 : : /* Initialize shared memory area */
148 [ + - ]: 6 : Assert(!found);
149 : :
150 : 6 : scan_locations->head = &scan_locations->items[0];
151 : 6 : scan_locations->tail = &scan_locations->items[SYNC_SCAN_NELEM - 1];
152 : :
153 [ + + ]: 126 : for (i = 0; i < SYNC_SCAN_NELEM; i++)
154 : : {
155 : 120 : ss_lru_item_t *item = &scan_locations->items[i];
156 : :
157 : : /*
158 : : * Initialize all slots with invalid values. As scans are started,
159 : : * these invalid entries will fall off the LRU list and get
160 : : * replaced with real entries.
161 : : */
162 : 120 : item->location.relfilelocator.spcOid = InvalidOid;
163 : 120 : item->location.relfilelocator.dbOid = InvalidOid;
164 : 120 : item->location.relfilelocator.relNumber = InvalidRelFileNumber;
165 : 120 : item->location.location = InvalidBlockNumber;
166 : :
167 [ + + ]: 120 : item->prev = (i > 0) ?
168 : 114 : (&scan_locations->items[i - 1]) : NULL;
169 [ + + ]: 120 : item->next = (i < SYNC_SCAN_NELEM - 1) ?
170 : 114 : (&scan_locations->items[i + 1]) : NULL;
171 : 120 : }
172 : 6 : }
173 : : else
174 [ # # ]: 0 : Assert(found);
175 : 6 : }
176 : :
177 : : /*
178 : : * ss_search --- search the scan_locations structure for an entry with the
179 : : * given relfilelocator.
180 : : *
181 : : * If "set" is true, the location is updated to the given location. If no
182 : : * entry for the given relfilelocator is found, it will be created at the head
183 : : * of the list with the given location, even if "set" is false.
184 : : *
185 : : * In any case, the location after possible update is returned.
186 : : *
187 : : * Caller is responsible for having acquired suitable lock on the shared
188 : : * data structure.
189 : : */
190 : : static BlockNumber
191 : 0 : ss_search(RelFileLocator relfilelocator, BlockNumber location, bool set)
192 : : {
193 : 0 : ss_lru_item_t *item;
194 : :
195 : 0 : item = scan_locations->head;
196 : 0 : for (;;)
197 : : {
198 : 0 : bool match;
199 : :
200 [ # # # # ]: 0 : match = RelFileLocatorEquals(item->location.relfilelocator,
201 : : relfilelocator);
202 : :
203 [ # # # # ]: 0 : if (match || item->next == NULL)
204 : : {
205 : : /*
206 : : * If we reached the end of list and no match was found, take over
207 : : * the last entry
208 : : */
209 [ # # ]: 0 : if (!match)
210 : : {
211 : 0 : item->location.relfilelocator = relfilelocator;
212 : 0 : item->location.location = location;
213 : 0 : }
214 [ # # ]: 0 : else if (set)
215 : 0 : item->location.location = location;
216 : :
217 : : /* Move the entry to the front of the LRU list */
218 [ # # ]: 0 : if (item != scan_locations->head)
219 : : {
220 : : /* unlink */
221 [ # # ]: 0 : if (item == scan_locations->tail)
222 : 0 : scan_locations->tail = item->prev;
223 : 0 : item->prev->next = item->next;
224 [ # # ]: 0 : if (item->next)
225 : 0 : item->next->prev = item->prev;
226 : :
227 : : /* link */
228 : 0 : item->prev = NULL;
229 : 0 : item->next = scan_locations->head;
230 : 0 : scan_locations->head->prev = item;
231 : 0 : scan_locations->head = item;
232 : 0 : }
233 : :
234 : 0 : return item->location.location;
235 : : }
236 : :
237 : 0 : item = item->next;
238 [ # # ]: 0 : }
239 : :
240 : : /* not reached */
241 : 0 : }
242 : :
243 : : /*
244 : : * ss_get_location --- get the optimal starting location for scan
245 : : *
246 : : * Returns the last-reported location of a sequential scan on the
247 : : * relation, or 0 if no valid location is found.
248 : : *
249 : : * We expect the caller has just done RelationGetNumberOfBlocks(), and
250 : : * so that number is passed in rather than computing it again. The result
251 : : * is guaranteed less than relnblocks (assuming that's > 0).
252 : : */
253 : : BlockNumber
254 : 0 : ss_get_location(Relation rel, BlockNumber relnblocks)
255 : : {
256 : 0 : BlockNumber startloc;
257 : :
258 : 0 : LWLockAcquire(SyncScanLock, LW_EXCLUSIVE);
259 : 0 : startloc = ss_search(rel->rd_locator, 0, false);
260 : 0 : LWLockRelease(SyncScanLock);
261 : :
262 : : /*
263 : : * If the location is not a valid block number for this scan, start at 0.
264 : : *
265 : : * This can happen if for instance a VACUUM truncated the table since the
266 : : * location was saved.
267 : : */
268 [ # # ]: 0 : if (startloc >= relnblocks)
269 : 0 : startloc = 0;
270 : :
271 : : #ifdef TRACE_SYNCSCAN
272 : : if (trace_syncscan)
273 : : elog(LOG,
274 : : "SYNC_SCAN: start \"%s\" (size %u) at %u",
275 : : RelationGetRelationName(rel), relnblocks, startloc);
276 : : #endif
277 : :
278 : 0 : return startloc;
279 : 0 : }
280 : :
281 : : /*
282 : : * ss_report_location --- update the current scan location
283 : : *
284 : : * Writes an entry into the shared Sync Scan state of the form
285 : : * (relfilelocator, blocknumber), overwriting any existing entry for the
286 : : * same relfilelocator.
287 : : */
288 : : void
289 : 0 : ss_report_location(Relation rel, BlockNumber location)
290 : : {
291 : : #ifdef TRACE_SYNCSCAN
292 : : if (trace_syncscan)
293 : : {
294 : : if ((location % 1024) == 0)
295 : : elog(LOG,
296 : : "SYNC_SCAN: scanning \"%s\" at %u",
297 : : RelationGetRelationName(rel), location);
298 : : }
299 : : #endif
300 : :
301 : : /*
302 : : * To reduce lock contention, only report scan progress every N pages. For
303 : : * the same reason, don't block if the lock isn't immediately available.
304 : : * Missing a few updates isn't critical, it just means that a new scan
305 : : * that wants to join the pack will start a little bit behind the head of
306 : : * the scan. Hopefully the pages are still in OS cache and the scan
307 : : * catches up quickly.
308 : : */
309 [ # # ]: 0 : if ((location % SYNC_SCAN_REPORT_INTERVAL) == 0)
310 : : {
311 [ # # ]: 0 : if (LWLockConditionalAcquire(SyncScanLock, LW_EXCLUSIVE))
312 : : {
313 : 0 : (void) ss_search(rel->rd_locator, location, true);
314 : 0 : LWLockRelease(SyncScanLock);
315 : 0 : }
316 : : #ifdef TRACE_SYNCSCAN
317 : : else if (trace_syncscan)
318 : : elog(LOG,
319 : : "SYNC_SCAN: missed update for \"%s\" at %u",
320 : : RelationGetRelationName(rel), location);
321 : : #endif
322 : 0 : }
323 : 0 : }
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