PostgreSQL Source Code  git master
clog.c
Go to the documentation of this file.
1 /*-------------------------------------------------------------------------
2  *
3  * clog.c
4  * PostgreSQL transaction-commit-log manager
5  *
6  * This module replaces the old "pg_log" access code, which treated pg_log
7  * essentially like a relation, in that it went through the regular buffer
8  * manager. The problem with that was that there wasn't any good way to
9  * recycle storage space for transactions so old that they'll never be
10  * looked up again. Now we use specialized access code so that the commit
11  * log can be broken into relatively small, independent segments.
12  *
13  * XLOG interactions: this module generates an XLOG record whenever a new
14  * CLOG page is initialized to zeroes. Other writes of CLOG come from
15  * recording of transaction commit or abort in xact.c, which generates its
16  * own XLOG records for these events and will re-perform the status update
17  * on redo; so we need make no additional XLOG entry here. For synchronous
18  * transaction commits, the XLOG is guaranteed flushed through the XLOG commit
19  * record before we are called to log a commit, so the WAL rule "write xlog
20  * before data" is satisfied automatically. However, for async commits we
21  * must track the latest LSN affecting each CLOG page, so that we can flush
22  * XLOG that far and satisfy the WAL rule. We don't have to worry about this
23  * for aborts (whether sync or async), since the post-crash assumption would
24  * be that such transactions failed anyway.
25  *
26  * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
27  * Portions Copyright (c) 1994, Regents of the University of California
28  *
29  * src/backend/access/transam/clog.c
30  *
31  *-------------------------------------------------------------------------
32  */
33 #include "postgres.h"
34 
35 #include "access/clog.h"
36 #include "access/slru.h"
37 #include "access/transam.h"
38 #include "access/xlog.h"
39 #include "access/xloginsert.h"
40 #include "access/xlogutils.h"
41 #include "miscadmin.h"
42 #include "pg_trace.h"
43 #include "pgstat.h"
44 #include "storage/proc.h"
45 #include "storage/sync.h"
46 
47 /*
48  * Defines for CLOG page sizes. A page is the same BLCKSZ as is used
49  * everywhere else in Postgres.
50  *
51  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
52  * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
53  * and CLOG segment numbering at
54  * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no
55  * explicit notice of that fact in this module, except when comparing segment
56  * and page numbers in TruncateCLOG (see CLOGPagePrecedes).
57  */
58 
59 /* We need two bits per xact, so four xacts fit in a byte */
60 #define CLOG_BITS_PER_XACT 2
61 #define CLOG_XACTS_PER_BYTE 4
62 #define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
63 #define CLOG_XACT_BITMASK ((1 << CLOG_BITS_PER_XACT) - 1)
64 
65 #define TransactionIdToPage(xid) ((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
66 #define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
67 #define TransactionIdToByte(xid) (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
68 #define TransactionIdToBIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
69 
70 /* We store the latest async LSN for each group of transactions */
71 #define CLOG_XACTS_PER_LSN_GROUP 32 /* keep this a power of 2 */
72 #define CLOG_LSNS_PER_PAGE (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)
73 
74 #define GetLSNIndex(slotno, xid) ((slotno) * CLOG_LSNS_PER_PAGE + \
75  ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)
76 
77 /*
78  * The number of subtransactions below which we consider to apply clog group
79  * update optimization. Testing reveals that the number higher than this can
80  * hurt performance.
81  */
82 #define THRESHOLD_SUBTRANS_CLOG_OPT 5
83 
84 /*
85  * Link to shared-memory data structures for CLOG control
86  */
88 
89 #define XactCtl (&XactCtlData)
90 
91 
92 static int ZeroCLOGPage(int pageno, bool writeXlog);
93 static bool CLOGPagePrecedes(int page1, int page2);
94 static void WriteZeroPageXlogRec(int pageno);
95 static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact,
96  Oid oldestXactDb);
97 static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
98  TransactionId *subxids, XidStatus status,
99  XLogRecPtr lsn, int pageno,
100  bool all_xact_same_page);
102  XLogRecPtr lsn, int slotno);
103 static void set_status_by_pages(int nsubxids, TransactionId *subxids,
106  XidStatus status, XLogRecPtr lsn, int pageno);
107 static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
108  TransactionId *subxids, XidStatus status,
109  XLogRecPtr lsn, int pageno);
110 
111 
112 /*
113  * TransactionIdSetTreeStatus
114  *
115  * Record the final state of transaction entries in the commit log for
116  * a transaction and its subtransaction tree. Take care to ensure this is
117  * efficient, and as atomic as possible.
118  *
119  * xid is a single xid to set status for. This will typically be
120  * the top level transactionid for a top level commit or abort. It can
121  * also be a subtransaction when we record transaction aborts.
122  *
123  * subxids is an array of xids of length nsubxids, representing subtransactions
124  * in the tree of xid. In various cases nsubxids may be zero.
125  *
126  * lsn must be the WAL location of the commit record when recording an async
127  * commit. For a synchronous commit it can be InvalidXLogRecPtr, since the
128  * caller guarantees the commit record is already flushed in that case. It
129  * should be InvalidXLogRecPtr for abort cases, too.
130  *
131  * In the commit case, atomicity is limited by whether all the subxids are in
132  * the same CLOG page as xid. If they all are, then the lock will be grabbed
133  * only once, and the status will be set to committed directly. Otherwise
134  * we must
135  * 1. set sub-committed all subxids that are not on the same page as the
136  * main xid
137  * 2. atomically set committed the main xid and the subxids on the same page
138  * 3. go over the first bunch again and set them committed
139  * Note that as far as concurrent checkers are concerned, main transaction
140  * commit as a whole is still atomic.
141  *
142  * Example:
143  * TransactionId t commits and has subxids t1, t2, t3, t4
144  * t is on page p1, t1 is also on p1, t2 and t3 are on p2, t4 is on p3
145  * 1. update pages2-3:
146  * page2: set t2,t3 as sub-committed
147  * page3: set t4 as sub-committed
148  * 2. update page1:
149  * page1: set t,t1 as committed
150  * 3. update pages2-3:
151  * page2: set t2,t3 as committed
152  * page3: set t4 as committed
153  *
154  * NB: this is a low-level routine and is NOT the preferred entry point
155  * for most uses; functions in transam.c are the intended callers.
156  *
157  * XXX Think about issuing POSIX_FADV_WILLNEED on pages that we will need,
158  * but aren't yet in cache, as well as hinting pages not to fall out of
159  * cache yet.
160  */
161 void
163  TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
164 {
165  int pageno = TransactionIdToPage(xid); /* get page of parent */
166  int i;
167 
170 
171  /*
172  * See how many subxids, if any, are on the same page as the parent, if
173  * any.
174  */
175  for (i = 0; i < nsubxids; i++)
176  {
177  if (TransactionIdToPage(subxids[i]) != pageno)
178  break;
179  }
180 
181  /*
182  * Do all items fit on a single page?
183  */
184  if (i == nsubxids)
185  {
186  /*
187  * Set the parent and all subtransactions in a single call
188  */
189  TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
190  pageno, true);
191  }
192  else
193  {
194  int nsubxids_on_first_page = i;
195 
196  /*
197  * If this is a commit then we care about doing this correctly (i.e.
198  * using the subcommitted intermediate status). By here, we know
199  * we're updating more than one page of clog, so we must mark entries
200  * that are *not* on the first page so that they show as subcommitted
201  * before we then return to update the status to fully committed.
202  *
203  * To avoid touching the first page twice, skip marking subcommitted
204  * for the subxids on that first page.
205  */
207  set_status_by_pages(nsubxids - nsubxids_on_first_page,
208  subxids + nsubxids_on_first_page,
210 
211  /*
212  * Now set the parent and subtransactions on same page as the parent,
213  * if any
214  */
215  pageno = TransactionIdToPage(xid);
216  TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
217  lsn, pageno, false);
218 
219  /*
220  * Now work through the rest of the subxids one clog page at a time,
221  * starting from the second page onwards, like we did above.
222  */
223  set_status_by_pages(nsubxids - nsubxids_on_first_page,
224  subxids + nsubxids_on_first_page,
225  status, lsn);
226  }
227 }
228 
229 /*
230  * Helper for TransactionIdSetTreeStatus: set the status for a bunch of
231  * transactions, chunking in the separate CLOG pages involved. We never
232  * pass the whole transaction tree to this function, only subtransactions
233  * that are on different pages to the top level transaction id.
234  */
235 static void
236 set_status_by_pages(int nsubxids, TransactionId *subxids,
238 {
239  int pageno = TransactionIdToPage(subxids[0]);
240  int offset = 0;
241  int i = 0;
242 
243  Assert(nsubxids > 0); /* else the pageno fetch above is unsafe */
244 
245  while (i < nsubxids)
246  {
247  int num_on_page = 0;
248  int nextpageno;
249 
250  do
251  {
252  nextpageno = TransactionIdToPage(subxids[i]);
253  if (nextpageno != pageno)
254  break;
255  num_on_page++;
256  i++;
257  } while (i < nsubxids);
258 
260  num_on_page, subxids + offset,
261  status, lsn, pageno, false);
262  offset = i;
263  pageno = nextpageno;
264  }
265 }
266 
267 /*
268  * Record the final state of transaction entries in the commit log for all
269  * entries on a single page. Atomic only on this page.
270  */
271 static void
273  TransactionId *subxids, XidStatus status,
274  XLogRecPtr lsn, int pageno,
275  bool all_xact_same_page)
276 {
277  /* Can't use group update when PGPROC overflows. */
279  "group clog threshold less than PGPROC cached subxids");
280 
281  /*
282  * When there is contention on XactSLRULock, we try to group multiple
283  * updates; a single leader process will perform transaction status
284  * updates for multiple backends so that the number of times XactSLRULock
285  * needs to be acquired is reduced.
286  *
287  * For this optimization to be safe, the XID and subxids in MyProc must be
288  * the same as the ones for which we're setting the status. Check that
289  * this is the case.
290  *
291  * For this optimization to be efficient, we shouldn't have too many
292  * sub-XIDs and all of the XIDs for which we're adjusting clog should be
293  * on the same page. Check those conditions, too.
294  */
295  if (all_xact_same_page && xid == MyProc->xid &&
296  nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
297  nsubxids == MyProc->subxidStatus.count &&
298  (nsubxids == 0 ||
299  memcmp(subxids, MyProc->subxids.xids,
300  nsubxids * sizeof(TransactionId)) == 0))
301  {
302  /*
303  * If we can immediately acquire XactSLRULock, we update the status of
304  * our own XID and release the lock. If not, try use group XID
305  * update. If that doesn't work out, fall back to waiting for the
306  * lock to perform an update for this transaction only.
307  */
308  if (LWLockConditionalAcquire(XactSLRULock, LW_EXCLUSIVE))
309  {
310  /* Got the lock without waiting! Do the update. */
311  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
312  lsn, pageno);
313  LWLockRelease(XactSLRULock);
314  return;
315  }
316  else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
317  {
318  /* Group update mechanism has done the work. */
319  return;
320  }
321 
322  /* Fall through only if update isn't done yet. */
323  }
324 
325  /* Group update not applicable, or couldn't accept this page number. */
326  LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
327  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
328  lsn, pageno);
329  LWLockRelease(XactSLRULock);
330 }
331 
332 /*
333  * Record the final state of transaction entry in the commit log
334  *
335  * We don't do any locking here; caller must handle that.
336  */
337 static void
339  TransactionId *subxids, XidStatus status,
340  XLogRecPtr lsn, int pageno)
341 {
342  int slotno;
343  int i;
344 
348  Assert(LWLockHeldByMeInMode(XactSLRULock, LW_EXCLUSIVE));
349 
350  /*
351  * If we're doing an async commit (ie, lsn is valid), then we must wait
352  * for any active write on the page slot to complete. Otherwise our
353  * update could reach disk in that write, which will not do since we
354  * mustn't let it reach disk until we've done the appropriate WAL flush.
355  * But when lsn is invalid, it's OK to scribble on a page while it is
356  * write-busy, since we don't care if the update reaches disk sooner than
357  * we think.
358  */
359  slotno = SimpleLruReadPage(XactCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
360 
361  /*
362  * Set the main transaction id, if any.
363  *
364  * If we update more than one xid on this page while it is being written
365  * out, we might find that some of the bits go to disk and others don't.
366  * If we are updating commits on the page with the top-level xid that
367  * could break atomicity, so we subcommit the subxids first before we mark
368  * the top-level commit.
369  */
370  if (TransactionIdIsValid(xid))
371  {
372  /* Subtransactions first, if needed ... */
374  {
375  for (i = 0; i < nsubxids; i++)
376  {
377  Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
378  TransactionIdSetStatusBit(subxids[i],
380  lsn, slotno);
381  }
382  }
383 
384  /* ... then the main transaction */
385  TransactionIdSetStatusBit(xid, status, lsn, slotno);
386  }
387 
388  /* Set the subtransactions */
389  for (i = 0; i < nsubxids; i++)
390  {
391  Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
392  TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
393  }
394 
395  XactCtl->shared->page_dirty[slotno] = true;
396 }
397 
398 /*
399  * When we cannot immediately acquire XactSLRULock in exclusive mode at
400  * commit time, add ourselves to a list of processes that need their XIDs
401  * status update. The first process to add itself to the list will acquire
402  * XactSLRULock in exclusive mode and set transaction status as required
403  * on behalf of all group members. This avoids a great deal of contention
404  * around XactSLRULock when many processes are trying to commit at once,
405  * since the lock need not be repeatedly handed off from one committing
406  * process to the next.
407  *
408  * Returns true when transaction status has been updated in clog; returns
409  * false if we decided against applying the optimization because the page
410  * number we need to update differs from those processes already waiting.
411  */
412 static bool
414  XLogRecPtr lsn, int pageno)
415 {
416  volatile PROC_HDR *procglobal = ProcGlobal;
417  PGPROC *proc = MyProc;
418  uint32 nextidx;
419  uint32 wakeidx;
420 
421  /* We should definitely have an XID whose status needs to be updated. */
423 
424  /*
425  * Add ourselves to the list of processes needing a group XID status
426  * update.
427  */
428  proc->clogGroupMember = true;
429  proc->clogGroupMemberXid = xid;
431  proc->clogGroupMemberPage = pageno;
432  proc->clogGroupMemberLsn = lsn;
433 
434  nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
435 
436  while (true)
437  {
438  /*
439  * Add the proc to list, if the clog page where we need to update the
440  * current transaction status is same as group leader's clog page.
441  *
442  * There is a race condition here, which is that after doing the below
443  * check and before adding this proc's clog update to a group, the
444  * group leader might have already finished the group update for this
445  * page and becomes group leader of another group. This will lead to a
446  * situation where a single group can have different clog page
447  * updates. This isn't likely and will still work, just maybe a bit
448  * less efficiently.
449  */
450  if (nextidx != INVALID_PGPROCNO &&
452  {
453  /*
454  * Ensure that this proc is not a member of any clog group that
455  * needs an XID status update.
456  */
457  proc->clogGroupMember = false;
459  return false;
460  }
461 
462  pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
463 
465  &nextidx,
466  (uint32) proc->pgprocno))
467  break;
468  }
469 
470  /*
471  * If the list was not empty, the leader will update the status of our
472  * XID. It is impossible to have followers without a leader because the
473  * first process that has added itself to the list will always have
474  * nextidx as INVALID_PGPROCNO.
475  */
476  if (nextidx != INVALID_PGPROCNO)
477  {
478  int extraWaits = 0;
479 
480  /* Sleep until the leader updates our XID status. */
482  for (;;)
483  {
484  /* acts as a read barrier */
485  PGSemaphoreLock(proc->sem);
486  if (!proc->clogGroupMember)
487  break;
488  extraWaits++;
489  }
491 
493 
494  /* Fix semaphore count for any absorbed wakeups */
495  while (extraWaits-- > 0)
496  PGSemaphoreUnlock(proc->sem);
497  return true;
498  }
499 
500  /* We are the leader. Acquire the lock on behalf of everyone. */
501  LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
502 
503  /*
504  * Now that we've got the lock, clear the list of processes waiting for
505  * group XID status update, saving a pointer to the head of the list.
506  * Trying to pop elements one at a time could lead to an ABA problem.
507  */
508  nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
510 
511  /* Remember head of list so we can perform wakeups after dropping lock. */
512  wakeidx = nextidx;
513 
514  /* Walk the list and update the status of all XIDs. */
515  while (nextidx != INVALID_PGPROCNO)
516  {
517  PGPROC *nextproc = &ProcGlobal->allProcs[nextidx];
518 
519  /*
520  * Transactions with more than THRESHOLD_SUBTRANS_CLOG_OPT sub-XIDs
521  * should not use group XID status update mechanism.
522  */
524 
526  nextproc->subxidStatus.count,
527  nextproc->subxids.xids,
528  nextproc->clogGroupMemberXidStatus,
529  nextproc->clogGroupMemberLsn,
530  nextproc->clogGroupMemberPage);
531 
532  /* Move to next proc in list. */
533  nextidx = pg_atomic_read_u32(&nextproc->clogGroupNext);
534  }
535 
536  /* We're done with the lock now. */
537  LWLockRelease(XactSLRULock);
538 
539  /*
540  * Now that we've released the lock, go back and wake everybody up. We
541  * don't do this under the lock so as to keep lock hold times to a
542  * minimum.
543  */
544  while (wakeidx != INVALID_PGPROCNO)
545  {
546  PGPROC *wakeproc = &ProcGlobal->allProcs[wakeidx];
547 
548  wakeidx = pg_atomic_read_u32(&wakeproc->clogGroupNext);
550 
551  /* ensure all previous writes are visible before follower continues. */
553 
554  wakeproc->clogGroupMember = false;
555 
556  if (wakeproc != MyProc)
557  PGSemaphoreUnlock(wakeproc->sem);
558  }
559 
560  return true;
561 }
562 
563 /*
564  * Sets the commit status of a single transaction.
565  *
566  * Must be called with XactSLRULock held
567  */
568 static void
570 {
571  int byteno = TransactionIdToByte(xid);
572  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
573  char *byteptr;
574  char byteval;
575  char curval;
576 
577  byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
578  curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
579 
580  /*
581  * When replaying transactions during recovery we still need to perform
582  * the two phases of subcommit and then commit. However, some transactions
583  * are already correctly marked, so we just treat those as a no-op which
584  * allows us to keep the following Assert as restrictive as possible.
585  */
588  return;
589 
590  /*
591  * Current state change should be from 0 or subcommitted to target state
592  * or we should already be there when replaying changes during recovery.
593  */
594  Assert(curval == 0 ||
597  curval == status);
598 
599  /* note this assumes exclusive access to the clog page */
600  byteval = *byteptr;
601  byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
602  byteval |= (status << bshift);
603  *byteptr = byteval;
604 
605  /*
606  * Update the group LSN if the transaction completion LSN is higher.
607  *
608  * Note: lsn will be invalid when supplied during InRecovery processing,
609  * so we don't need to do anything special to avoid LSN updates during
610  * recovery. After recovery completes the next clog change will set the
611  * LSN correctly.
612  */
613  if (!XLogRecPtrIsInvalid(lsn))
614  {
615  int lsnindex = GetLSNIndex(slotno, xid);
616 
617  if (XactCtl->shared->group_lsn[lsnindex] < lsn)
618  XactCtl->shared->group_lsn[lsnindex] = lsn;
619  }
620 }
621 
622 /*
623  * Interrogate the state of a transaction in the commit log.
624  *
625  * Aside from the actual commit status, this function returns (into *lsn)
626  * an LSN that is late enough to be able to guarantee that if we flush up to
627  * that LSN then we will have flushed the transaction's commit record to disk.
628  * The result is not necessarily the exact LSN of the transaction's commit
629  * record! For example, for long-past transactions (those whose clog pages
630  * already migrated to disk), we'll return InvalidXLogRecPtr. Also, because
631  * we group transactions on the same clog page to conserve storage, we might
632  * return the LSN of a later transaction that falls into the same group.
633  *
634  * NB: this is a low-level routine and is NOT the preferred entry point
635  * for most uses; TransactionLogFetch() in transam.c is the intended caller.
636  */
637 XidStatus
639 {
640  int pageno = TransactionIdToPage(xid);
641  int byteno = TransactionIdToByte(xid);
642  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
643  int slotno;
644  int lsnindex;
645  char *byteptr;
647 
648  /* lock is acquired by SimpleLruReadPage_ReadOnly */
649 
650  slotno = SimpleLruReadPage_ReadOnly(XactCtl, pageno, xid);
651  byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
652 
653  status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
654 
655  lsnindex = GetLSNIndex(slotno, xid);
656  *lsn = XactCtl->shared->group_lsn[lsnindex];
657 
658  LWLockRelease(XactSLRULock);
659 
660  return status;
661 }
662 
663 /*
664  * Number of shared CLOG buffers.
665  *
666  * On larger multi-processor systems, it is possible to have many CLOG page
667  * requests in flight at one time which could lead to disk access for CLOG
668  * page if the required page is not found in memory. Testing revealed that we
669  * can get the best performance by having 128 CLOG buffers, more than that it
670  * doesn't improve performance.
671  *
672  * Unconditionally keeping the number of CLOG buffers to 128 did not seem like
673  * a good idea, because it would increase the minimum amount of shared memory
674  * required to start, which could be a problem for people running very small
675  * configurations. The following formula seems to represent a reasonable
676  * compromise: people with very low values for shared_buffers will get fewer
677  * CLOG buffers as well, and everyone else will get 128.
678  */
679 Size
681 {
682  return Min(128, Max(4, NBuffers / 512));
683 }
684 
685 /*
686  * Initialization of shared memory for CLOG
687  */
688 Size
690 {
692 }
693 
694 void
696 {
697  XactCtl->PagePrecedes = CLOGPagePrecedes;
699  XactSLRULock, "pg_xact", LWTRANCHE_XACT_BUFFER,
702 }
703 
704 /*
705  * This func must be called ONCE on system install. It creates
706  * the initial CLOG segment. (The CLOG directory is assumed to
707  * have been created by initdb, and CLOGShmemInit must have been
708  * called already.)
709  */
710 void
712 {
713  int slotno;
714 
715  LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
716 
717  /* Create and zero the first page of the commit log */
718  slotno = ZeroCLOGPage(0, false);
719 
720  /* Make sure it's written out */
721  SimpleLruWritePage(XactCtl, slotno);
722  Assert(!XactCtl->shared->page_dirty[slotno]);
723 
724  LWLockRelease(XactSLRULock);
725 }
726 
727 /*
728  * Initialize (or reinitialize) a page of CLOG to zeroes.
729  * If writeXlog is true, also emit an XLOG record saying we did this.
730  *
731  * The page is not actually written, just set up in shared memory.
732  * The slot number of the new page is returned.
733  *
734  * Control lock must be held at entry, and will be held at exit.
735  */
736 static int
737 ZeroCLOGPage(int pageno, bool writeXlog)
738 {
739  int slotno;
740 
741  slotno = SimpleLruZeroPage(XactCtl, pageno);
742 
743  if (writeXlog)
744  WriteZeroPageXlogRec(pageno);
745 
746  return slotno;
747 }
748 
749 /*
750  * This must be called ONCE during postmaster or standalone-backend startup,
751  * after StartupXLOG has initialized ShmemVariableCache->nextXid.
752  */
753 void
755 {
757  int pageno = TransactionIdToPage(xid);
758 
759  LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
760 
761  /*
762  * Initialize our idea of the latest page number.
763  */
764  XactCtl->shared->latest_page_number = pageno;
765 
766  LWLockRelease(XactSLRULock);
767 }
768 
769 /*
770  * This must be called ONCE at the end of startup/recovery.
771  */
772 void
773 TrimCLOG(void)
774 {
776  int pageno = TransactionIdToPage(xid);
777 
778  LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
779 
780  /*
781  * Zero out the remainder of the current clog page. Under normal
782  * circumstances it should be zeroes already, but it seems at least
783  * theoretically possible that XLOG replay will have settled on a nextXID
784  * value that is less than the last XID actually used and marked by the
785  * previous database lifecycle (since subtransaction commit writes clog
786  * but makes no WAL entry). Let's just be safe. (We need not worry about
787  * pages beyond the current one, since those will be zeroed when first
788  * used. For the same reason, there is no need to do anything when
789  * nextXid is exactly at a page boundary; and it's likely that the
790  * "current" page doesn't exist yet in that case.)
791  */
792  if (TransactionIdToPgIndex(xid) != 0)
793  {
794  int byteno = TransactionIdToByte(xid);
795  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
796  int slotno;
797  char *byteptr;
798 
799  slotno = SimpleLruReadPage(XactCtl, pageno, false, xid);
800  byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
801 
802  /* Zero so-far-unused positions in the current byte */
803  *byteptr &= (1 << bshift) - 1;
804  /* Zero the rest of the page */
805  MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
806 
807  XactCtl->shared->page_dirty[slotno] = true;
808  }
809 
810  LWLockRelease(XactSLRULock);
811 }
812 
813 /*
814  * Perform a checkpoint --- either during shutdown, or on-the-fly
815  */
816 void
818 {
819  /*
820  * Write dirty CLOG pages to disk. This may result in sync requests
821  * queued for later handling by ProcessSyncRequests(), as part of the
822  * checkpoint.
823  */
824  TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
825  SimpleLruWriteAll(XactCtl, true);
826  TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
827 }
828 
829 
830 /*
831  * Make sure that CLOG has room for a newly-allocated XID.
832  *
833  * NB: this is called while holding XidGenLock. We want it to be very fast
834  * most of the time; even when it's not so fast, no actual I/O need happen
835  * unless we're forced to write out a dirty clog or xlog page to make room
836  * in shared memory.
837  */
838 void
840 {
841  int pageno;
842 
843  /*
844  * No work except at first XID of a page. But beware: just after
845  * wraparound, the first XID of page zero is FirstNormalTransactionId.
846  */
847  if (TransactionIdToPgIndex(newestXact) != 0 &&
849  return;
850 
851  pageno = TransactionIdToPage(newestXact);
852 
853  LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
854 
855  /* Zero the page and make an XLOG entry about it */
856  ZeroCLOGPage(pageno, true);
857 
858  LWLockRelease(XactSLRULock);
859 }
860 
861 
862 /*
863  * Remove all CLOG segments before the one holding the passed transaction ID
864  *
865  * Before removing any CLOG data, we must flush XLOG to disk, to ensure
866  * that any recently-emitted FREEZE_PAGE records have reached disk; otherwise
867  * a crash and restart might leave us with some unfrozen tuples referencing
868  * removed CLOG data. We choose to emit a special TRUNCATE XLOG record too.
869  * Replaying the deletion from XLOG is not critical, since the files could
870  * just as well be removed later, but doing so prevents a long-running hot
871  * standby server from acquiring an unreasonably bloated CLOG directory.
872  *
873  * Since CLOG segments hold a large number of transactions, the opportunity to
874  * actually remove a segment is fairly rare, and so it seems best not to do
875  * the XLOG flush unless we have confirmed that there is a removable segment.
876  */
877 void
878 TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
879 {
880  int cutoffPage;
881 
882  /*
883  * The cutoff point is the start of the segment containing oldestXact. We
884  * pass the *page* containing oldestXact to SimpleLruTruncate.
885  */
886  cutoffPage = TransactionIdToPage(oldestXact);
887 
888  /* Check to see if there's any files that could be removed */
890  return; /* nothing to remove */
891 
892  /*
893  * Advance oldestClogXid before truncating clog, so concurrent xact status
894  * lookups can ensure they don't attempt to access truncated-away clog.
895  *
896  * It's only necessary to do this if we will actually truncate away clog
897  * pages.
898  */
899  AdvanceOldestClogXid(oldestXact);
900 
901  /*
902  * Write XLOG record and flush XLOG to disk. We record the oldest xid
903  * we're keeping information about here so we can ensure that it's always
904  * ahead of clog truncation in case we crash, and so a standby finds out
905  * the new valid xid before the next checkpoint.
906  */
907  WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
908 
909  /* Now we can remove the old CLOG segment(s) */
910  SimpleLruTruncate(XactCtl, cutoffPage);
911 }
912 
913 
914 /*
915  * Decide whether a CLOG page number is "older" for truncation purposes.
916  *
917  * We need to use comparison of TransactionIds here in order to do the right
918  * thing with wraparound XID arithmetic. However, TransactionIdPrecedes()
919  * would get weird about permanent xact IDs. So, offset both such that xid1,
920  * xid2, and xid2 + CLOG_XACTS_PER_PAGE - 1 are all normal XIDs; this offset
921  * is relevant to page 0 and to the page preceding page 0.
922  *
923  * The page containing oldestXact-2^31 is the important edge case. The
924  * portion of that page equaling or following oldestXact-2^31 is expendable,
925  * but the portion preceding oldestXact-2^31 is not. When oldestXact-2^31 is
926  * the first XID of a page and segment, the entire page and segment is
927  * expendable, and we could truncate the segment. Recognizing that case would
928  * require making oldestXact, not just the page containing oldestXact,
929  * available to this callback. The benefit would be rare and small, so we
930  * don't optimize that edge case.
931  */
932 static bool
933 CLOGPagePrecedes(int page1, int page2)
934 {
935  TransactionId xid1;
936  TransactionId xid2;
937 
938  xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
939  xid1 += FirstNormalTransactionId + 1;
940  xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
941  xid2 += FirstNormalTransactionId + 1;
942 
943  return (TransactionIdPrecedes(xid1, xid2) &&
944  TransactionIdPrecedes(xid1, xid2 + CLOG_XACTS_PER_PAGE - 1));
945 }
946 
947 
948 /*
949  * Write a ZEROPAGE xlog record
950  */
951 static void
953 {
954  XLogBeginInsert();
955  XLogRegisterData((char *) (&pageno), sizeof(int));
956  (void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE);
957 }
958 
959 /*
960  * Write a TRUNCATE xlog record
961  *
962  * We must flush the xlog record to disk before returning --- see notes
963  * in TruncateCLOG().
964  */
965 static void
966 WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXactDb)
967 {
968  XLogRecPtr recptr;
969  xl_clog_truncate xlrec;
970 
971  xlrec.pageno = pageno;
972  xlrec.oldestXact = oldestXact;
973  xlrec.oldestXactDb = oldestXactDb;
974 
975  XLogBeginInsert();
976  XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
977  recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
978  XLogFlush(recptr);
979 }
980 
981 /*
982  * CLOG resource manager's routines
983  */
984 void
986 {
987  uint8 info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
988 
989  /* Backup blocks are not used in clog records */
990  Assert(!XLogRecHasAnyBlockRefs(record));
991 
992  if (info == CLOG_ZEROPAGE)
993  {
994  int pageno;
995  int slotno;
996 
997  memcpy(&pageno, XLogRecGetData(record), sizeof(int));
998 
999  LWLockAcquire(XactSLRULock, LW_EXCLUSIVE);
1000 
1001  slotno = ZeroCLOGPage(pageno, false);
1002  SimpleLruWritePage(XactCtl, slotno);
1003  Assert(!XactCtl->shared->page_dirty[slotno]);
1004 
1005  LWLockRelease(XactSLRULock);
1006  }
1007  else if (info == CLOG_TRUNCATE)
1008  {
1009  xl_clog_truncate xlrec;
1010 
1011  memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
1012 
1014 
1016  }
1017  else
1018  elog(PANIC, "clog_redo: unknown op code %u", info);
1019 }
1020 
1021 /*
1022  * Entrypoint for sync.c to sync clog files.
1023  */
1024 int
1025 clogsyncfiletag(const FileTag *ftag, char *path)
1026 {
1027  return SlruSyncFileTag(XactCtl, ftag, path);
1028 }
static bool pg_atomic_compare_exchange_u32(volatile pg_atomic_uint32 *ptr, uint32 *expected, uint32 newval)
Definition: atomics.h:306
#define pg_write_barrier()
Definition: atomics.h:154
static void pg_atomic_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
Definition: atomics.h:253
static uint32 pg_atomic_read_u32(volatile pg_atomic_uint32 *ptr)
Definition: atomics.h:236
static uint32 pg_atomic_exchange_u32(volatile pg_atomic_uint32 *ptr, uint32 newval)
Definition: atomics.h:287
unsigned int uint32
Definition: c.h:442
#define Min(x, y)
Definition: c.h:937
#define Max(x, y)
Definition: c.h:931
unsigned char uint8
Definition: c.h:440
#define MemSet(start, val, len)
Definition: c.h:953
#define StaticAssertStmt(condition, errmessage)
Definition: c.h:869
uint32 TransactionId
Definition: c.h:588
size_t Size
Definition: c.h:541
#define CLOG_XACT_BITMASK
Definition: clog.c:63
#define CLOG_XACTS_PER_PAGE
Definition: clog.c:62
static void WriteZeroPageXlogRec(int pageno)
Definition: clog.c:952
static bool TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status, XLogRecPtr lsn, int pageno)
Definition: clog.c:413
static bool CLOGPagePrecedes(int page1, int page2)
Definition: clog.c:933
#define THRESHOLD_SUBTRANS_CLOG_OPT
Definition: clog.c:82
static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status, XLogRecPtr lsn, int slotno)
Definition: clog.c:569
XidStatus TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
Definition: clog.c:638
void TransactionIdSetTreeStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
Definition: clog.c:162
void CLOGShmemInit(void)
Definition: clog.c:695
#define TransactionIdToBIndex(xid)
Definition: clog.c:68
Size CLOGShmemBuffers(void)
Definition: clog.c:680
void ExtendCLOG(TransactionId newestXact)
Definition: clog.c:839
void clog_redo(XLogReaderState *record)
Definition: clog.c:985
void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
Definition: clog.c:878
Size CLOGShmemSize(void)
Definition: clog.c:689
static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int pageno, bool all_xact_same_page)
Definition: clog.c:272
int clogsyncfiletag(const FileTag *ftag, char *path)
Definition: clog.c:1025
void BootStrapCLOG(void)
Definition: clog.c:711
#define CLOG_BITS_PER_XACT
Definition: clog.c:60
#define CLOG_LSNS_PER_PAGE
Definition: clog.c:72
#define TransactionIdToByte(xid)
Definition: clog.c:67
#define TransactionIdToPgIndex(xid)
Definition: clog.c:66
void StartupCLOG(void)
Definition: clog.c:754
static void set_status_by_pages(int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
Definition: clog.c:236
#define GetLSNIndex(slotno, xid)
Definition: clog.c:74
void CheckPointCLOG(void)
Definition: clog.c:817
static SlruCtlData XactCtlData
Definition: clog.c:87
#define XactCtl
Definition: clog.c:89
void TrimCLOG(void)
Definition: clog.c:773
static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int pageno)
Definition: clog.c:338
static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXactDb)
Definition: clog.c:966
#define TransactionIdToPage(xid)
Definition: clog.c:65
static int ZeroCLOGPage(int pageno, bool writeXlog)
Definition: clog.c:737
#define TRANSACTION_STATUS_IN_PROGRESS
Definition: clog.h:27
int XidStatus
Definition: clog.h:25
#define CLOG_ZEROPAGE
Definition: clog.h:56
#define TRANSACTION_STATUS_ABORTED
Definition: clog.h:29
#define TRANSACTION_STATUS_SUB_COMMITTED
Definition: clog.h:30
#define CLOG_TRUNCATE
Definition: clog.h:57
#define TRANSACTION_STATUS_COMMITTED
Definition: clog.h:28
#define PANIC
Definition: elog.h:38
int NBuffers
Definition: globals.c:136
int i
Definition: isn.c:73
Assert(fmt[strlen(fmt) - 1] !='\n')
bool LWLockAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1194
bool LWLockHeldByMeInMode(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1962
void LWLockRelease(LWLock *lock)
Definition: lwlock.c:1802
bool LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1365
@ LWTRANCHE_XACT_BUFFER
Definition: lwlock.h:177
@ LW_EXCLUSIVE
Definition: lwlock.h:112
static void static void status(const char *fmt,...) pg_attribute_printf(1
Definition: pg_regress.c:225
void PGSemaphoreUnlock(PGSemaphore sema)
Definition: posix_sema.c:340
void PGSemaphoreLock(PGSemaphore sema)
Definition: posix_sema.c:320
unsigned int Oid
Definition: postgres_ext.h:31
#define INVALID_PGPROCNO
Definition: proc.h:85
#define PGPROC_MAX_CACHED_SUBXIDS
Definition: proc.h:38
void SimpleLruWritePage(SlruCtl ctl, int slotno)
Definition: slru.c:615
void SimpleLruWriteAll(SlruCtl ctl, bool allow_redirtied)
Definition: slru.c:1157
bool SlruScanDirCbReportPresence(SlruCtl ctl, char *filename, int segpage, void *data)
Definition: slru.c:1501
void SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns, LWLock *ctllock, const char *subdir, int tranche_id, SyncRequestHandler sync_handler)
Definition: slru.c:188
void SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
Definition: slru.c:1227
bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
Definition: slru.c:1554
int SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path)
Definition: slru.c:1594
int SimpleLruZeroPage(SlruCtl ctl, int pageno)
Definition: slru.c:281
int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
Definition: slru.c:496
int SimpleLruReadPage(SlruCtl ctl, int pageno, bool write_ok, TransactionId xid)
Definition: slru.c:396
Size SimpleLruShmemSize(int nslots, int nlsns)
Definition: slru.c:156
#define SlruPagePrecedesUnitTests(ctl, per_page)
Definition: slru.h:156
PGPROC * MyProc
Definition: proc.c:68
PROC_HDR * ProcGlobal
Definition: proc.c:80
Definition: sync.h:51
Definition: proc.h:162
XLogRecPtr clogGroupMemberLsn
Definition: proc.h:280
int clogGroupMemberPage
Definition: proc.h:278
TransactionId clogGroupMemberXid
Definition: proc.h:275
bool clogGroupMember
Definition: proc.h:273
pg_atomic_uint32 clogGroupNext
Definition: proc.h:274
XidStatus clogGroupMemberXidStatus
Definition: proc.h:276
XidCacheStatus subxidStatus
Definition: proc.h:254
TransactionId xid
Definition: proc.h:173
int pgprocno
Definition: proc.h:191
struct XidCache subxids
Definition: proc.h:256
PGSemaphore sem
Definition: proc.h:167
Definition: proc.h:360
PGPROC * allProcs
Definition: proc.h:362
pg_atomic_uint32 clogGroupFirst
Definition: proc.h:392
FullTransactionId nextXid
Definition: transam.h:220
uint8 count
Definition: proc.h:43
TransactionId xids[PGPROC_MAX_CACHED_SUBXIDS]
Definition: proc.h:50
Oid oldestXactDb
Definition: clog.h:36
int pageno
Definition: clog.h:34
TransactionId oldestXact
Definition: clog.h:35
@ SYNC_HANDLER_CLOG
Definition: sync.h:38
bool TransactionIdPrecedes(TransactionId id1, TransactionId id2)
Definition: transam.c:273
#define InvalidTransactionId
Definition: transam.h:31
#define TransactionIdEquals(id1, id2)
Definition: transam.h:43
#define XidFromFullTransactionId(x)
Definition: transam.h:48
#define FirstNormalTransactionId
Definition: transam.h:34
#define TransactionIdIsValid(xid)
Definition: transam.h:41
void AdvanceOldestClogXid(TransactionId oldest_datfrozenxid)
Definition: varsup.c:328
VariableCache ShmemVariableCache
Definition: varsup.c:34
@ WAIT_EVENT_XACT_GROUP_UPDATE
Definition: wait_event.h:131
static void pgstat_report_wait_start(uint32 wait_event_info)
Definition: wait_event.h:268
static void pgstat_report_wait_end(void)
Definition: wait_event.h:284
void XLogFlush(XLogRecPtr record)
Definition: xlog.c:2512
#define XLogRecPtrIsInvalid(r)
Definition: xlogdefs.h:29
uint64 XLogRecPtr
Definition: xlogdefs.h:21
void XLogRegisterData(char *data, uint32 len)
Definition: xloginsert.c:351
XLogRecPtr XLogInsert(RmgrId rmid, uint8 info)
Definition: xloginsert.c:451
void XLogBeginInsert(void)
Definition: xloginsert.c:150
#define XLogRecGetInfo(decoder)
Definition: xlogreader.h:411
#define XLogRecGetData(decoder)
Definition: xlogreader.h:416
#define XLogRecHasAnyBlockRefs(decoder)
Definition: xlogreader.h:418
#define XLR_INFO_MASK
Definition: xlogrecord.h:62
bool InRecovery
Definition: xlogutils.c:53