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clog.c
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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) 1996-2017, 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 "pgstat.h"
43 #include "pg_trace.h"
44 #include "storage/proc.h"
45 
46 /*
47  * Defines for CLOG page sizes. A page is the same BLCKSZ as is used
48  * everywhere else in Postgres.
49  *
50  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
51  * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
52  * and CLOG segment numbering at
53  * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no
54  * explicit notice of that fact in this module, except when comparing segment
55  * and page numbers in TruncateCLOG (see CLOGPagePrecedes).
56  */
57 
58 /* We need two bits per xact, so four xacts fit in a byte */
59 #define CLOG_BITS_PER_XACT 2
60 #define CLOG_XACTS_PER_BYTE 4
61 #define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
62 #define CLOG_XACT_BITMASK ((1 << CLOG_BITS_PER_XACT) - 1)
63 
64 #define TransactionIdToPage(xid) ((xid) / (TransactionId) CLOG_XACTS_PER_PAGE)
65 #define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
66 #define TransactionIdToByte(xid) (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
67 #define TransactionIdToBIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
68 
69 /* We store the latest async LSN for each group of transactions */
70 #define CLOG_XACTS_PER_LSN_GROUP 32 /* keep this a power of 2 */
71 #define CLOG_LSNS_PER_PAGE (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)
72 
73 #define GetLSNIndex(slotno, xid) ((slotno) * CLOG_LSNS_PER_PAGE + \
74  ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)
75 
76 /*
77  * The number of subtransactions below which we consider to apply clog group
78  * update optimization. Testing reveals that the number higher than this can
79  * hurt performance.
80  */
81 #define THRESHOLD_SUBTRANS_CLOG_OPT 5
82 
83 /*
84  * Link to shared-memory data structures for CLOG control
85  */
87 
88 #define ClogCtl (&ClogCtlData)
89 
90 
91 static int ZeroCLOGPage(int pageno, bool writeXlog);
92 static bool CLOGPagePrecedes(int page1, int page2);
93 static void WriteZeroPageXlogRec(int pageno);
94 static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact,
95  Oid oldestXidDb);
96 static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
97  TransactionId *subxids, XidStatus status,
98  XLogRecPtr lsn, int pageno,
99  bool all_xact_same_page);
101  XLogRecPtr lsn, int slotno);
102 static void set_status_by_pages(int nsubxids, TransactionId *subxids,
105  XidStatus status, XLogRecPtr lsn, int pageno);
106 static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
107  TransactionId *subxids, XidStatus status,
108  XLogRecPtr lsn, int pageno);
109 
110 
111 /*
112  * TransactionIdSetTreeStatus
113  *
114  * Record the final state of transaction entries in the commit log for
115  * a transaction and its subtransaction tree. Take care to ensure this is
116  * efficient, and as atomic as possible.
117  *
118  * xid is a single xid to set status for. This will typically be
119  * the top level transactionid for a top level commit or abort. It can
120  * also be a subtransaction when we record transaction aborts.
121  *
122  * subxids is an array of xids of length nsubxids, representing subtransactions
123  * in the tree of xid. In various cases nsubxids may be zero.
124  *
125  * lsn must be the WAL location of the commit record when recording an async
126  * commit. For a synchronous commit it can be InvalidXLogRecPtr, since the
127  * caller guarantees the commit record is already flushed in that case. It
128  * should be InvalidXLogRecPtr for abort cases, too.
129  *
130  * In the commit case, atomicity is limited by whether all the subxids are in
131  * the same CLOG page as xid. If they all are, then the lock will be grabbed
132  * only once, and the status will be set to committed directly. Otherwise
133  * we must
134  * 1. set sub-committed all subxids that are not on the same page as the
135  * main xid
136  * 2. atomically set committed the main xid and the subxids on the same page
137  * 3. go over the first bunch again and set them committed
138  * Note that as far as concurrent checkers are concerned, main transaction
139  * commit as a whole is still atomic.
140  *
141  * Example:
142  * TransactionId t commits and has subxids t1, t2, t3, t4
143  * t is on page p1, t1 is also on p1, t2 and t3 are on p2, t4 is on p3
144  * 1. update pages2-3:
145  * page2: set t2,t3 as sub-committed
146  * page3: set t4 as sub-committed
147  * 2. update page1:
148  * set t1 as sub-committed,
149  * then set t as committed,
150  then set t1 as committed
151  * 3. update pages2-3:
152  * page2: set t2,t3 as committed
153  * page3: set t4 as committed
154  *
155  * NB: this is a low-level routine and is NOT the preferred entry point
156  * for most uses; functions in transam.c are the intended callers.
157  *
158  * XXX Think about issuing FADVISE_WILLNEED on pages that we will need,
159  * but aren't yet in cache, as well as hinting pages not to fall out of
160  * cache yet.
161  */
162 void
164  TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
165 {
166  int pageno = TransactionIdToPage(xid); /* get page of parent */
167  int i;
168 
170  status == TRANSACTION_STATUS_ABORTED);
171 
172  /*
173  * See how many subxids, if any, are on the same page as the parent, if
174  * any.
175  */
176  for (i = 0; i < nsubxids; i++)
177  {
178  if (TransactionIdToPage(subxids[i]) != pageno)
179  break;
180  }
181 
182  /*
183  * Do all items fit on a single page?
184  */
185  if (i == nsubxids)
186  {
187  /*
188  * Set the parent and all subtransactions in a single call
189  */
190  TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
191  pageno, true);
192  }
193  else
194  {
195  int nsubxids_on_first_page = i;
196 
197  /*
198  * If this is a commit then we care about doing this correctly (i.e.
199  * using the subcommitted intermediate status). By here, we know
200  * we're updating more than one page of clog, so we must mark entries
201  * that are *not* on the first page so that they show as subcommitted
202  * before we then return to update the status to fully committed.
203  *
204  * To avoid touching the first page twice, skip marking subcommitted
205  * for the subxids on that first page.
206  */
207  if (status == TRANSACTION_STATUS_COMMITTED)
208  set_status_by_pages(nsubxids - nsubxids_on_first_page,
209  subxids + nsubxids_on_first_page,
211 
212  /*
213  * Now set the parent and subtransactions on same page as the parent,
214  * if any
215  */
216  pageno = TransactionIdToPage(xid);
217  TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
218  lsn, pageno, false);
219 
220  /*
221  * Now work through the rest of the subxids one clog page at a time,
222  * starting from the second page onwards, like we did above.
223  */
224  set_status_by_pages(nsubxids - nsubxids_on_first_page,
225  subxids + nsubxids_on_first_page,
226  status, lsn);
227  }
228 }
229 
230 /*
231  * Helper for TransactionIdSetTreeStatus: set the status for a bunch of
232  * transactions, chunking in the separate CLOG pages involved. We never
233  * pass the whole transaction tree to this function, only subtransactions
234  * that are on different pages to the top level transaction id.
235  */
236 static void
237 set_status_by_pages(int nsubxids, TransactionId *subxids,
239 {
240  int pageno = TransactionIdToPage(subxids[0]);
241  int offset = 0;
242  int i = 0;
243 
244  Assert(nsubxids > 0); /* else the pageno fetch above is unsafe */
245 
246  while (i < nsubxids)
247  {
248  int num_on_page = 0;
249  int nextpageno;
250 
251  do
252  {
253  nextpageno = TransactionIdToPage(subxids[i]);
254  if (nextpageno != pageno)
255  break;
256  num_on_page++;
257  i++;
258  } while (i < nsubxids);
259 
261  num_on_page, subxids + offset,
262  status, lsn, pageno, false);
263  offset = i;
264  pageno = nextpageno;
265  }
266 }
267 
268 /*
269  * Record the final state of transaction entries in the commit log for all
270  * entries on a single page. Atomic only on this page.
271  */
272 static void
274  TransactionId *subxids, XidStatus status,
275  XLogRecPtr lsn, int pageno,
276  bool all_xact_same_page)
277 {
278  /* Can't use group update when PGPROC overflows. */
280  "group clog threshold less than PGPROC cached subxids");
281 
282  /*
283  * When there is contention on CLogControlLock, we try to group multiple
284  * updates; a single leader process will perform transaction status
285  * updates for multiple backends so that the number of times
286  * CLogControlLock needs to be acquired is reduced.
287  *
288  * For this optimization to be safe, the XID in MyPgXact and the subxids
289  * in MyProc must be the same as the ones for which we're setting the
290  * status. Check that this is the case.
291  *
292  * For this optimization to be efficient, we shouldn't have too many
293  * sub-XIDs and all of the XIDs for which we're adjusting clog should be
294  * on the same page. Check those conditions, too.
295  */
296  if (all_xact_same_page && xid == MyPgXact->xid &&
297  nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
298  nsubxids == MyPgXact->nxids &&
299  memcmp(subxids, MyProc->subxids.xids,
300  nsubxids * sizeof(TransactionId)) == 0)
301  {
302  /*
303  * We don't try to do group update optimization if a process has
304  * overflowed the subxids array in its PGPROC, since in that case we
305  * don't have a complete list of XIDs for it.
306  */
308 
309  /*
310  * If we can immediately acquire CLogControlLock, we update the status
311  * of our own XID and release the lock. If not, try use group XID
312  * update. If that doesn't work out, fall back to waiting for the
313  * lock to perform an update for this transaction only.
314  */
315  if (LWLockConditionalAcquire(CLogControlLock, LW_EXCLUSIVE))
316  {
317  /* Got the lock without waiting! Do the update. */
318  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
319  lsn, pageno);
320  LWLockRelease(CLogControlLock);
321  return;
322  }
323  else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
324  {
325  /* Group update mechanism has done the work. */
326  return;
327  }
328 
329  /* Fall through only if update isn't done yet. */
330  }
331 
332  /* Group update not applicable, or couldn't accept this page number. */
333  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
334  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
335  lsn, pageno);
336  LWLockRelease(CLogControlLock);
337 }
338 
339 /*
340  * Record the final state of transaction entry in the commit log
341  *
342  * We don't do any locking here; caller must handle that.
343  */
344 static void
346  TransactionId *subxids, XidStatus status,
347  XLogRecPtr lsn, int pageno)
348 {
349  int slotno;
350  int i;
351 
353  status == TRANSACTION_STATUS_ABORTED ||
355  Assert(LWLockHeldByMeInMode(CLogControlLock, LW_EXCLUSIVE));
356 
357  /*
358  * If we're doing an async commit (ie, lsn is valid), then we must wait
359  * for any active write on the page slot to complete. Otherwise our
360  * update could reach disk in that write, which will not do since we
361  * mustn't let it reach disk until we've done the appropriate WAL flush.
362  * But when lsn is invalid, it's OK to scribble on a page while it is
363  * write-busy, since we don't care if the update reaches disk sooner than
364  * we think.
365  */
366  slotno = SimpleLruReadPage(ClogCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
367 
368  /*
369  * Set the main transaction id, if any.
370  *
371  * If we update more than one xid on this page while it is being written
372  * out, we might find that some of the bits go to disk and others don't.
373  * If we are updating commits on the page with the top-level xid that
374  * could break atomicity, so we subcommit the subxids first before we mark
375  * the top-level commit.
376  */
377  if (TransactionIdIsValid(xid))
378  {
379  /* Subtransactions first, if needed ... */
380  if (status == TRANSACTION_STATUS_COMMITTED)
381  {
382  for (i = 0; i < nsubxids; i++)
383  {
384  Assert(ClogCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
385  TransactionIdSetStatusBit(subxids[i],
387  lsn, slotno);
388  }
389  }
390 
391  /* ... then the main transaction */
392  TransactionIdSetStatusBit(xid, status, lsn, slotno);
393  }
394 
395  /* Set the subtransactions */
396  for (i = 0; i < nsubxids; i++)
397  {
398  Assert(ClogCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
399  TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
400  }
401 
402  ClogCtl->shared->page_dirty[slotno] = true;
403 }
404 
405 /*
406  * When we cannot immediately acquire CLogControlLock in exclusive mode at
407  * commit time, add ourselves to a list of processes that need their XIDs
408  * status update. The first process to add itself to the list will acquire
409  * CLogControlLock in exclusive mode and set transaction status as required
410  * on behalf of all group members. This avoids a great deal of contention
411  * around CLogControlLock when many processes are trying to commit at once,
412  * since the lock need not be repeatedly handed off from one committing
413  * process to the next.
414  *
415  * Returns true when transaction status has been updated in clog; returns
416  * false if we decided against applying the optimization because the page
417  * number we need to update differs from those processes already waiting.
418  */
419 static bool
421  XLogRecPtr lsn, int pageno)
422 {
423  volatile PROC_HDR *procglobal = ProcGlobal;
424  PGPROC *proc = MyProc;
425  uint32 nextidx;
426  uint32 wakeidx;
427 
428  /* We should definitely have an XID whose status needs to be updated. */
430 
431  /*
432  * Add ourselves to the list of processes needing a group XID status
433  * update.
434  */
435  proc->clogGroupMember = true;
436  proc->clogGroupMemberXid = xid;
438  proc->clogGroupMemberPage = pageno;
439  proc->clogGroupMemberLsn = lsn;
440 
441  nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
442 
443  while (true)
444  {
445  /*
446  * Add the proc to list, if the clog page where we need to update the
447  * current transaction status is same as group leader's clog page.
448  *
449  * There is a race condition here, which is that after doing the below
450  * check and before adding this proc's clog update to a group, the
451  * group leader might have already finished the group update for this
452  * page and becomes group leader of another group. This will lead to a
453  * situation where a single group can have different clog page
454  * updates. This isn't likely and will still work, just maybe a bit
455  * less efficiently.
456  */
457  if (nextidx != INVALID_PGPROCNO &&
459  {
460  proc->clogGroupMember = false;
461  return false;
462  }
463 
464  pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
465 
467  &nextidx,
468  (uint32) proc->pgprocno))
469  break;
470  }
471 
472  /*
473  * If the list was not empty, the leader will update the status of our
474  * XID. It is impossible to have followers without a leader because the
475  * first process that has added itself to the list will always have
476  * nextidx as INVALID_PGPROCNO.
477  */
478  if (nextidx != INVALID_PGPROCNO)
479  {
480  int extraWaits = 0;
481 
482  /* Sleep until the leader updates our XID status. */
484  for (;;)
485  {
486  /* acts as a read barrier */
487  PGSemaphoreLock(proc->sem);
488  if (!proc->clogGroupMember)
489  break;
490  extraWaits++;
491  }
493 
495 
496  /* Fix semaphore count for any absorbed wakeups */
497  while (extraWaits-- > 0)
498  PGSemaphoreUnlock(proc->sem);
499  return true;
500  }
501 
502  /* We are the leader. Acquire the lock on behalf of everyone. */
503  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
504 
505  /*
506  * Now that we've got the lock, clear the list of processes waiting for
507  * group XID status update, saving a pointer to the head of the list.
508  * Trying to pop elements one at a time could lead to an ABA problem.
509  */
510  nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
512 
513  /* Remember head of list so we can perform wakeups after dropping lock. */
514  wakeidx = nextidx;
515 
516  /* Walk the list and update the status of all XIDs. */
517  while (nextidx != INVALID_PGPROCNO)
518  {
519  PGPROC *proc = &ProcGlobal->allProcs[nextidx];
520  PGXACT *pgxact = &ProcGlobal->allPgXact[nextidx];
521 
522  /*
523  * Overflowed transactions should not use group XID status update
524  * mechanism.
525  */
526  Assert(!pgxact->overflowed);
527 
529  pgxact->nxids,
530  proc->subxids.xids,
532  proc->clogGroupMemberLsn,
533  proc->clogGroupMemberPage);
534 
535  /* Move to next proc in list. */
536  nextidx = pg_atomic_read_u32(&proc->clogGroupNext);
537  }
538 
539  /* We're done with the lock now. */
540  LWLockRelease(CLogControlLock);
541 
542  /*
543  * Now that we've released the lock, go back and wake everybody up. We
544  * don't do this under the lock so as to keep lock hold times to a
545  * minimum.
546  */
547  while (wakeidx != INVALID_PGPROCNO)
548  {
549  PGPROC *proc = &ProcGlobal->allProcs[wakeidx];
550 
551  wakeidx = pg_atomic_read_u32(&proc->clogGroupNext);
553 
554  /* ensure all previous writes are visible before follower continues. */
556 
557  proc->clogGroupMember = false;
558 
559  if (proc != MyProc)
560  PGSemaphoreUnlock(proc->sem);
561  }
562 
563  return true;
564 }
565 
566 /*
567  * Sets the commit status of a single transaction.
568  *
569  * Must be called with CLogControlLock held
570  */
571 static void
573 {
574  int byteno = TransactionIdToByte(xid);
575  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
576  char *byteptr;
577  char byteval;
578  char curval;
579 
580  byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
581  curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
582 
583  /*
584  * When replaying transactions during recovery we still need to perform
585  * the two phases of subcommit and then commit. However, some transactions
586  * are already correctly marked, so we just treat those as a no-op which
587  * allows us to keep the following Assert as restrictive as possible.
588  */
589  if (InRecovery && status == TRANSACTION_STATUS_SUB_COMMITTED &&
591  return;
592 
593  /*
594  * Current state change should be from 0 or subcommitted to target state
595  * or we should already be there when replaying changes during recovery.
596  */
597  Assert(curval == 0 ||
599  status != TRANSACTION_STATUS_IN_PROGRESS) ||
600  curval == status);
601 
602  /* note this assumes exclusive access to the clog page */
603  byteval = *byteptr;
604  byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
605  byteval |= (status << bshift);
606  *byteptr = byteval;
607 
608  /*
609  * Update the group LSN if the transaction completion LSN is higher.
610  *
611  * Note: lsn will be invalid when supplied during InRecovery processing,
612  * so we don't need to do anything special to avoid LSN updates during
613  * recovery. After recovery completes the next clog change will set the
614  * LSN correctly.
615  */
616  if (!XLogRecPtrIsInvalid(lsn))
617  {
618  int lsnindex = GetLSNIndex(slotno, xid);
619 
620  if (ClogCtl->shared->group_lsn[lsnindex] < lsn)
621  ClogCtl->shared->group_lsn[lsnindex] = lsn;
622  }
623 }
624 
625 /*
626  * Interrogate the state of a transaction in the commit log.
627  *
628  * Aside from the actual commit status, this function returns (into *lsn)
629  * an LSN that is late enough to be able to guarantee that if we flush up to
630  * that LSN then we will have flushed the transaction's commit record to disk.
631  * The result is not necessarily the exact LSN of the transaction's commit
632  * record! For example, for long-past transactions (those whose clog pages
633  * already migrated to disk), we'll return InvalidXLogRecPtr. Also, because
634  * we group transactions on the same clog page to conserve storage, we might
635  * return the LSN of a later transaction that falls into the same group.
636  *
637  * NB: this is a low-level routine and is NOT the preferred entry point
638  * for most uses; TransactionLogFetch() in transam.c is the intended caller.
639  */
640 XidStatus
642 {
643  int pageno = TransactionIdToPage(xid);
644  int byteno = TransactionIdToByte(xid);
645  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
646  int slotno;
647  int lsnindex;
648  char *byteptr;
650 
651  /* lock is acquired by SimpleLruReadPage_ReadOnly */
652 
653  slotno = SimpleLruReadPage_ReadOnly(ClogCtl, pageno, xid);
654  byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
655 
656  status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
657 
658  lsnindex = GetLSNIndex(slotno, xid);
659  *lsn = ClogCtl->shared->group_lsn[lsnindex];
660 
661  LWLockRelease(CLogControlLock);
662 
663  return status;
664 }
665 
666 /*
667  * Number of shared CLOG buffers.
668  *
669  * On larger multi-processor systems, it is possible to have many CLOG page
670  * requests in flight at one time which could lead to disk access for CLOG
671  * page if the required page is not found in memory. Testing revealed that we
672  * can get the best performance by having 128 CLOG buffers, more than that it
673  * doesn't improve performance.
674  *
675  * Unconditionally keeping the number of CLOG buffers to 128 did not seem like
676  * a good idea, because it would increase the minimum amount of shared memory
677  * required to start, which could be a problem for people running very small
678  * configurations. The following formula seems to represent a reasonable
679  * compromise: people with very low values for shared_buffers will get fewer
680  * CLOG buffers as well, and everyone else will get 128.
681  */
682 Size
684 {
685  return Min(128, Max(4, NBuffers / 512));
686 }
687 
688 /*
689  * Initialization of shared memory for CLOG
690  */
691 Size
693 {
695 }
696 
697 void
699 {
700  ClogCtl->PagePrecedes = CLOGPagePrecedes;
702  CLogControlLock, "pg_xact", LWTRANCHE_CLOG_BUFFERS);
703 }
704 
705 /*
706  * This func must be called ONCE on system install. It creates
707  * the initial CLOG segment. (The CLOG directory is assumed to
708  * have been created by initdb, and CLOGShmemInit must have been
709  * called already.)
710  */
711 void
713 {
714  int slotno;
715 
716  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
717 
718  /* Create and zero the first page of the commit log */
719  slotno = ZeroCLOGPage(0, false);
720 
721  /* Make sure it's written out */
722  SimpleLruWritePage(ClogCtl, slotno);
723  Assert(!ClogCtl->shared->page_dirty[slotno]);
724 
725  LWLockRelease(CLogControlLock);
726 }
727 
728 /*
729  * Initialize (or reinitialize) a page of CLOG to zeroes.
730  * If writeXlog is true, also emit an XLOG record saying we did this.
731  *
732  * The page is not actually written, just set up in shared memory.
733  * The slot number of the new page is returned.
734  *
735  * Control lock must be held at entry, and will be held at exit.
736  */
737 static int
738 ZeroCLOGPage(int pageno, bool writeXlog)
739 {
740  int slotno;
741 
742  slotno = SimpleLruZeroPage(ClogCtl, pageno);
743 
744  if (writeXlog)
745  WriteZeroPageXlogRec(pageno);
746 
747  return slotno;
748 }
749 
750 /*
751  * This must be called ONCE during postmaster or standalone-backend startup,
752  * after StartupXLOG has initialized ShmemVariableCache->nextXid.
753  */
754 void
756 {
758  int pageno = TransactionIdToPage(xid);
759 
760  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
761 
762  /*
763  * Initialize our idea of the latest page number.
764  */
765  ClogCtl->shared->latest_page_number = pageno;
766 
767  LWLockRelease(CLogControlLock);
768 }
769 
770 /*
771  * This must be called ONCE at the end of startup/recovery.
772  */
773 void
774 TrimCLOG(void)
775 {
777  int pageno = TransactionIdToPage(xid);
778 
779  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
780 
781  /*
782  * Re-Initialize our idea of the latest page number.
783  */
784  ClogCtl->shared->latest_page_number = pageno;
785 
786  /*
787  * Zero out the remainder of the current clog page. Under normal
788  * circumstances it should be zeroes already, but it seems at least
789  * theoretically possible that XLOG replay will have settled on a nextXID
790  * value that is less than the last XID actually used and marked by the
791  * previous database lifecycle (since subtransaction commit writes clog
792  * but makes no WAL entry). Let's just be safe. (We need not worry about
793  * pages beyond the current one, since those will be zeroed when first
794  * used. For the same reason, there is no need to do anything when
795  * nextXid is exactly at a page boundary; and it's likely that the
796  * "current" page doesn't exist yet in that case.)
797  */
798  if (TransactionIdToPgIndex(xid) != 0)
799  {
800  int byteno = TransactionIdToByte(xid);
801  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
802  int slotno;
803  char *byteptr;
804 
805  slotno = SimpleLruReadPage(ClogCtl, pageno, false, xid);
806  byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
807 
808  /* Zero so-far-unused positions in the current byte */
809  *byteptr &= (1 << bshift) - 1;
810  /* Zero the rest of the page */
811  MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
812 
813  ClogCtl->shared->page_dirty[slotno] = true;
814  }
815 
816  LWLockRelease(CLogControlLock);
817 }
818 
819 /*
820  * This must be called ONCE during postmaster or standalone-backend shutdown
821  */
822 void
824 {
825  /* Flush dirty CLOG pages to disk */
826  TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(false);
827  SimpleLruFlush(ClogCtl, false);
828 
829  /*
830  * fsync pg_xact to ensure that any files flushed previously are durably
831  * on disk.
832  */
833  fsync_fname("pg_xact", true);
834 
835  TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(false);
836 }
837 
838 /*
839  * Perform a checkpoint --- either during shutdown, or on-the-fly
840  */
841 void
843 {
844  /* Flush dirty CLOG pages to disk */
845  TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
846  SimpleLruFlush(ClogCtl, true);
847 
848  /*
849  * fsync pg_xact to ensure that any files flushed previously are durably
850  * on disk.
851  */
852  fsync_fname("pg_xact", true);
853 
854  TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
855 }
856 
857 
858 /*
859  * Make sure that CLOG has room for a newly-allocated XID.
860  *
861  * NB: this is called while holding XidGenLock. We want it to be very fast
862  * most of the time; even when it's not so fast, no actual I/O need happen
863  * unless we're forced to write out a dirty clog or xlog page to make room
864  * in shared memory.
865  */
866 void
868 {
869  int pageno;
870 
871  /*
872  * No work except at first XID of a page. But beware: just after
873  * wraparound, the first XID of page zero is FirstNormalTransactionId.
874  */
875  if (TransactionIdToPgIndex(newestXact) != 0 &&
877  return;
878 
879  pageno = TransactionIdToPage(newestXact);
880 
881  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
882 
883  /* Zero the page and make an XLOG entry about it */
884  ZeroCLOGPage(pageno, true);
885 
886  LWLockRelease(CLogControlLock);
887 }
888 
889 
890 /*
891  * Remove all CLOG segments before the one holding the passed transaction ID
892  *
893  * Before removing any CLOG data, we must flush XLOG to disk, to ensure
894  * that any recently-emitted HEAP_FREEZE records have reached disk; otherwise
895  * a crash and restart might leave us with some unfrozen tuples referencing
896  * removed CLOG data. We choose to emit a special TRUNCATE XLOG record too.
897  * Replaying the deletion from XLOG is not critical, since the files could
898  * just as well be removed later, but doing so prevents a long-running hot
899  * standby server from acquiring an unreasonably bloated CLOG directory.
900  *
901  * Since CLOG segments hold a large number of transactions, the opportunity to
902  * actually remove a segment is fairly rare, and so it seems best not to do
903  * the XLOG flush unless we have confirmed that there is a removable segment.
904  */
905 void
906 TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
907 {
908  int cutoffPage;
909 
910  /*
911  * The cutoff point is the start of the segment containing oldestXact. We
912  * pass the *page* containing oldestXact to SimpleLruTruncate.
913  */
914  cutoffPage = TransactionIdToPage(oldestXact);
915 
916  /* Check to see if there's any files that could be removed */
918  return; /* nothing to remove */
919 
920  /*
921  * Advance oldestClogXid before truncating clog, so concurrent xact status
922  * lookups can ensure they don't attempt to access truncated-away clog.
923  *
924  * It's only necessary to do this if we will actually truncate away clog
925  * pages.
926  */
927  AdvanceOldestClogXid(oldestXact);
928 
929  /*
930  * Write XLOG record and flush XLOG to disk. We record the oldest xid
931  * we're keeping information about here so we can ensure that it's always
932  * ahead of clog truncation in case we crash, and so a standby finds out
933  * the new valid xid before the next checkpoint.
934  */
935  WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
936 
937  /* Now we can remove the old CLOG segment(s) */
938  SimpleLruTruncate(ClogCtl, cutoffPage);
939 }
940 
941 
942 /*
943  * Decide which of two CLOG page numbers is "older" for truncation purposes.
944  *
945  * We need to use comparison of TransactionIds here in order to do the right
946  * thing with wraparound XID arithmetic. However, if we are asked about
947  * page number zero, we don't want to hand InvalidTransactionId to
948  * TransactionIdPrecedes: it'll get weird about permanent xact IDs. So,
949  * offset both xids by FirstNormalTransactionId to avoid that.
950  */
951 static bool
952 CLOGPagePrecedes(int page1, int page2)
953 {
954  TransactionId xid1;
955  TransactionId xid2;
956 
957  xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
958  xid1 += FirstNormalTransactionId;
959  xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
960  xid2 += FirstNormalTransactionId;
961 
962  return TransactionIdPrecedes(xid1, xid2);
963 }
964 
965 
966 /*
967  * Write a ZEROPAGE xlog record
968  */
969 static void
971 {
972  XLogBeginInsert();
973  XLogRegisterData((char *) (&pageno), sizeof(int));
974  (void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE);
975 }
976 
977 /*
978  * Write a TRUNCATE xlog record
979  *
980  * We must flush the xlog record to disk before returning --- see notes
981  * in TruncateCLOG().
982  */
983 static void
984 WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXactDb)
985 {
986  XLogRecPtr recptr;
987  xl_clog_truncate xlrec;
988 
989  xlrec.pageno = pageno;
990  xlrec.oldestXact = oldestXact;
991  xlrec.oldestXactDb = oldestXactDb;
992 
993  XLogBeginInsert();
994  XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
995  recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
996  XLogFlush(recptr);
997 }
998 
999 /*
1000  * CLOG resource manager's routines
1001  */
1002 void
1004 {
1005  uint8 info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
1006 
1007  /* Backup blocks are not used in clog records */
1008  Assert(!XLogRecHasAnyBlockRefs(record));
1009 
1010  if (info == CLOG_ZEROPAGE)
1011  {
1012  int pageno;
1013  int slotno;
1014 
1015  memcpy(&pageno, XLogRecGetData(record), sizeof(int));
1016 
1017  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
1018 
1019  slotno = ZeroCLOGPage(pageno, false);
1020  SimpleLruWritePage(ClogCtl, slotno);
1021  Assert(!ClogCtl->shared->page_dirty[slotno]);
1022 
1023  LWLockRelease(CLogControlLock);
1024  }
1025  else if (info == CLOG_TRUNCATE)
1026  {
1027  xl_clog_truncate xlrec;
1028 
1029  memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
1030 
1031  /*
1032  * During XLOG replay, latest_page_number isn't set up yet; insert a
1033  * suitable value to bypass the sanity test in SimpleLruTruncate.
1034  */
1035  ClogCtl->shared->latest_page_number = xlrec.pageno;
1036 
1038 
1040  }
1041  else
1042  elog(PANIC, "clog_redo: unknown op code %u", info);
1043 }
#define TransactionIdToBIndex(xid)
Definition: clog.c:67
#define CLOG_XACT_BITMASK
Definition: clog.c:62
#define CLOG_XACTS_PER_PAGE
Definition: clog.c:61
#define TransactionIdToPage(xid)
Definition: clog.c:64
XidStatus clogGroupMemberXidStatus
Definition: proc.h:179
bool LWLockHeldByMeInMode(LWLock *l, LWLockMode mode)
Definition: lwlock.c:1855
static void WriteZeroPageXlogRec(int pageno)
Definition: clog.c:970
void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
Definition: clog.c:906
#define TransactionIdEquals(id1, id2)
Definition: transam.h:43
#define TRANSACTION_STATUS_COMMITTED
Definition: clog.h:27
uint32 TransactionId
Definition: c.h:445
#define GetLSNIndex(slotno, xid)
Definition: clog.c:73
void PGSemaphoreUnlock(PGSemaphore sema)
Definition: posix_sema.c:323
Definition: proc.h:219
void clog_redo(XLogReaderState *record)
Definition: clog.c:1003
static bool TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status, XLogRecPtr lsn, int pageno)
Definition: clog.c:420
XLogRecPtr clogGroupMemberLsn
Definition: proc.h:183
static bool pg_atomic_compare_exchange_u32(volatile pg_atomic_uint32 *ptr, uint32 *expected, uint32 newval)
Definition: atomics.h:322
PGXACT * allPgXact
Definition: proc.h:246
PGPROC * MyProc
Definition: proc.c:67
static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status, XLogRecPtr lsn, int slotno)
Definition: clog.c:572
TransactionId xid
Definition: proc.h:221
void SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
Definition: slru.c:1168
void ExtendCLOG(TransactionId newestXact)
Definition: clog.c:867
bool InRecovery
Definition: xlog.c:194
#define Min(x, y)
Definition: c.h:802
#define TransactionIdToPgIndex(xid)
Definition: clog.c:65
void ShutdownCLOG(void)
Definition: clog.c:823
void fsync_fname(const char *fname, bool isdir)
Definition: fd.c:572
unsigned char uint8
Definition: c.h:294
int XidStatus
Definition: clog.h:24
PROC_HDR * ProcGlobal
Definition: proc.c:80
#define MemSet(start, val, len)
Definition: c.h:853
pg_atomic_uint32 clogGroupNext
Definition: proc.h:177
unsigned int Oid
Definition: postgres_ext.h:31
static SlruCtlData ClogCtlData
Definition: clog.c:86
#define PANIC
Definition: elog.h:53
Size SimpleLruShmemSize(int nslots, int nlsns)
Definition: slru.c:145
void SimpleLruFlush(SlruCtl ctl, bool allow_redirtied)
Definition: slru.c:1103
void XLogFlush(XLogRecPtr record)
Definition: xlog.c:2763
PGXACT * MyPgXact
Definition: proc.c:68
void LWLockRelease(LWLock *lock)
Definition: lwlock.c:1721
static uint32 pg_atomic_exchange_u32(volatile pg_atomic_uint32 *ptr, uint32 newval)
Definition: atomics.h:303
#define ClogCtl
Definition: clog.c:88
#define CLOG_LSNS_PER_PAGE
Definition: clog.c:71
#define StaticAssertStmt(condition, errmessage)
Definition: c.h:753
#define CLOG_ZEROPAGE
Definition: clog.h:54
void AdvanceOldestClogXid(TransactionId oldest_datfrozenxid)
Definition: varsup.c:271
void TransactionIdSetTreeStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
Definition: clog.c:163
#define XLogRecGetData(decoder)
Definition: xlogreader.h:226
#define FirstNormalTransactionId
Definition: transam.h:34
TransactionId nextXid
Definition: transam.h:117
int SimpleLruReadPage(SlruCtl ctl, int pageno, bool write_ok, TransactionId xid)
Definition: slru.c:375
#define TRANSACTION_STATUS_ABORTED
Definition: clog.h:28
#define THRESHOLD_SUBTRANS_CLOG_OPT
Definition: clog.c:81
static bool CLOGPagePrecedes(int page1, int page2)
Definition: clog.c:952
bool LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1289
int clogGroupMemberPage
Definition: proc.h:181
uint8 nxids
Definition: proc.h:235
VariableCache ShmemVariableCache
Definition: varsup.c:34
Size CLOGShmemBuffers(void)
Definition: clog.c:683
#define TRANSACTION_STATUS_SUB_COMMITTED
Definition: clog.h:29
#define InvalidTransactionId
Definition: transam.h:31
TransactionId clogGroupMemberXid
Definition: proc.h:178
unsigned int uint32
Definition: c.h:296
#define CLOG_BITS_PER_XACT
Definition: clog.c:59
void SimpleLruWritePage(SlruCtl ctl, int slotno)
Definition: slru.c:578
int pageno
Definition: clog.h:33
static void pgstat_report_wait_end(void)
Definition: pgstat.h:1244
struct XidCache subxids
Definition: proc.h:159
void CheckPointCLOG(void)
Definition: clog.c:842
#define CLOG_TRUNCATE
Definition: clog.h:55
#define XLogRecGetInfo(decoder)
Definition: xlogreader.h:222
#define INVALID_PGPROCNO
Definition: proc.h:77
bool TransactionIdPrecedes(TransactionId id1, TransactionId id2)
Definition: transam.c:300
Definition: proc.h:241
#define XLogRecPtrIsInvalid(r)
Definition: xlogdefs.h:29
void XLogRegisterData(char *data, int len)
Definition: xloginsert.c:323
TransactionId oldestXact
Definition: clog.h:34
XLogRecPtr XLogInsert(RmgrId rmid, uint8 info)
Definition: xloginsert.c:415
bool SlruScanDirCbReportPresence(SlruCtl ctl, char *filename, int segpage, void *data)
Definition: slru.c:1321
int SimpleLruReadPage_ReadOnly(SlruCtl ctl, int pageno, TransactionId xid)
Definition: slru.c:467
bool overflowed
Definition: proc.h:231
Size CLOGShmemSize(void)
Definition: clog.c:692
static void set_status_by_pages(int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
Definition: clog.c:237
#define PGPROC_MAX_CACHED_SUBXIDS
Definition: proc.h:36
void BootStrapCLOG(void)
Definition: clog.c:712
#define Max(x, y)
Definition: c.h:796
TransactionId xids[PGPROC_MAX_CACHED_SUBXIDS]
Definition: proc.h:40
uint64 XLogRecPtr
Definition: xlogdefs.h:21
#define Assert(condition)
Definition: c.h:670
#define XLR_INFO_MASK
Definition: xlogrecord.h:62
bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
Definition: slru.c:1376
size_t Size
Definition: c.h:404
static void pgstat_report_wait_start(uint32 wait_event_info)
Definition: pgstat.h:1220
static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int pageno, bool all_xact_same_page)
Definition: clog.c:273
bool LWLockAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1117
void StartupCLOG(void)
Definition: clog.c:755
void PGSemaphoreLock(PGSemaphore sema)
Definition: posix_sema.c:303
XidStatus TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
Definition: clog.c:641
pg_atomic_uint32 clogGroupFirst
Definition: proc.h:258
#define TransactionIdToByte(xid)
Definition: clog.c:66
int pgprocno
Definition: proc.h:110
static int ZeroCLOGPage(int pageno, bool writeXlog)
Definition: clog.c:738
bool clogGroupMember
Definition: proc.h:176
int i
void CLOGShmemInit(void)
Definition: clog.c:698
#define pg_write_barrier()
Definition: atomics.h:162
int NBuffers
Definition: globals.c:122
Oid oldestXactDb
Definition: clog.h:35
static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int pageno)
Definition: clog.c:345
#define XLogRecHasAnyBlockRefs(decoder)
Definition: xlogreader.h:228
PGPROC * allProcs
Definition: proc.h:244
#define elog
Definition: elog.h:219
#define TransactionIdIsValid(xid)
Definition: transam.h:41
static void pg_atomic_write_u32(volatile pg_atomic_uint32 *ptr, uint32 val)
Definition: atomics.h:269
PGSemaphore sem
Definition: proc.h:101
static void WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXidDb)
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static void static void status(const char *fmt,...) pg_attribute_printf(1
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void XLogBeginInsert(void)
Definition: xloginsert.c:120
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Definition: slru.c:263
void TrimCLOG(void)
Definition: clog.c:774
#define TRANSACTION_STATUS_IN_PROGRESS
Definition: clog.h:26
static uint32 pg_atomic_read_u32(volatile pg_atomic_uint32 *ptr)
Definition: atomics.h:252
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Definition: slru.c:165