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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  while (i < nsubxids)
245  {
246  int num_on_page = 0;
247 
248  while (TransactionIdToPage(subxids[i]) == pageno && i < nsubxids)
249  {
250  num_on_page++;
251  i++;
252  }
253 
255  num_on_page, subxids + offset,
256  status, lsn, pageno, false);
257  offset = i;
258  pageno = TransactionIdToPage(subxids[offset]);
259  }
260 }
261 
262 /*
263  * Record the final state of transaction entries in the commit log for all
264  * entries on a single page. Atomic only on this page.
265  */
266 static void
268  TransactionId *subxids, XidStatus status,
269  XLogRecPtr lsn, int pageno,
270  bool all_xact_same_page)
271 {
272  /* Can't use group update when PGPROC overflows. */
274  "group clog threshold less than PGPROC cached subxids");
275 
276  /*
277  * When there is contention on CLogControlLock, we try to group multiple
278  * updates; a single leader process will perform transaction status
279  * updates for multiple backends so that the number of times
280  * CLogControlLock needs to be acquired is reduced.
281  *
282  * For this optimization to be safe, the XID in MyPgXact and the subxids
283  * in MyProc must be the same as the ones for which we're setting the
284  * status. Check that this is the case.
285  *
286  * For this optimization to be efficient, we shouldn't have too many
287  * sub-XIDs and all of the XIDs for which we're adjusting clog should be
288  * on the same page. Check those conditions, too.
289  */
290  if (all_xact_same_page && xid == MyPgXact->xid &&
291  nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
292  nsubxids == MyPgXact->nxids &&
293  memcmp(subxids, MyProc->subxids.xids,
294  nsubxids * sizeof(TransactionId)) == 0)
295  {
296  /*
297  * We don't try to do group update optimization if a process has
298  * overflowed the subxids array in its PGPROC, since in that case we
299  * don't have a complete list of XIDs for it.
300  */
302 
303  /*
304  * If we can immediately acquire CLogControlLock, we update the status
305  * of our own XID and release the lock. If not, try use group XID
306  * update. If that doesn't work out, fall back to waiting for the
307  * lock to perform an update for this transaction only.
308  */
309  if (LWLockConditionalAcquire(CLogControlLock, LW_EXCLUSIVE))
310  {
311  /* Got the lock without waiting! Do the update. */
312  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
313  lsn, pageno);
314  LWLockRelease(CLogControlLock);
315  return;
316  }
317  else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
318  {
319  /* Group update mechanism has done the work. */
320  return;
321  }
322 
323  /* Fall through only if update isn't done yet. */
324  }
325 
326  /* Group update not applicable, or couldn't accept this page number. */
327  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
328  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
329  lsn, pageno);
330  LWLockRelease(CLogControlLock);
331 }
332 
333 /*
334  * Record the final state of transaction entry in the commit log
335  *
336  * We don't do any locking here; caller must handle that.
337  */
338 static void
340  TransactionId *subxids, XidStatus status,
341  XLogRecPtr lsn, int pageno)
342 {
343  int slotno;
344  int i;
345 
347  status == TRANSACTION_STATUS_ABORTED ||
349  Assert(LWLockHeldByMeInMode(CLogControlLock, LW_EXCLUSIVE));
350 
351  /*
352  * If we're doing an async commit (ie, lsn is valid), then we must wait
353  * for any active write on the page slot to complete. Otherwise our
354  * update could reach disk in that write, which will not do since we
355  * mustn't let it reach disk until we've done the appropriate WAL flush.
356  * But when lsn is invalid, it's OK to scribble on a page while it is
357  * write-busy, since we don't care if the update reaches disk sooner than
358  * we think.
359  */
360  slotno = SimpleLruReadPage(ClogCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
361 
362  /*
363  * Set the main transaction id, if any.
364  *
365  * If we update more than one xid on this page while it is being written
366  * out, we might find that some of the bits go to disk and others don't.
367  * If we are updating commits on the page with the top-level xid that
368  * could break atomicity, so we subcommit the subxids first before we mark
369  * the top-level commit.
370  */
371  if (TransactionIdIsValid(xid))
372  {
373  /* Subtransactions first, if needed ... */
374  if (status == TRANSACTION_STATUS_COMMITTED)
375  {
376  for (i = 0; i < nsubxids; i++)
377  {
378  Assert(ClogCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
379  TransactionIdSetStatusBit(subxids[i],
381  lsn, slotno);
382  }
383  }
384 
385  /* ... then the main transaction */
386  TransactionIdSetStatusBit(xid, status, lsn, slotno);
387  }
388 
389  /* Set the subtransactions */
390  for (i = 0; i < nsubxids; i++)
391  {
392  Assert(ClogCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
393  TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
394  }
395 
396  ClogCtl->shared->page_dirty[slotno] = true;
397 }
398 
399 /*
400  * When we cannot immediately acquire CLogControlLock in exclusive mode at
401  * commit time, add ourselves to a list of processes that need their XIDs
402  * status update. The first process to add itself to the list will acquire
403  * CLogControlLock in exclusive mode and set transaction status as required
404  * on behalf of all group members. This avoids a great deal of contention
405  * around CLogControlLock when many processes are trying to commit at once,
406  * since the lock need not be repeatedly handed off from one committing
407  * process to the next.
408  *
409  * Returns true when transaction status has been updated in clog; returns
410  * false if we decided against applying the optimization because the page
411  * number we need to update differs from those processes already waiting.
412  */
413 static bool
415  XLogRecPtr lsn, int pageno)
416 {
417  volatile PROC_HDR *procglobal = ProcGlobal;
418  PGPROC *proc = MyProc;
419  uint32 nextidx;
420  uint32 wakeidx;
421 
422  /* We should definitely have an XID whose status needs to be updated. */
424 
425  /*
426  * Add ourselves to the list of processes needing a group XID status
427  * update.
428  */
429  proc->clogGroupMember = true;
430  proc->clogGroupMemberXid = xid;
432  proc->clogGroupMemberPage = pageno;
433  proc->clogGroupMemberLsn = lsn;
434 
435  nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
436 
437  while (true)
438  {
439  /*
440  * Add the proc to list, if the clog page where we need to update the
441  * current transaction status is same as group leader's clog page.
442  *
443  * There is a race condition here, which is that after doing the below
444  * check and before adding this proc's clog update to a group, the
445  * group leader might have already finished the group update for this
446  * page and becomes group leader of another group. This will lead to a
447  * situation where a single group can have different clog page
448  * updates. This isn't likely and will still work, just maybe a bit
449  * less efficiently.
450  */
451  if (nextidx != INVALID_PGPROCNO &&
453  {
454  proc->clogGroupMember = false;
455  return false;
456  }
457 
458  pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
459 
461  &nextidx,
462  (uint32) proc->pgprocno))
463  break;
464  }
465 
466  /*
467  * If the list was not empty, the leader will update the status of our
468  * XID. It is impossible to have followers without a leader because the
469  * first process that has added itself to the list will always have
470  * nextidx as INVALID_PGPROCNO.
471  */
472  if (nextidx != INVALID_PGPROCNO)
473  {
474  int extraWaits = 0;
475 
476  /* Sleep until the leader updates our XID status. */
478  for (;;)
479  {
480  /* acts as a read barrier */
481  PGSemaphoreLock(proc->sem);
482  if (!proc->clogGroupMember)
483  break;
484  extraWaits++;
485  }
487 
489 
490  /* Fix semaphore count for any absorbed wakeups */
491  while (extraWaits-- > 0)
492  PGSemaphoreUnlock(proc->sem);
493  return true;
494  }
495 
496  /* We are the leader. Acquire the lock on behalf of everyone. */
497  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
498 
499  /*
500  * Now that we've got the lock, clear the list of processes waiting for
501  * group XID status update, saving a pointer to the head of the list.
502  * Trying to pop elements one at a time could lead to an ABA problem.
503  */
504  nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
506 
507  /* Remember head of list so we can perform wakeups after dropping lock. */
508  wakeidx = nextidx;
509 
510  /* Walk the list and update the status of all XIDs. */
511  while (nextidx != INVALID_PGPROCNO)
512  {
513  PGPROC *proc = &ProcGlobal->allProcs[nextidx];
514  PGXACT *pgxact = &ProcGlobal->allPgXact[nextidx];
515 
516  /*
517  * Overflowed transactions should not use group XID status update
518  * mechanism.
519  */
520  Assert(!pgxact->overflowed);
521 
523  pgxact->nxids,
524  proc->subxids.xids,
526  proc->clogGroupMemberLsn,
527  proc->clogGroupMemberPage);
528 
529  /* Move to next proc in list. */
530  nextidx = pg_atomic_read_u32(&proc->clogGroupNext);
531  }
532 
533  /* We're done with the lock now. */
534  LWLockRelease(CLogControlLock);
535 
536  /*
537  * Now that we've released the lock, go back and wake everybody up. We
538  * don't do this under the lock so as to keep lock hold times to a
539  * minimum.
540  */
541  while (wakeidx != INVALID_PGPROCNO)
542  {
543  PGPROC *proc = &ProcGlobal->allProcs[wakeidx];
544 
545  wakeidx = pg_atomic_read_u32(&proc->clogGroupNext);
547 
548  /* ensure all previous writes are visible before follower continues. */
550 
551  proc->clogGroupMember = false;
552 
553  if (proc != MyProc)
554  PGSemaphoreUnlock(proc->sem);
555  }
556 
557  return true;
558 }
559 
560 /*
561  * Sets the commit status of a single transaction.
562  *
563  * Must be called with CLogControlLock held
564  */
565 static void
567 {
568  int byteno = TransactionIdToByte(xid);
569  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
570  char *byteptr;
571  char byteval;
572  char curval;
573 
574  byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
575  curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
576 
577  /*
578  * When replaying transactions during recovery we still need to perform
579  * the two phases of subcommit and then commit. However, some transactions
580  * are already correctly marked, so we just treat those as a no-op which
581  * allows us to keep the following Assert as restrictive as possible.
582  */
583  if (InRecovery && status == TRANSACTION_STATUS_SUB_COMMITTED &&
585  return;
586 
587  /*
588  * Current state change should be from 0 or subcommitted to target state
589  * or we should already be there when replaying changes during recovery.
590  */
591  Assert(curval == 0 ||
593  status != TRANSACTION_STATUS_IN_PROGRESS) ||
594  curval == status);
595 
596  /* note this assumes exclusive access to the clog page */
597  byteval = *byteptr;
598  byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
599  byteval |= (status << bshift);
600  *byteptr = byteval;
601 
602  /*
603  * Update the group LSN if the transaction completion LSN is higher.
604  *
605  * Note: lsn will be invalid when supplied during InRecovery processing,
606  * so we don't need to do anything special to avoid LSN updates during
607  * recovery. After recovery completes the next clog change will set the
608  * LSN correctly.
609  */
610  if (!XLogRecPtrIsInvalid(lsn))
611  {
612  int lsnindex = GetLSNIndex(slotno, xid);
613 
614  if (ClogCtl->shared->group_lsn[lsnindex] < lsn)
615  ClogCtl->shared->group_lsn[lsnindex] = lsn;
616  }
617 }
618 
619 /*
620  * Interrogate the state of a transaction in the commit log.
621  *
622  * Aside from the actual commit status, this function returns (into *lsn)
623  * an LSN that is late enough to be able to guarantee that if we flush up to
624  * that LSN then we will have flushed the transaction's commit record to disk.
625  * The result is not necessarily the exact LSN of the transaction's commit
626  * record! For example, for long-past transactions (those whose clog pages
627  * already migrated to disk), we'll return InvalidXLogRecPtr. Also, because
628  * we group transactions on the same clog page to conserve storage, we might
629  * return the LSN of a later transaction that falls into the same group.
630  *
631  * NB: this is a low-level routine and is NOT the preferred entry point
632  * for most uses; TransactionLogFetch() in transam.c is the intended caller.
633  */
634 XidStatus
636 {
637  int pageno = TransactionIdToPage(xid);
638  int byteno = TransactionIdToByte(xid);
639  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
640  int slotno;
641  int lsnindex;
642  char *byteptr;
644 
645  /* lock is acquired by SimpleLruReadPage_ReadOnly */
646 
647  slotno = SimpleLruReadPage_ReadOnly(ClogCtl, pageno, xid);
648  byteptr = ClogCtl->shared->page_buffer[slotno] + byteno;
649 
650  status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
651 
652  lsnindex = GetLSNIndex(slotno, xid);
653  *lsn = ClogCtl->shared->group_lsn[lsnindex];
654 
655  LWLockRelease(CLogControlLock);
656 
657  return status;
658 }
659 
660 /*
661  * Number of shared CLOG buffers.
662  *
663  * On larger multi-processor systems, it is possible to have many CLOG page
664  * requests in flight at one time which could lead to disk access for CLOG
665  * page if the required page is not found in memory. Testing revealed that we
666  * can get the best performance by having 128 CLOG buffers, more than that it
667  * doesn't improve performance.
668  *
669  * Unconditionally keeping the number of CLOG buffers to 128 did not seem like
670  * a good idea, because it would increase the minimum amount of shared memory
671  * required to start, which could be a problem for people running very small
672  * configurations. The following formula seems to represent a reasonable
673  * compromise: people with very low values for shared_buffers will get fewer
674  * CLOG buffers as well, and everyone else will get 128.
675  */
676 Size
678 {
679  return Min(128, Max(4, NBuffers / 512));
680 }
681 
682 /*
683  * Initialization of shared memory for CLOG
684  */
685 Size
687 {
689 }
690 
691 void
693 {
694  ClogCtl->PagePrecedes = CLOGPagePrecedes;
696  CLogControlLock, "pg_xact", LWTRANCHE_CLOG_BUFFERS);
697 }
698 
699 /*
700  * This func must be called ONCE on system install. It creates
701  * the initial CLOG segment. (The CLOG directory is assumed to
702  * have been created by initdb, and CLOGShmemInit must have been
703  * called already.)
704  */
705 void
707 {
708  int slotno;
709 
710  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
711 
712  /* Create and zero the first page of the commit log */
713  slotno = ZeroCLOGPage(0, false);
714 
715  /* Make sure it's written out */
716  SimpleLruWritePage(ClogCtl, slotno);
717  Assert(!ClogCtl->shared->page_dirty[slotno]);
718 
719  LWLockRelease(CLogControlLock);
720 }
721 
722 /*
723  * Initialize (or reinitialize) a page of CLOG to zeroes.
724  * If writeXlog is TRUE, also emit an XLOG record saying we did this.
725  *
726  * The page is not actually written, just set up in shared memory.
727  * The slot number of the new page is returned.
728  *
729  * Control lock must be held at entry, and will be held at exit.
730  */
731 static int
732 ZeroCLOGPage(int pageno, bool writeXlog)
733 {
734  int slotno;
735 
736  slotno = SimpleLruZeroPage(ClogCtl, pageno);
737 
738  if (writeXlog)
739  WriteZeroPageXlogRec(pageno);
740 
741  return slotno;
742 }
743 
744 /*
745  * This must be called ONCE during postmaster or standalone-backend startup,
746  * after StartupXLOG has initialized ShmemVariableCache->nextXid.
747  */
748 void
750 {
752  int pageno = TransactionIdToPage(xid);
753 
754  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
755 
756  /*
757  * Initialize our idea of the latest page number.
758  */
759  ClogCtl->shared->latest_page_number = pageno;
760 
761  LWLockRelease(CLogControlLock);
762 }
763 
764 /*
765  * This must be called ONCE at the end of startup/recovery.
766  */
767 void
768 TrimCLOG(void)
769 {
771  int pageno = TransactionIdToPage(xid);
772 
773  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
774 
775  /*
776  * Re-Initialize our idea of the latest page number.
777  */
778  ClogCtl->shared->latest_page_number = pageno;
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(ClogCtl, pageno, false, xid);
800  byteptr = ClogCtl->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  ClogCtl->shared->page_dirty[slotno] = true;
808  }
809 
810  LWLockRelease(CLogControlLock);
811 }
812 
813 /*
814  * This must be called ONCE during postmaster or standalone-backend shutdown
815  */
816 void
818 {
819  /* Flush dirty CLOG pages to disk */
820  TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(false);
821  SimpleLruFlush(ClogCtl, false);
822 
823  /*
824  * fsync pg_xact to ensure that any files flushed previously are durably
825  * on disk.
826  */
827  fsync_fname("pg_xact", true);
828 
829  TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(false);
830 }
831 
832 /*
833  * Perform a checkpoint --- either during shutdown, or on-the-fly
834  */
835 void
837 {
838  /* Flush dirty CLOG pages to disk */
839  TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
840  SimpleLruFlush(ClogCtl, true);
841 
842  /*
843  * fsync pg_xact to ensure that any files flushed previously are durably
844  * on disk.
845  */
846  fsync_fname("pg_xact", true);
847 
848  TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
849 }
850 
851 
852 /*
853  * Make sure that CLOG has room for a newly-allocated XID.
854  *
855  * NB: this is called while holding XidGenLock. We want it to be very fast
856  * most of the time; even when it's not so fast, no actual I/O need happen
857  * unless we're forced to write out a dirty clog or xlog page to make room
858  * in shared memory.
859  */
860 void
862 {
863  int pageno;
864 
865  /*
866  * No work except at first XID of a page. But beware: just after
867  * wraparound, the first XID of page zero is FirstNormalTransactionId.
868  */
869  if (TransactionIdToPgIndex(newestXact) != 0 &&
871  return;
872 
873  pageno = TransactionIdToPage(newestXact);
874 
875  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
876 
877  /* Zero the page and make an XLOG entry about it */
878  ZeroCLOGPage(pageno, true);
879 
880  LWLockRelease(CLogControlLock);
881 }
882 
883 
884 /*
885  * Remove all CLOG segments before the one holding the passed transaction ID
886  *
887  * Before removing any CLOG data, we must flush XLOG to disk, to ensure
888  * that any recently-emitted HEAP_FREEZE records have reached disk; otherwise
889  * a crash and restart might leave us with some unfrozen tuples referencing
890  * removed CLOG data. We choose to emit a special TRUNCATE XLOG record too.
891  * Replaying the deletion from XLOG is not critical, since the files could
892  * just as well be removed later, but doing so prevents a long-running hot
893  * standby server from acquiring an unreasonably bloated CLOG directory.
894  *
895  * Since CLOG segments hold a large number of transactions, the opportunity to
896  * actually remove a segment is fairly rare, and so it seems best not to do
897  * the XLOG flush unless we have confirmed that there is a removable segment.
898  */
899 void
900 TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
901 {
902  int cutoffPage;
903 
904  /*
905  * The cutoff point is the start of the segment containing oldestXact. We
906  * pass the *page* containing oldestXact to SimpleLruTruncate.
907  */
908  cutoffPage = TransactionIdToPage(oldestXact);
909 
910  /* Check to see if there's any files that could be removed */
912  return; /* nothing to remove */
913 
914  /*
915  * Advance oldestClogXid before truncating clog, so concurrent xact status
916  * lookups can ensure they don't attempt to access truncated-away clog.
917  *
918  * It's only necessary to do this if we will actually truncate away clog
919  * pages.
920  */
921  AdvanceOldestClogXid(oldestXact);
922 
923  /*
924  * Write XLOG record and flush XLOG to disk. We record the oldest xid
925  * we're keeping information about here so we can ensure that it's always
926  * ahead of clog truncation in case we crash, and so a standby finds out
927  * the new valid xid before the next checkpoint.
928  */
929  WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
930 
931  /* Now we can remove the old CLOG segment(s) */
932  SimpleLruTruncate(ClogCtl, cutoffPage);
933 }
934 
935 
936 /*
937  * Decide which of two CLOG page numbers is "older" for truncation purposes.
938  *
939  * We need to use comparison of TransactionIds here in order to do the right
940  * thing with wraparound XID arithmetic. However, if we are asked about
941  * page number zero, we don't want to hand InvalidTransactionId to
942  * TransactionIdPrecedes: it'll get weird about permanent xact IDs. So,
943  * offset both xids by FirstNormalTransactionId to avoid that.
944  */
945 static bool
946 CLOGPagePrecedes(int page1, int page2)
947 {
948  TransactionId xid1;
949  TransactionId xid2;
950 
951  xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
952  xid1 += FirstNormalTransactionId;
953  xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
954  xid2 += FirstNormalTransactionId;
955 
956  return TransactionIdPrecedes(xid1, xid2);
957 }
958 
959 
960 /*
961  * Write a ZEROPAGE xlog record
962  */
963 static void
965 {
966  XLogBeginInsert();
967  XLogRegisterData((char *) (&pageno), sizeof(int));
968  (void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE);
969 }
970 
971 /*
972  * Write a TRUNCATE xlog record
973  *
974  * We must flush the xlog record to disk before returning --- see notes
975  * in TruncateCLOG().
976  */
977 static void
978 WriteTruncateXlogRec(int pageno, TransactionId oldestXact, Oid oldestXactDb)
979 {
980  XLogRecPtr recptr;
981  xl_clog_truncate xlrec;
982 
983  xlrec.pageno = pageno;
984  xlrec.oldestXact = oldestXact;
985  xlrec.oldestXactDb = oldestXactDb;
986 
987  XLogBeginInsert();
988  XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
989  recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
990  XLogFlush(recptr);
991 }
992 
993 /*
994  * CLOG resource manager's routines
995  */
996 void
998 {
999  uint8 info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
1000 
1001  /* Backup blocks are not used in clog records */
1002  Assert(!XLogRecHasAnyBlockRefs(record));
1003 
1004  if (info == CLOG_ZEROPAGE)
1005  {
1006  int pageno;
1007  int slotno;
1008 
1009  memcpy(&pageno, XLogRecGetData(record), sizeof(int));
1010 
1011  LWLockAcquire(CLogControlLock, LW_EXCLUSIVE);
1012 
1013  slotno = ZeroCLOGPage(pageno, false);
1014  SimpleLruWritePage(ClogCtl, slotno);
1015  Assert(!ClogCtl->shared->page_dirty[slotno]);
1016 
1017  LWLockRelease(CLogControlLock);
1018  }
1019  else if (info == CLOG_TRUNCATE)
1020  {
1021  xl_clog_truncate xlrec;
1022 
1023  memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
1024 
1025  /*
1026  * During XLOG replay, latest_page_number isn't set up yet; insert a
1027  * suitable value to bypass the sanity test in SimpleLruTruncate.
1028  */
1029  ClogCtl->shared->latest_page_number = xlrec.pageno;
1030 
1032 
1034  }
1035  else
1036  elog(PANIC, "clog_redo: unknown op code %u", info);
1037 }
#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:964
void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
Definition: clog.c:900
#define TransactionIdEquals(id1, id2)
Definition: transam.h:43
#define TRANSACTION_STATUS_COMMITTED
Definition: clog.h:27
uint32 TransactionId
Definition: c.h:391
#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:997
static bool TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status, XLogRecPtr lsn, int pageno)
Definition: clog.c:414
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:566
TransactionId xid
Definition: proc.h:221
void SimpleLruTruncate(SlruCtl ctl, int cutoffPage)
Definition: slru.c:1169
void ExtendCLOG(TransactionId newestXact)
Definition: clog.c:861
bool InRecovery
Definition: xlog.c:192
#define Min(x, y)
Definition: c.h:795
#define TransactionIdToPgIndex(xid)
Definition: clog.c:65
void ShutdownCLOG(void)
Definition: clog.c:817
void fsync_fname(const char *fname, bool isdir)
Definition: fd.c:567
unsigned char uint8
Definition: c.h:256
int XidStatus
Definition: clog.h:24
PROC_HDR * ProcGlobal
Definition: proc.c:80
#define MemSet(start, val, len)
Definition: c.h:846
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:1104
void XLogFlush(XLogRecPtr record)
Definition: xlog.c:2757
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:746
#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:220
#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:946
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:677
#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:258
#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:1236
struct XidCache subxids
Definition: proc.h:159
void CheckPointCLOG(void)
Definition: clog.c:836
#define CLOG_TRUNCATE
Definition: clog.h:55
#define XLogRecGetInfo(decoder)
Definition: xlogreader.h:216
#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:1322
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:686
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:706
#define Max(x, y)
Definition: c.h:789
TransactionId xids[PGPROC_MAX_CACHED_SUBXIDS]
Definition: proc.h:40
uint64 XLogRecPtr
Definition: xlogdefs.h:21
#define Assert(condition)
Definition: c.h:664
#define XLR_INFO_MASK
Definition: xlogrecord.h:62
bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
Definition: slru.c:1377
size_t Size
Definition: c.h:350
static void pgstat_report_wait_start(uint32 wait_event_info)
Definition: pgstat.h:1212
static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int pageno, bool all_xact_same_page)
Definition: clog.c:267
bool LWLockAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1117
void StartupCLOG(void)
Definition: clog.c:749
void PGSemaphoreLock(PGSemaphore sema)
Definition: posix_sema.c:303
XidStatus TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
Definition: clog.c:635
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:732
bool clogGroupMember
Definition: proc.h:176
int i
void CLOGShmemInit(void)
Definition: clog.c:692
#define pg_write_barrier()
Definition: atomics.h:162
int NBuffers
Definition: globals.c:123
Oid oldestXactDb
Definition: clog.h:35
static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int pageno)
Definition: clog.c:339
#define XLogRecHasAnyBlockRefs(decoder)
Definition: xlogreader.h:222
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)
Definition: clog.c:978
static void static void status(const char *fmt,...) pg_attribute_printf(1
Definition: pg_regress.c:224
void XLogBeginInsert(void)
Definition: xloginsert.c:120
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int SimpleLruZeroPage(SlruCtl ctl, int pageno)
Definition: slru.c:263
void TrimCLOG(void)
Definition: clog.c:768
#define TRANSACTION_STATUS_IN_PROGRESS
Definition: clog.h:26
static uint32 pg_atomic_read_u32(volatile pg_atomic_uint32 *ptr)
Definition: atomics.h:252
void SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns, LWLock *ctllock, const char *subdir, int tranche_id)
Definition: slru.c:165