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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 stores two bits per transaction regarding its commit/abort
7  * status; the status for four transactions fit in a byte.
8  *
9  * This would be a pretty simple abstraction on top of slru.c, except that
10  * for performance reasons we allow multiple transactions that are
11  * committing concurrently to form a queue, so that a single process can
12  * update the status for all of them within a single lock acquisition run.
13  *
14  * XLOG interactions: this module generates an XLOG record whenever a new
15  * CLOG page is initialized to zeroes. Other writes of CLOG come from
16  * recording of transaction commit or abort in xact.c, which generates its
17  * own XLOG records for these events and will re-perform the status update
18  * on redo; so we need make no additional XLOG entry here. For synchronous
19  * transaction commits, the XLOG is guaranteed flushed through the XLOG commit
20  * record before we are called to log a commit, so the WAL rule "write xlog
21  * before data" is satisfied automatically. However, for async commits we
22  * must track the latest LSN affecting each CLOG page, so that we can flush
23  * XLOG that far and satisfy the WAL rule. We don't have to worry about this
24  * for aborts (whether sync or async), since the post-crash assumption would
25  * be that such transactions failed anyway.
26  *
27  * Portions Copyright (c) 1996-2024, PostgreSQL Global Development Group
28  * Portions Copyright (c) 1994, Regents of the University of California
29  *
30  * src/backend/access/transam/clog.c
31  *
32  *-------------------------------------------------------------------------
33  */
34 #include "postgres.h"
35 
36 #include "access/clog.h"
37 #include "access/slru.h"
38 #include "access/transam.h"
39 #include "access/xlog.h"
40 #include "access/xloginsert.h"
41 #include "access/xlogutils.h"
42 #include "miscadmin.h"
43 #include "pg_trace.h"
44 #include "pgstat.h"
45 #include "storage/proc.h"
46 #include "storage/sync.h"
47 #include "utils/guc_hooks.h"
48 
49 /*
50  * Defines for CLOG page sizes. A page is the same BLCKSZ as is used
51  * everywhere else in Postgres.
52  *
53  * Note: because TransactionIds are 32 bits and wrap around at 0xFFFFFFFF,
54  * CLOG page numbering also wraps around at 0xFFFFFFFF/CLOG_XACTS_PER_PAGE,
55  * and CLOG segment numbering at
56  * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE/SLRU_PAGES_PER_SEGMENT. We need take no
57  * explicit notice of that fact in this module, except when comparing segment
58  * and page numbers in TruncateCLOG (see CLOGPagePrecedes).
59  */
60 
61 /* We need two bits per xact, so four xacts fit in a byte */
62 #define CLOG_BITS_PER_XACT 2
63 #define CLOG_XACTS_PER_BYTE 4
64 #define CLOG_XACTS_PER_PAGE (BLCKSZ * CLOG_XACTS_PER_BYTE)
65 #define CLOG_XACT_BITMASK ((1 << CLOG_BITS_PER_XACT) - 1)
66 
67 /*
68  * Because space used in CLOG by each transaction is so small, we place a
69  * smaller limit on the number of CLOG buffers than SLRU allows. No other
70  * SLRU needs this.
71  */
72 #define CLOG_MAX_ALLOWED_BUFFERS \
73  Min(SLRU_MAX_ALLOWED_BUFFERS, \
74  (((MaxTransactionId / 2) + (CLOG_XACTS_PER_PAGE - 1)) / CLOG_XACTS_PER_PAGE))
75 
76 
77 /*
78  * Although we return an int64 the actual value can't currently exceed
79  * 0xFFFFFFFF/CLOG_XACTS_PER_PAGE.
80  */
81 static inline int64
83 {
84  return xid / (int64) CLOG_XACTS_PER_PAGE;
85 }
86 
87 #define TransactionIdToPgIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE)
88 #define TransactionIdToByte(xid) (TransactionIdToPgIndex(xid) / CLOG_XACTS_PER_BYTE)
89 #define TransactionIdToBIndex(xid) ((xid) % (TransactionId) CLOG_XACTS_PER_BYTE)
90 
91 /* We store the latest async LSN for each group of transactions */
92 #define CLOG_XACTS_PER_LSN_GROUP 32 /* keep this a power of 2 */
93 #define CLOG_LSNS_PER_PAGE (CLOG_XACTS_PER_PAGE / CLOG_XACTS_PER_LSN_GROUP)
94 
95 #define GetLSNIndex(slotno, xid) ((slotno) * CLOG_LSNS_PER_PAGE + \
96  ((xid) % (TransactionId) CLOG_XACTS_PER_PAGE) / CLOG_XACTS_PER_LSN_GROUP)
97 
98 /*
99  * The number of subtransactions below which we consider to apply clog group
100  * update optimization. Testing reveals that the number higher than this can
101  * hurt performance.
102  */
103 #define THRESHOLD_SUBTRANS_CLOG_OPT 5
104 
105 /*
106  * Link to shared-memory data structures for CLOG control
107  */
109 
110 #define XactCtl (&XactCtlData)
111 
112 
113 static int ZeroCLOGPage(int64 pageno, bool writeXlog);
114 static bool CLOGPagePrecedes(int64 page1, int64 page2);
115 static void WriteZeroPageXlogRec(int64 pageno);
116 static void WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact,
117  Oid oldestXactDb);
118 static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids,
119  TransactionId *subxids, XidStatus status,
120  XLogRecPtr lsn, int64 pageno,
121  bool all_xact_same_page);
122 static void TransactionIdSetStatusBit(TransactionId xid, XidStatus status,
123  XLogRecPtr lsn, int slotno);
124 static void set_status_by_pages(int nsubxids, TransactionId *subxids,
125  XidStatus status, XLogRecPtr lsn);
127  XidStatus status, XLogRecPtr lsn, int64 pageno);
128 static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids,
129  TransactionId *subxids, XidStatus status,
130  XLogRecPtr lsn, int64 pageno);
131 
132 
133 /*
134  * TransactionIdSetTreeStatus
135  *
136  * Record the final state of transaction entries in the commit log for
137  * a transaction and its subtransaction tree. Take care to ensure this is
138  * efficient, and as atomic as possible.
139  *
140  * xid is a single xid to set status for. This will typically be
141  * the top level transactionid for a top level commit or abort. It can
142  * also be a subtransaction when we record transaction aborts.
143  *
144  * subxids is an array of xids of length nsubxids, representing subtransactions
145  * in the tree of xid. In various cases nsubxids may be zero.
146  *
147  * lsn must be the WAL location of the commit record when recording an async
148  * commit. For a synchronous commit it can be InvalidXLogRecPtr, since the
149  * caller guarantees the commit record is already flushed in that case. It
150  * should be InvalidXLogRecPtr for abort cases, too.
151  *
152  * In the commit case, atomicity is limited by whether all the subxids are in
153  * the same CLOG page as xid. If they all are, then the lock will be grabbed
154  * only once, and the status will be set to committed directly. Otherwise
155  * we must
156  * 1. set sub-committed all subxids that are not on the same page as the
157  * main xid
158  * 2. atomically set committed the main xid and the subxids on the same page
159  * 3. go over the first bunch again and set them committed
160  * Note that as far as concurrent checkers are concerned, main transaction
161  * commit as a whole is still atomic.
162  *
163  * Example:
164  * TransactionId t commits and has subxids t1, t2, t3, t4
165  * t is on page p1, t1 is also on p1, t2 and t3 are on p2, t4 is on p3
166  * 1. update pages2-3:
167  * page2: set t2,t3 as sub-committed
168  * page3: set t4 as sub-committed
169  * 2. update page1:
170  * page1: set t,t1 as committed
171  * 3. update pages2-3:
172  * page2: set t2,t3 as committed
173  * page3: set t4 as committed
174  *
175  * NB: this is a low-level routine and is NOT the preferred entry point
176  * for most uses; functions in transam.c are the intended callers.
177  *
178  * XXX Think about issuing POSIX_FADV_WILLNEED on pages that we will need,
179  * but aren't yet in cache, as well as hinting pages not to fall out of
180  * cache yet.
181  */
182 void
184  TransactionId *subxids, XidStatus status, XLogRecPtr lsn)
185 {
186  int64 pageno = TransactionIdToPage(xid); /* get page of parent */
187  int i;
188 
190  status == TRANSACTION_STATUS_ABORTED);
191 
192  /*
193  * See how many subxids, if any, are on the same page as the parent, if
194  * any.
195  */
196  for (i = 0; i < nsubxids; i++)
197  {
198  if (TransactionIdToPage(subxids[i]) != pageno)
199  break;
200  }
201 
202  /*
203  * Do all items fit on a single page?
204  */
205  if (i == nsubxids)
206  {
207  /*
208  * Set the parent and all subtransactions in a single call
209  */
210  TransactionIdSetPageStatus(xid, nsubxids, subxids, status, lsn,
211  pageno, true);
212  }
213  else
214  {
215  int nsubxids_on_first_page = i;
216 
217  /*
218  * If this is a commit then we care about doing this correctly (i.e.
219  * using the subcommitted intermediate status). By here, we know
220  * we're updating more than one page of clog, so we must mark entries
221  * that are *not* on the first page so that they show as subcommitted
222  * before we then return to update the status to fully committed.
223  *
224  * To avoid touching the first page twice, skip marking subcommitted
225  * for the subxids on that first page.
226  */
227  if (status == TRANSACTION_STATUS_COMMITTED)
228  set_status_by_pages(nsubxids - nsubxids_on_first_page,
229  subxids + nsubxids_on_first_page,
231 
232  /*
233  * Now set the parent and subtransactions on same page as the parent,
234  * if any
235  */
236  pageno = TransactionIdToPage(xid);
237  TransactionIdSetPageStatus(xid, nsubxids_on_first_page, subxids, status,
238  lsn, pageno, false);
239 
240  /*
241  * Now work through the rest of the subxids one clog page at a time,
242  * starting from the second page onwards, like we did above.
243  */
244  set_status_by_pages(nsubxids - nsubxids_on_first_page,
245  subxids + nsubxids_on_first_page,
246  status, lsn);
247  }
248 }
249 
250 /*
251  * Helper for TransactionIdSetTreeStatus: set the status for a bunch of
252  * transactions, chunking in the separate CLOG pages involved. We never
253  * pass the whole transaction tree to this function, only subtransactions
254  * that are on different pages to the top level transaction id.
255  */
256 static void
257 set_status_by_pages(int nsubxids, TransactionId *subxids,
258  XidStatus status, XLogRecPtr lsn)
259 {
260  int64 pageno = TransactionIdToPage(subxids[0]);
261  int offset = 0;
262  int i = 0;
263 
264  Assert(nsubxids > 0); /* else the pageno fetch above is unsafe */
265 
266  while (i < nsubxids)
267  {
268  int num_on_page = 0;
269  int64 nextpageno;
270 
271  do
272  {
273  nextpageno = TransactionIdToPage(subxids[i]);
274  if (nextpageno != pageno)
275  break;
276  num_on_page++;
277  i++;
278  } while (i < nsubxids);
279 
281  num_on_page, subxids + offset,
282  status, lsn, pageno, false);
283  offset = i;
284  pageno = nextpageno;
285  }
286 }
287 
288 /*
289  * Record the final state of transaction entries in the commit log for all
290  * entries on a single page. Atomic only on this page.
291  */
292 static void
294  TransactionId *subxids, XidStatus status,
295  XLogRecPtr lsn, int64 pageno,
296  bool all_xact_same_page)
297 {
298  LWLock *lock;
299 
300  /* Can't use group update when PGPROC overflows. */
302  "group clog threshold less than PGPROC cached subxids");
303 
304  /* Get the SLRU bank lock for the page we are going to access. */
305  lock = SimpleLruGetBankLock(XactCtl, pageno);
306 
307  /*
308  * When there is contention on the SLRU bank lock we need, we try to group
309  * multiple updates; a single leader process will perform transaction
310  * status updates for multiple backends so that the number of times the
311  * bank lock needs to be acquired is reduced.
312  *
313  * For this optimization to be safe, the XID and subxids in MyProc must be
314  * the same as the ones for which we're setting the status. Check that
315  * this is the case.
316  *
317  * For this optimization to be efficient, we shouldn't have too many
318  * sub-XIDs and all of the XIDs for which we're adjusting clog should be
319  * on the same page. Check those conditions, too.
320  */
321  if (all_xact_same_page && xid == MyProc->xid &&
322  nsubxids <= THRESHOLD_SUBTRANS_CLOG_OPT &&
323  nsubxids == MyProc->subxidStatus.count &&
324  (nsubxids == 0 ||
325  memcmp(subxids, MyProc->subxids.xids,
326  nsubxids * sizeof(TransactionId)) == 0))
327  {
328  /*
329  * If we can immediately acquire the lock, we update the status of our
330  * own XID and release the lock. If not, try use group XID update. If
331  * that doesn't work out, fall back to waiting for the lock to perform
332  * an update for this transaction only.
333  */
335  {
336  /* Got the lock without waiting! Do the update. */
337  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
338  lsn, pageno);
339  LWLockRelease(lock);
340  return;
341  }
342  else if (TransactionGroupUpdateXidStatus(xid, status, lsn, pageno))
343  {
344  /* Group update mechanism has done the work. */
345  return;
346  }
347 
348  /* Fall through only if update isn't done yet. */
349  }
350 
351  /* Group update not applicable, or couldn't accept this page number. */
353  TransactionIdSetPageStatusInternal(xid, nsubxids, subxids, status,
354  lsn, pageno);
355  LWLockRelease(lock);
356 }
357 
358 /*
359  * Record the final state of transaction entry in the commit log
360  *
361  * We don't do any locking here; caller must handle that.
362  */
363 static void
365  TransactionId *subxids, XidStatus status,
366  XLogRecPtr lsn, int64 pageno)
367 {
368  int slotno;
369  int i;
370 
372  status == TRANSACTION_STATUS_ABORTED ||
375  LW_EXCLUSIVE));
376 
377  /*
378  * If we're doing an async commit (ie, lsn is valid), then we must wait
379  * for any active write on the page slot to complete. Otherwise our
380  * update could reach disk in that write, which will not do since we
381  * mustn't let it reach disk until we've done the appropriate WAL flush.
382  * But when lsn is invalid, it's OK to scribble on a page while it is
383  * write-busy, since we don't care if the update reaches disk sooner than
384  * we think.
385  */
386  slotno = SimpleLruReadPage(XactCtl, pageno, XLogRecPtrIsInvalid(lsn), xid);
387 
388  /*
389  * Set the main transaction id, if any.
390  *
391  * If we update more than one xid on this page while it is being written
392  * out, we might find that some of the bits go to disk and others don't.
393  * If we are updating commits on the page with the top-level xid that
394  * could break atomicity, so we subcommit the subxids first before we mark
395  * the top-level commit.
396  */
397  if (TransactionIdIsValid(xid))
398  {
399  /* Subtransactions first, if needed ... */
400  if (status == TRANSACTION_STATUS_COMMITTED)
401  {
402  for (i = 0; i < nsubxids; i++)
403  {
404  Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
405  TransactionIdSetStatusBit(subxids[i],
407  lsn, slotno);
408  }
409  }
410 
411  /* ... then the main transaction */
412  TransactionIdSetStatusBit(xid, status, lsn, slotno);
413  }
414 
415  /* Set the subtransactions */
416  for (i = 0; i < nsubxids; i++)
417  {
418  Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(subxids[i]));
419  TransactionIdSetStatusBit(subxids[i], status, lsn, slotno);
420  }
421 
422  XactCtl->shared->page_dirty[slotno] = true;
423 }
424 
425 /*
426  * Subroutine for TransactionIdSetPageStatus, q.v.
427  *
428  * When we cannot immediately acquire the SLRU bank lock in exclusive mode at
429  * commit time, add ourselves to a list of processes that need their XIDs
430  * status update. The first process to add itself to the list will acquire
431  * the lock in exclusive mode and set transaction status as required on behalf
432  * of all group members. This avoids a great deal of contention when many
433  * processes are trying to commit at once, since the lock need not be
434  * repeatedly handed off from one committing process to the next.
435  *
436  * Returns true when transaction status has been updated in clog; returns
437  * false if we decided against applying the optimization because the page
438  * number we need to update differs from those processes already waiting.
439  */
440 static bool
442  XLogRecPtr lsn, int64 pageno)
443 {
444  volatile PROC_HDR *procglobal = ProcGlobal;
445  PGPROC *proc = MyProc;
446  uint32 nextidx;
447  uint32 wakeidx;
448  int prevpageno;
449  LWLock *prevlock = NULL;
450 
451  /* We should definitely have an XID whose status needs to be updated. */
453 
454  /*
455  * Prepare to add ourselves to the list of processes needing a group XID
456  * status update.
457  */
458  proc->clogGroupMember = true;
459  proc->clogGroupMemberXid = xid;
460  proc->clogGroupMemberXidStatus = status;
461  proc->clogGroupMemberPage = pageno;
462  proc->clogGroupMemberLsn = lsn;
463 
464  /*
465  * We put ourselves in the queue by writing MyProcNumber to
466  * ProcGlobal->clogGroupFirst. However, if there's already a process
467  * listed there, we compare our pageno with that of that process; if it
468  * differs, we cannot participate in the group, so we return for caller to
469  * update pg_xact in the normal way.
470  *
471  * If we're not the first process in the list, we must follow the leader.
472  * We do this by storing the data we want updated in our PGPROC entry
473  * where the leader can find it, then going to sleep.
474  *
475  * If no process is already in the list, we're the leader; our first step
476  * is to lock the SLRU bank to which our page belongs, then we close out
477  * the group by resetting the list pointer from ProcGlobal->clogGroupFirst
478  * (this lets other processes set up other groups later); finally we do
479  * the SLRU updates, release the SLRU bank lock, and wake up the sleeping
480  * processes.
481  *
482  * If another group starts to update a page in a different SLRU bank, they
483  * can proceed concurrently, since the bank lock they're going to use is
484  * different from ours. If another group starts to update a page in the
485  * same bank as ours, they wait until we release the lock.
486  */
487  nextidx = pg_atomic_read_u32(&procglobal->clogGroupFirst);
488 
489  while (true)
490  {
491  /*
492  * Add the proc to list, if the clog page where we need to update the
493  * current transaction status is same as group leader's clog page.
494  *
495  * There is a race condition here, which is that after doing the below
496  * check and before adding this proc's clog update to a group, the
497  * group leader might have already finished the group update for this
498  * page and becomes group leader of another group, updating a
499  * different page. This will lead to a situation where a single group
500  * can have different clog page updates. This isn't likely and will
501  * still work, just less efficiently -- we handle this case by
502  * switching to a different bank lock in the loop below.
503  */
504  if (nextidx != INVALID_PROC_NUMBER &&
505  GetPGProcByNumber(nextidx)->clogGroupMemberPage != proc->clogGroupMemberPage)
506  {
507  /*
508  * Ensure that this proc is not a member of any clog group that
509  * needs an XID status update.
510  */
511  proc->clogGroupMember = false;
513  return false;
514  }
515 
516  pg_atomic_write_u32(&proc->clogGroupNext, nextidx);
517 
519  &nextidx,
520  (uint32) MyProcNumber))
521  break;
522  }
523 
524  /*
525  * If the list was not empty, the leader will update the status of our
526  * XID. It is impossible to have followers without a leader because the
527  * first process that has added itself to the list will always have
528  * nextidx as INVALID_PROC_NUMBER.
529  */
530  if (nextidx != INVALID_PROC_NUMBER)
531  {
532  int extraWaits = 0;
533 
534  /* Sleep until the leader updates our XID status. */
535  pgstat_report_wait_start(WAIT_EVENT_XACT_GROUP_UPDATE);
536  for (;;)
537  {
538  /* acts as a read barrier */
539  PGSemaphoreLock(proc->sem);
540  if (!proc->clogGroupMember)
541  break;
542  extraWaits++;
543  }
545 
547 
548  /* Fix semaphore count for any absorbed wakeups */
549  while (extraWaits-- > 0)
550  PGSemaphoreUnlock(proc->sem);
551  return true;
552  }
553 
554  /*
555  * By here, we know we're the leader process. Acquire the SLRU bank lock
556  * that corresponds to the page we originally wanted to modify.
557  */
558  prevpageno = proc->clogGroupMemberPage;
559  prevlock = SimpleLruGetBankLock(XactCtl, prevpageno);
560  LWLockAcquire(prevlock, LW_EXCLUSIVE);
561 
562  /*
563  * Now that we've got the lock, clear the list of processes waiting for
564  * group XID status update, saving a pointer to the head of the list.
565  * (Trying to pop elements one at a time could lead to an ABA problem.)
566  *
567  * At this point, any processes trying to do this would create a separate
568  * group.
569  */
570  nextidx = pg_atomic_exchange_u32(&procglobal->clogGroupFirst,
572 
573  /* Remember head of list so we can perform wakeups after dropping lock. */
574  wakeidx = nextidx;
575 
576  /* Walk the list and update the status of all XIDs. */
577  while (nextidx != INVALID_PROC_NUMBER)
578  {
579  PGPROC *nextproc = &ProcGlobal->allProcs[nextidx];
580  int thispageno = nextproc->clogGroupMemberPage;
581 
582  /*
583  * If the page to update belongs to a different bank than the previous
584  * one, exchange bank lock to the new one. This should be quite rare,
585  * as described above.
586  *
587  * (We could try to optimize this by waking up the processes for which
588  * we have already updated the status while we exchange the lock, but
589  * the code doesn't do that at present. I think it'd require
590  * additional bookkeeping, making the common path slower in order to
591  * improve an infrequent case.)
592  */
593  if (thispageno != prevpageno)
594  {
595  LWLock *lock = SimpleLruGetBankLock(XactCtl, thispageno);
596 
597  if (prevlock != lock)
598  {
599  LWLockRelease(prevlock);
601  }
602  prevlock = lock;
603  prevpageno = thispageno;
604  }
605 
606  /*
607  * Transactions with more than THRESHOLD_SUBTRANS_CLOG_OPT sub-XIDs
608  * should not use group XID status update mechanism.
609  */
611 
613  nextproc->subxidStatus.count,
614  nextproc->subxids.xids,
615  nextproc->clogGroupMemberXidStatus,
616  nextproc->clogGroupMemberLsn,
617  nextproc->clogGroupMemberPage);
618 
619  /* Move to next proc in list. */
620  nextidx = pg_atomic_read_u32(&nextproc->clogGroupNext);
621  }
622 
623  /* We're done with the lock now. */
624  if (prevlock != NULL)
625  LWLockRelease(prevlock);
626 
627  /*
628  * Now that we've released the lock, go back and wake everybody up. We
629  * don't do this under the lock so as to keep lock hold times to a
630  * minimum.
631  *
632  * (Perhaps we could do this in two passes, the first setting
633  * clogGroupNext to invalid while saving the semaphores to an array, then
634  * a single write barrier, then another pass unlocking the semaphores.)
635  */
636  while (wakeidx != INVALID_PROC_NUMBER)
637  {
638  PGPROC *wakeproc = &ProcGlobal->allProcs[wakeidx];
639 
640  wakeidx = pg_atomic_read_u32(&wakeproc->clogGroupNext);
642 
643  /* ensure all previous writes are visible before follower continues. */
645 
646  wakeproc->clogGroupMember = false;
647 
648  if (wakeproc != MyProc)
649  PGSemaphoreUnlock(wakeproc->sem);
650  }
651 
652  return true;
653 }
654 
655 /*
656  * Sets the commit status of a single transaction.
657  *
658  * Caller must hold the corresponding SLRU bank lock, will be held at exit.
659  */
660 static void
662 {
663  int byteno = TransactionIdToByte(xid);
664  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
665  char *byteptr;
666  char byteval;
667  char curval;
668 
669  Assert(XactCtl->shared->page_number[slotno] == TransactionIdToPage(xid));
671  XactCtl->shared->page_number[slotno]),
672  LW_EXCLUSIVE));
673 
674  byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
675  curval = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
676 
677  /*
678  * When replaying transactions during recovery we still need to perform
679  * the two phases of subcommit and then commit. However, some transactions
680  * are already correctly marked, so we just treat those as a no-op which
681  * allows us to keep the following Assert as restrictive as possible.
682  */
683  if (InRecovery && status == TRANSACTION_STATUS_SUB_COMMITTED &&
685  return;
686 
687  /*
688  * Current state change should be from 0 or subcommitted to target state
689  * or we should already be there when replaying changes during recovery.
690  */
691  Assert(curval == 0 ||
693  status != TRANSACTION_STATUS_IN_PROGRESS) ||
694  curval == status);
695 
696  /* note this assumes exclusive access to the clog page */
697  byteval = *byteptr;
698  byteval &= ~(((1 << CLOG_BITS_PER_XACT) - 1) << bshift);
699  byteval |= (status << bshift);
700  *byteptr = byteval;
701 
702  /*
703  * Update the group LSN if the transaction completion LSN is higher.
704  *
705  * Note: lsn will be invalid when supplied during InRecovery processing,
706  * so we don't need to do anything special to avoid LSN updates during
707  * recovery. After recovery completes the next clog change will set the
708  * LSN correctly.
709  */
710  if (!XLogRecPtrIsInvalid(lsn))
711  {
712  int lsnindex = GetLSNIndex(slotno, xid);
713 
714  if (XactCtl->shared->group_lsn[lsnindex] < lsn)
715  XactCtl->shared->group_lsn[lsnindex] = lsn;
716  }
717 }
718 
719 /*
720  * Interrogate the state of a transaction in the commit log.
721  *
722  * Aside from the actual commit status, this function returns (into *lsn)
723  * an LSN that is late enough to be able to guarantee that if we flush up to
724  * that LSN then we will have flushed the transaction's commit record to disk.
725  * The result is not necessarily the exact LSN of the transaction's commit
726  * record! For example, for long-past transactions (those whose clog pages
727  * already migrated to disk), we'll return InvalidXLogRecPtr. Also, because
728  * we group transactions on the same clog page to conserve storage, we might
729  * return the LSN of a later transaction that falls into the same group.
730  *
731  * NB: this is a low-level routine and is NOT the preferred entry point
732  * for most uses; TransactionLogFetch() in transam.c is the intended caller.
733  */
734 XidStatus
736 {
737  int64 pageno = TransactionIdToPage(xid);
738  int byteno = TransactionIdToByte(xid);
739  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
740  int slotno;
741  int lsnindex;
742  char *byteptr;
743  XidStatus status;
744 
745  /* lock is acquired by SimpleLruReadPage_ReadOnly */
746 
747  slotno = SimpleLruReadPage_ReadOnly(XactCtl, pageno, xid);
748  byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
749 
750  status = (*byteptr >> bshift) & CLOG_XACT_BITMASK;
751 
752  lsnindex = GetLSNIndex(slotno, xid);
753  *lsn = XactCtl->shared->group_lsn[lsnindex];
754 
756 
757  return status;
758 }
759 
760 /*
761  * Number of shared CLOG buffers.
762  *
763  * If asked to autotune, use 2MB for every 1GB of shared buffers, up to 8MB.
764  * Otherwise just cap the configured amount to be between 16 and the maximum
765  * allowed.
766  */
767 static int
769 {
770  /* auto-tune based on shared buffers */
771  if (transaction_buffers == 0)
772  return SimpleLruAutotuneBuffers(512, 1024);
773 
775 }
776 
777 /*
778  * Initialization of shared memory for CLOG
779  */
780 Size
782 {
784 }
785 
786 void
788 {
789  /* If auto-tuning is requested, now is the time to do it */
790  if (transaction_buffers == 0)
791  {
792  char buf[32];
793 
794  snprintf(buf, sizeof(buf), "%d", CLOGShmemBuffers());
795  SetConfigOption("transaction_buffers", buf, PGC_POSTMASTER,
797 
798  /*
799  * We prefer to report this value's source as PGC_S_DYNAMIC_DEFAULT.
800  * However, if the DBA explicitly set transaction_buffers = 0 in the
801  * config file, then PGC_S_DYNAMIC_DEFAULT will fail to override that
802  * and we must force the matter with PGC_S_OVERRIDE.
803  */
804  if (transaction_buffers == 0) /* failed to apply it? */
805  SetConfigOption("transaction_buffers", buf, PGC_POSTMASTER,
807  }
809 
810  XactCtl->PagePrecedes = CLOGPagePrecedes;
812  "pg_xact", LWTRANCHE_XACT_BUFFER,
815 }
816 
817 /*
818  * GUC check_hook for transaction_buffers
819  */
820 bool
822 {
823  return check_slru_buffers("transaction_buffers", newval);
824 }
825 
826 /*
827  * This func must be called ONCE on system install. It creates
828  * the initial CLOG segment. (The CLOG directory is assumed to
829  * have been created by initdb, and CLOGShmemInit must have been
830  * called already.)
831  */
832 void
834 {
835  int slotno;
836  LWLock *lock = SimpleLruGetBankLock(XactCtl, 0);
837 
839 
840  /* Create and zero the first page of the commit log */
841  slotno = ZeroCLOGPage(0, false);
842 
843  /* Make sure it's written out */
844  SimpleLruWritePage(XactCtl, slotno);
845  Assert(!XactCtl->shared->page_dirty[slotno]);
846 
847  LWLockRelease(lock);
848 }
849 
850 /*
851  * Initialize (or reinitialize) a page of CLOG to zeroes.
852  * If writeXlog is true, also emit an XLOG record saying we did this.
853  *
854  * The page is not actually written, just set up in shared memory.
855  * The slot number of the new page is returned.
856  *
857  * Control lock must be held at entry, and will be held at exit.
858  */
859 static int
860 ZeroCLOGPage(int64 pageno, bool writeXlog)
861 {
862  int slotno;
863 
864  slotno = SimpleLruZeroPage(XactCtl, pageno);
865 
866  if (writeXlog)
867  WriteZeroPageXlogRec(pageno);
868 
869  return slotno;
870 }
871 
872 /*
873  * This must be called ONCE during postmaster or standalone-backend startup,
874  * after StartupXLOG has initialized TransamVariables->nextXid.
875  */
876 void
878 {
880  int64 pageno = TransactionIdToPage(xid);
881 
882  /*
883  * Initialize our idea of the latest page number.
884  */
885  pg_atomic_write_u64(&XactCtl->shared->latest_page_number, pageno);
886 }
887 
888 /*
889  * This must be called ONCE at the end of startup/recovery.
890  */
891 void
892 TrimCLOG(void)
893 {
895  int64 pageno = TransactionIdToPage(xid);
896  LWLock *lock = SimpleLruGetBankLock(XactCtl, pageno);
897 
899 
900  /*
901  * Zero out the remainder of the current clog page. Under normal
902  * circumstances it should be zeroes already, but it seems at least
903  * theoretically possible that XLOG replay will have settled on a nextXID
904  * value that is less than the last XID actually used and marked by the
905  * previous database lifecycle (since subtransaction commit writes clog
906  * but makes no WAL entry). Let's just be safe. (We need not worry about
907  * pages beyond the current one, since those will be zeroed when first
908  * used. For the same reason, there is no need to do anything when
909  * nextXid is exactly at a page boundary; and it's likely that the
910  * "current" page doesn't exist yet in that case.)
911  */
912  if (TransactionIdToPgIndex(xid) != 0)
913  {
914  int byteno = TransactionIdToByte(xid);
915  int bshift = TransactionIdToBIndex(xid) * CLOG_BITS_PER_XACT;
916  int slotno;
917  char *byteptr;
918 
919  slotno = SimpleLruReadPage(XactCtl, pageno, false, xid);
920  byteptr = XactCtl->shared->page_buffer[slotno] + byteno;
921 
922  /* Zero so-far-unused positions in the current byte */
923  *byteptr &= (1 << bshift) - 1;
924  /* Zero the rest of the page */
925  MemSet(byteptr + 1, 0, BLCKSZ - byteno - 1);
926 
927  XactCtl->shared->page_dirty[slotno] = true;
928  }
929 
930  LWLockRelease(lock);
931 }
932 
933 /*
934  * Perform a checkpoint --- either during shutdown, or on-the-fly
935  */
936 void
938 {
939  /*
940  * Write dirty CLOG pages to disk. This may result in sync requests
941  * queued for later handling by ProcessSyncRequests(), as part of the
942  * checkpoint.
943  */
944  TRACE_POSTGRESQL_CLOG_CHECKPOINT_START(true);
945  SimpleLruWriteAll(XactCtl, true);
946  TRACE_POSTGRESQL_CLOG_CHECKPOINT_DONE(true);
947 }
948 
949 
950 /*
951  * Make sure that CLOG has room for a newly-allocated XID.
952  *
953  * NB: this is called while holding XidGenLock. We want it to be very fast
954  * most of the time; even when it's not so fast, no actual I/O need happen
955  * unless we're forced to write out a dirty clog or xlog page to make room
956  * in shared memory.
957  */
958 void
960 {
961  int64 pageno;
962  LWLock *lock;
963 
964  /*
965  * No work except at first XID of a page. But beware: just after
966  * wraparound, the first XID of page zero is FirstNormalTransactionId.
967  */
968  if (TransactionIdToPgIndex(newestXact) != 0 &&
970  return;
971 
972  pageno = TransactionIdToPage(newestXact);
973  lock = SimpleLruGetBankLock(XactCtl, pageno);
974 
976 
977  /* Zero the page and make an XLOG entry about it */
978  ZeroCLOGPage(pageno, true);
979 
980  LWLockRelease(lock);
981 }
982 
983 
984 /*
985  * Remove all CLOG segments before the one holding the passed transaction ID
986  *
987  * Before removing any CLOG data, we must flush XLOG to disk, to ensure
988  * that any recently-emitted FREEZE_PAGE records have reached disk; otherwise
989  * a crash and restart might leave us with some unfrozen tuples referencing
990  * removed CLOG data. We choose to emit a special TRUNCATE XLOG record too.
991  * Replaying the deletion from XLOG is not critical, since the files could
992  * just as well be removed later, but doing so prevents a long-running hot
993  * standby server from acquiring an unreasonably bloated CLOG directory.
994  *
995  * Since CLOG segments hold a large number of transactions, the opportunity to
996  * actually remove a segment is fairly rare, and so it seems best not to do
997  * the XLOG flush unless we have confirmed that there is a removable segment.
998  */
999 void
1000 TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
1001 {
1002  int64 cutoffPage;
1003 
1004  /*
1005  * The cutoff point is the start of the segment containing oldestXact. We
1006  * pass the *page* containing oldestXact to SimpleLruTruncate.
1007  */
1008  cutoffPage = TransactionIdToPage(oldestXact);
1009 
1010  /* Check to see if there's any files that could be removed */
1012  return; /* nothing to remove */
1013 
1014  /*
1015  * Advance oldestClogXid before truncating clog, so concurrent xact status
1016  * lookups can ensure they don't attempt to access truncated-away clog.
1017  *
1018  * It's only necessary to do this if we will actually truncate away clog
1019  * pages.
1020  */
1021  AdvanceOldestClogXid(oldestXact);
1022 
1023  /*
1024  * Write XLOG record and flush XLOG to disk. We record the oldest xid
1025  * we're keeping information about here so we can ensure that it's always
1026  * ahead of clog truncation in case we crash, and so a standby finds out
1027  * the new valid xid before the next checkpoint.
1028  */
1029  WriteTruncateXlogRec(cutoffPage, oldestXact, oldestxid_datoid);
1030 
1031  /* Now we can remove the old CLOG segment(s) */
1032  SimpleLruTruncate(XactCtl, cutoffPage);
1033 }
1034 
1035 
1036 /*
1037  * Decide whether a CLOG page number is "older" for truncation purposes.
1038  *
1039  * We need to use comparison of TransactionIds here in order to do the right
1040  * thing with wraparound XID arithmetic. However, TransactionIdPrecedes()
1041  * would get weird about permanent xact IDs. So, offset both such that xid1,
1042  * xid2, and xid2 + CLOG_XACTS_PER_PAGE - 1 are all normal XIDs; this offset
1043  * is relevant to page 0 and to the page preceding page 0.
1044  *
1045  * The page containing oldestXact-2^31 is the important edge case. The
1046  * portion of that page equaling or following oldestXact-2^31 is expendable,
1047  * but the portion preceding oldestXact-2^31 is not. When oldestXact-2^31 is
1048  * the first XID of a page and segment, the entire page and segment is
1049  * expendable, and we could truncate the segment. Recognizing that case would
1050  * require making oldestXact, not just the page containing oldestXact,
1051  * available to this callback. The benefit would be rare and small, so we
1052  * don't optimize that edge case.
1053  */
1054 static bool
1055 CLOGPagePrecedes(int64 page1, int64 page2)
1056 {
1057  TransactionId xid1;
1058  TransactionId xid2;
1059 
1060  xid1 = ((TransactionId) page1) * CLOG_XACTS_PER_PAGE;
1061  xid1 += FirstNormalTransactionId + 1;
1062  xid2 = ((TransactionId) page2) * CLOG_XACTS_PER_PAGE;
1063  xid2 += FirstNormalTransactionId + 1;
1064 
1065  return (TransactionIdPrecedes(xid1, xid2) &&
1066  TransactionIdPrecedes(xid1, xid2 + CLOG_XACTS_PER_PAGE - 1));
1067 }
1068 
1069 
1070 /*
1071  * Write a ZEROPAGE xlog record
1072  */
1073 static void
1075 {
1076  XLogBeginInsert();
1077  XLogRegisterData((char *) (&pageno), sizeof(pageno));
1078  (void) XLogInsert(RM_CLOG_ID, CLOG_ZEROPAGE);
1079 }
1080 
1081 /*
1082  * Write a TRUNCATE xlog record
1083  *
1084  * We must flush the xlog record to disk before returning --- see notes
1085  * in TruncateCLOG().
1086  */
1087 static void
1088 WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact, Oid oldestXactDb)
1089 {
1090  XLogRecPtr recptr;
1091  xl_clog_truncate xlrec;
1092 
1093  xlrec.pageno = pageno;
1094  xlrec.oldestXact = oldestXact;
1095  xlrec.oldestXactDb = oldestXactDb;
1096 
1097  XLogBeginInsert();
1098  XLogRegisterData((char *) (&xlrec), sizeof(xl_clog_truncate));
1099  recptr = XLogInsert(RM_CLOG_ID, CLOG_TRUNCATE);
1100  XLogFlush(recptr);
1101 }
1102 
1103 /*
1104  * CLOG resource manager's routines
1105  */
1106 void
1108 {
1109  uint8 info = XLogRecGetInfo(record) & ~XLR_INFO_MASK;
1110 
1111  /* Backup blocks are not used in clog records */
1112  Assert(!XLogRecHasAnyBlockRefs(record));
1113 
1114  if (info == CLOG_ZEROPAGE)
1115  {
1116  int64 pageno;
1117  int slotno;
1118  LWLock *lock;
1119 
1120  memcpy(&pageno, XLogRecGetData(record), sizeof(pageno));
1121 
1122  lock = SimpleLruGetBankLock(XactCtl, pageno);
1123  LWLockAcquire(lock, LW_EXCLUSIVE);
1124 
1125  slotno = ZeroCLOGPage(pageno, false);
1126  SimpleLruWritePage(XactCtl, slotno);
1127  Assert(!XactCtl->shared->page_dirty[slotno]);
1128 
1129  LWLockRelease(lock);
1130  }
1131  else if (info == CLOG_TRUNCATE)
1132  {
1133  xl_clog_truncate xlrec;
1134 
1135  memcpy(&xlrec, XLogRecGetData(record), sizeof(xl_clog_truncate));
1136 
1138 
1140  }
1141  else
1142  elog(PANIC, "clog_redo: unknown op code %u", info);
1143 }
1144 
1145 /*
1146  * Entrypoint for sync.c to sync clog files.
1147  */
1148 int
1149 clogsyncfiletag(const FileTag *ftag, char *path)
1150 {
1151  return SlruSyncFileTag(XactCtl, ftag, path);
1152 }
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Definition: atomics.h:344
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Definition: atomics.h:480
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Definition: atomics.h:271
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unsigned int uint32
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#define Min(x, y)
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#define Max(x, y)
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#define Assert(condition)
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unsigned char uint8
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uint32 TransactionId
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size_t Size
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#define CLOG_MAX_ALLOWED_BUFFERS
Definition: clog.c:72
static void WriteZeroPageXlogRec(int64 pageno)
Definition: clog.c:1074
static int ZeroCLOGPage(int64 pageno, bool writeXlog)
Definition: clog.c:860
#define CLOG_XACT_BITMASK
Definition: clog.c:65
#define CLOG_XACTS_PER_PAGE
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#define THRESHOLD_SUBTRANS_CLOG_OPT
Definition: clog.c:103
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Definition: clog.c:661
XidStatus TransactionIdGetStatus(TransactionId xid, XLogRecPtr *lsn)
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#define TransactionIdToBIndex(xid)
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static int CLOGShmemBuffers(void)
Definition: clog.c:768
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Definition: clog.c:959
void clog_redo(XLogReaderState *record)
Definition: clog.c:1107
static bool CLOGPagePrecedes(int64 page1, int64 page2)
Definition: clog.c:1055
void TruncateCLOG(TransactionId oldestXact, Oid oldestxid_datoid)
Definition: clog.c:1000
Size CLOGShmemSize(void)
Definition: clog.c:781
bool check_transaction_buffers(int *newval, void **extra, GucSource source)
Definition: clog.c:821
static void TransactionIdSetPageStatus(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int64 pageno, bool all_xact_same_page)
Definition: clog.c:293
int clogsyncfiletag(const FileTag *ftag, char *path)
Definition: clog.c:1149
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Definition: clog.c:833
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Definition: clog.c:62
#define CLOG_LSNS_PER_PAGE
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static int64 TransactionIdToPage(TransactionId xid)
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#define TransactionIdToByte(xid)
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Definition: clog.c:257
static bool TransactionGroupUpdateXidStatus(TransactionId xid, XidStatus status, XLogRecPtr lsn, int64 pageno)
Definition: clog.c:441
static void TransactionIdSetPageStatusInternal(TransactionId xid, int nsubxids, TransactionId *subxids, XidStatus status, XLogRecPtr lsn, int64 pageno)
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#define GetLSNIndex(slotno, xid)
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static SlruCtlData XactCtlData
Definition: clog.c:108
#define XactCtl
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void TrimCLOG(void)
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static void WriteTruncateXlogRec(int64 pageno, TransactionId oldestXact, Oid oldestXactDb)
Definition: clog.c:1088
#define TRANSACTION_STATUS_IN_PROGRESS
Definition: clog.h:27
int XidStatus
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#define CLOG_ZEROPAGE
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#define TRANSACTION_STATUS_ABORTED
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#define TRANSACTION_STATUS_SUB_COMMITTED
Definition: clog.h:30
#define CLOG_TRUNCATE
Definition: clog.h:56
#define TRANSACTION_STATUS_COMMITTED
Definition: clog.h:28
#define PANIC
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#define elog(elevel,...)
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int transaction_buffers
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ProcNumber MyProcNumber
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#define newval
GucSource
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@ PGC_S_DYNAMIC_DEFAULT
Definition: guc.h:110
@ PGC_S_OVERRIDE
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@ PGC_POSTMASTER
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int i
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bool LWLockAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1170
bool LWLockHeldByMeInMode(LWLock *lock, LWLockMode mode)
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void LWLockRelease(LWLock *lock)
Definition: lwlock.c:1783
bool LWLockConditionalAcquire(LWLock *lock, LWLockMode mode)
Definition: lwlock.c:1341
@ LWTRANCHE_XACT_BUFFER
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@ LWTRANCHE_XACT_SLRU
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@ LW_EXCLUSIVE
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bool SlruScanDirectory(SlruCtl ctl, SlruScanCallback callback, void *data)
Definition: slru.c:1773
int SimpleLruReadPage(SlruCtl ctl, int64 pageno, bool write_ok, TransactionId xid)
Definition: slru.c:487
int SlruSyncFileTag(SlruCtl ctl, const FileTag *ftag, char *path)
Definition: slru.c:1813
int SimpleLruZeroPage(SlruCtl ctl, int64 pageno)
Definition: slru.c:360
void SimpleLruTruncate(SlruCtl ctl, int64 cutoffPage)
Definition: slru.c:1390
Size SimpleLruShmemSize(int nslots, int nlsns)
Definition: slru.c:184
bool SlruScanDirCbReportPresence(SlruCtl ctl, char *filename, int64 segpage, void *data)
Definition: slru.c:1694
bool check_slru_buffers(const char *name, int *newval)
Definition: slru.c:340
static LWLock * SimpleLruGetBankLock(SlruCtl ctl, int64 pageno)
Definition: slru.h:179
#define SlruPagePrecedesUnitTests(ctl, per_page)
Definition: slru.h:203
PGPROC * MyProc
Definition: proc.c:66
PROC_HDR * ProcGlobal
Definition: proc.c:78
Definition: sync.h:51
Definition: lwlock.h:42
Definition: proc.h:157
XLogRecPtr clogGroupMemberLsn
Definition: proc.h:285
TransactionId clogGroupMemberXid
Definition: proc.h:280
int64 clogGroupMemberPage
Definition: proc.h:283
bool clogGroupMember
Definition: proc.h:278
pg_atomic_uint32 clogGroupNext
Definition: proc.h:279
XidStatus clogGroupMemberXidStatus
Definition: proc.h:281
XidCacheStatus subxidStatus
Definition: proc.h:259
TransactionId xid
Definition: proc.h:168
struct XidCache subxids
Definition: proc.h:261
PGSemaphore sem
Definition: proc.h:162
Definition: proc.h:378
PGPROC * allProcs
Definition: proc.h:380
pg_atomic_uint32 clogGroupFirst
Definition: proc.h:410
FullTransactionId nextXid
Definition: transam.h:220
uint8 count
Definition: proc.h:44
TransactionId xids[PGPROC_MAX_CACHED_SUBXIDS]
Definition: proc.h:51
int64 pageno
Definition: clog.h:34
Oid oldestXactDb
Definition: clog.h:36
TransactionId oldestXact
Definition: clog.h:35
@ SYNC_HANDLER_CLOG
Definition: sync.h:38
bool TransactionIdPrecedes(TransactionId id1, TransactionId id2)
Definition: transam.c:280
#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:355
TransamVariablesData * TransamVariables
Definition: varsup.c:34
static void pgstat_report_wait_start(uint32 wait_event_info)
Definition: wait_event.h:82
static void pgstat_report_wait_end(void)
Definition: wait_event.h:98
void XLogFlush(XLogRecPtr record)
Definition: xlog.c:2791
#define XLogRecPtrIsInvalid(r)
Definition: xlogdefs.h:29
uint64 XLogRecPtr
Definition: xlogdefs.h:21
void XLogRegisterData(char *data, uint32 len)
Definition: xloginsert.c:364
XLogRecPtr XLogInsert(RmgrId rmid, uint8 info)
Definition: xloginsert.c:474
void XLogBeginInsert(void)
Definition: xloginsert.c:149
#define XLogRecGetInfo(decoder)
Definition: xlogreader.h:410
#define XLogRecGetData(decoder)
Definition: xlogreader.h:415
#define XLogRecHasAnyBlockRefs(decoder)
Definition: xlogreader.h:417
#define XLR_INFO_MASK
Definition: xlogrecord.h:62
bool InRecovery
Definition: xlogutils.c:50