Postgresql源码（28）获取快照GetSnapshotData流程分析和性能问题

--------活跃--------提交--------活跃--------回滚--------活跃--------提交
事务ID：1840251    1840252     1840253    1840254     1840255   1840256

五个事务ID执行内容：
1840251：begin;insert into a values (199);
1840252：begin;insert into a values (199);commit;
1840253：begin;insert into a values (199);
1840254：begin;insert into a values (200);rollback;
1840255：begin;insert into a values (199);
1840256：begin;insert into a values (199);commit;

五个活跃进程pid
1840251：18004：活跃事务一
1840253：18257：活跃事务二
1840255：18523：活跃事务三
1840256：19656：已提交事务一
1840257：24492：调试进程

【调试进程】执行begin;insert into a values (199) 获取快照，观察快照获取过程
begin;insert into a values (199);

Snapshot
GetSnapshotData(Snapshot snapshot)
{
	ProcArrayStruct *arrayP = procArray;
	TransactionId xmin;
	TransactionId xmax;
	TransactionId globalxmin;
	int			index;
	int			count = 0;
	int			subcount = 0;
	bool		suboverflowed = false;
	volatile TransactionId replication_slot_xmin = InvalidTransactionId;
	volatile TransactionId replication_slot_catalog_xmin = InvalidTransactionId;
  
  ...
  
	LWLockAcquire(ProcArrayLock, LW_SHARED);

	xmax = ShmemVariableCache->latestCompletedXid;
	TransactionIdAdvance(xmax);

	globalxmin = xmin = xmax;
  
  ...

	if (!snapshot->takenDuringRecovery)
	{
		int		   *pgprocnos = arrayP->pgprocnos;
		int			numProcs;

		/*
		 * Spin over procArray checking xid, xmin, and subxids.  The goal is
		 * to gather all active xids, find the lowest xmin, and try to record
		 * subxids.
		 */
		numProcs = arrayP->numProcs;
		for (index = 0; index < numProcs; index++)
		{
			int			pgprocno = pgprocnos[index];
			volatile PGXACT *pgxact = &allPgXact[pgprocno];
			TransactionId xid;
      
      ...

			/* Update globalxmin to be the smallest valid xmin */
			xid = pgxact->xmin; /* fetch just once */

			if (TransactionIdIsNormal(xid) &&
				NormalTransactionIdPrecedes(xid, globalxmin))
				globalxmin = xid;

			/* Fetch xid just once - see GetNewTransactionId */
			xid = pgxact->xid;

			/*
			 * If the transaction has no XID assigned, we can skip it; it
			 * won't have sub-XIDs either.  If the XID is >= xmax, we can also
			 * skip it; such transactions will be treated as running anyway
			 * (and any sub-XIDs will also be >= xmax).
			 */

			if (!TransactionIdIsNormal(xid)
				|| !NormalTransactionIdPrecedes(xid, xmax))
				continue;

			/*
			 * We don't include our own XIDs (if any) in the snapshot, but we
			 * must include them in xmin.
			 */

			if (NormalTransactionIdPrecedes(xid, xmin))
				xmin = xid;
			if (pgxact == MyPgXact)
				continue;

			/* Add XID to snapshot. */
			snapshot->xip[count++] = xid;

			if (!suboverflowed)
			{
				if (pgxact->overflowed)
					suboverflowed = true;
				else
				{
					int			nxids = pgxact->nxids;

					if (nxids > 0)
					{
						volatile PGPROC *proc = &allProcs[pgprocno];

						memcpy(snapshot->subxip + subcount,
							   (void *) proc->subxids.xids,
							   nxids * sizeof(TransactionId));
						subcount += nxids;
					}
				}
			}
		}
	}
	else
	{
		...
    // in hot standby
	}

  ...

	if (!TransactionIdIsValid(MyPgXact->xmin))
		MyPgXact->xmin = TransactionXmin = xmin;

	LWLockRelease(ProcArrayLock);

	if (TransactionIdPrecedes(xmin, globalxmin))
		globalxmin = xmin;

...

	RecentXmin = xmin;

	snapshot->xmin = xmin;
	snapshot->xmax = xmax;
	snapshot->xcnt = count;
	snapshot->subxcnt = subcount;
	snapshot->suboverflowed = suboverflowed;

	snapshot->curcid = GetCurrentCommandId(false);
	snapshot->active_count = 0;
	snapshot->regd_count = 0;
	snapshot->copied = false;

...

	return snapshot;
}

(gdb) p *snapshot
$34 = {satisfies = 0xa2f5bb <HeapTupleSatisfiesMVCC>, xmin = 1840251, xmax = 1840257, xip = 0x12c81c0, xcnt = 3, 
  subxip = 0x7fecacadc010, subxcnt = 0, suboverflowed = 0 '\000', takenDuringRecovery = 0 '\000', copied = 0 '\000', curcid = 1, 
  speculativeToken = 0, active_count = 0, regd_count = 0, ph_node = {first_child = 0x0, next_sibling = 0x0, prev_or_parent = 0x0}, 
  whenTaken = 0, lsn = 0}

- 第一步：ProcArrayLock加读锁，在锁内通过xmax = ShmemVariableCache->latestCompletedXid拿到上一个使用过的xid，xmax=xid+1
- 第二步：遍历所有活跃进程，最小的xid记录到xmin中，最小的xmin记录到globalxmin中。
- 第三步：如果MyXact中的xmin无效，更新MyPgXact->xmin = TransactionXmin = xmin。（造成cache line invalid，性能问题点，针对这点PG13有大幅度优化）

exec_simple_query
  GetSnapshotData
    MyPgXact->xmin = 0 --> 1840251

exec_simple_query
  PopActiveSnapshot
    SnapshotResetXmin
      MyPgXact->xmin = 1840251 --> 0

exec_simple_query
  PortalRun
    PortalRunMulti 
      GetTransactionSnapshot
        GetSnapshotData
          MyPgXact->xmin = 0 --> 1840251

exec_simple_query
  PortalRun
    PortalRunMulti 
      PopActiveSnapshot
        SnapshotResetXmin
          MyPgXact->xmin = 1840251 --> 0