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snapshot scalability: Introduce dense array of in-progress xids.

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The new array contains the xids for all connected backends / in-use
PGPROC entries in a dense manner (in contrast to the PGPROC/PGXACT
arrays which can have unused entries interspersed).

This improves performance because GetSnapshotData() always needs to
scan the xids of all live procarray entries and now there's no need to
go through the procArray->pgprocnos indirection anymore.

As the set of running top-level xids changes rarely, compared to the
number of snapshots taken, this substantially increases the likelihood
of most data required for a snapshot being in l2 cache.  In
read-mostly workloads scanning the xids[] array will sufficient to
build a snapshot, as most backends will not have an xid assigned.

To keep the xid array dense ProcArrayRemove() needs to move entries
behind the to-be-removed proc's one further up in the array. Obviously
moving array entries cannot happen while a backend sets it
xid. I.e. locking needs to prevent that array entries are moved while
a backend modifies its xid.

To avoid locking ProcArrayLock in GetNewTransactionId() - a fairly hot
spot already - ProcArrayAdd() / ProcArrayRemove() now needs to hold
XidGenLock in addition to ProcArrayLock. Adding / Removing a procarray
entry is not a very frequent operation, even taking 2PC into account.

Due to the above, the dense array entries can only be read or modified
while holding ProcArrayLock and/or XidGenLock. This prevents a
concurrent ProcArrayRemove() from shifting the dense array while it is
accessed concurrently.

While the new dense array is very good when needing to look at all
xids it is less suitable when accessing a single backend's xid. In
particular it would be problematic to have to acquire a lock to access
a backend's own xid. Therefore a backend's xid is not just stored in
the dense array, but also in PGPROC. This also allows a backend to
only access the shared xid value when the backend had acquired an
xid.

The infrastructure added in this commit will be used for the remaining
PGXACT fields in subsequent commits. They are kept separate to make
review easier.

Author: Andres Freund <andres@anarazel.de>
Reviewed-By: Robert Haas <robertmhaas@gmail.com>
Reviewed-By: Thomas Munro <thomas.munro@gmail.com>
Reviewed-By: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/20200301083601.ews6hz5dduc3w2se@alap3.anarazel.de
This commit is contained in:
Andres Freund
2020-08-14 12:15:38 -07:00
parent 2ba5b2db79
commit 941697c3c1
11 changed files with 332 additions and 159 deletions
Download Patch File Download Diff File Expand all files Collapse all files

31
src/backend/access/transam/twophase.c
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@@ -351,7 +351,7 @@ AtAbort_Twophase(void)
/*
* This is called after we have finished transferring state to the prepared
* PGXACT entry.
* PGPROC entry.
*/
void
PostPrepare_Twophase(void)
@@ -463,7 +463,7 @@ MarkAsPreparingGuts(GlobalTransaction gxact, TransactionId xid, const char *gid,
proc->waitStatus = PROC_WAIT_STATUS_OK;
/* We set up the gxact's VXID as InvalidBackendId/XID */
proc->lxid = (LocalTransactionId) xid;
pgxact->xid = xid;
proc->xid = xid;
Assert(proc->xmin == InvalidTransactionId);
proc->delayChkpt = false;
pgxact->vacuumFlags = 0;
@@ -768,7 +768,6 @@ pg_prepared_xact(PG_FUNCTION_ARGS)
{
GlobalTransaction gxact = &status->array[status->currIdx++];
PGPROC *proc = &ProcGlobal->allProcs[gxact->pgprocno];
PGXACT *pgxact = &ProcGlobal->allPgXact[gxact->pgprocno];
Datum values[5];
bool nulls[5];
HeapTuple tuple;
@@ -783,7 +782,7 @@ pg_prepared_xact(PG_FUNCTION_ARGS)
MemSet(values, 0, sizeof(values));
MemSet(nulls, 0, sizeof(nulls));
values[0] = TransactionIdGetDatum(pgxact->xid);
values[0] = TransactionIdGetDatum(proc->xid);
values[1] = CStringGetTextDatum(gxact->gid);
values[2] = TimestampTzGetDatum(gxact->prepared_at);
values[3] = ObjectIdGetDatum(gxact->owner);
@@ -829,9 +828,8 @@ TwoPhaseGetGXact(TransactionId xid, bool lock_held)
for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
{
GlobalTransaction gxact = TwoPhaseState->prepXacts[i];
PGXACT *pgxact = &ProcGlobal->allPgXact[gxact->pgprocno];
if (pgxact->xid == xid)
if (gxact->xid == xid)
{
result = gxact;
break;
@@ -987,8 +985,7 @@ void
StartPrepare(GlobalTransaction gxact)
{
PGPROC *proc = &ProcGlobal->allProcs[gxact->pgprocno];
PGXACT *pgxact = &ProcGlobal->allPgXact[gxact->pgprocno];
TransactionId xid = pgxact->xid;
TransactionId xid = gxact->xid;
TwoPhaseFileHeader hdr;
TransactionId *children;
RelFileNode *commitrels;
@@ -1140,15 +1137,15 @@ EndPrepare(GlobalTransaction gxact)
/*
* Mark the prepared transaction as valid. As soon as xact.c marks
* MyPgXact as not running our XID (which it will do immediately after
* MyProc as not running our XID (which it will do immediately after
* this function returns), others can commit/rollback the xact.
*
* NB: a side effect of this is to make a dummy ProcArray entry for the
* prepared XID. This must happen before we clear the XID from MyPgXact,
* else there is a window where the XID is not running according to
* TransactionIdIsInProgress, and onlookers would be entitled to assume
* the xact crashed. Instead we have a window where the same XID appears
* twice in ProcArray, which is OK.
* prepared XID. This must happen before we clear the XID from MyProc /
* ProcGlobal->xids[], else there is a window where the XID is not running
* according to TransactionIdIsInProgress, and onlookers would be entitled
* to assume the xact crashed. Instead we have a window where the same
* XID appears twice in ProcArray, which is OK.
*/
MarkAsPrepared(gxact, false);
@@ -1404,7 +1401,6 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
{
GlobalTransaction gxact;
PGPROC *proc;
PGXACT *pgxact;
TransactionId xid;
char *buf;
char *bufptr;
@@ -1423,8 +1419,7 @@ FinishPreparedTransaction(const char *gid, bool isCommit)
*/
gxact = LockGXact(gid, GetUserId());
proc = &ProcGlobal->allProcs[gxact->pgprocno];
pgxact = &ProcGlobal->allPgXact[gxact->pgprocno];
xid = pgxact->xid;
xid = gxact->xid;
/*
* Read and validate 2PC state data. State data will typically be stored
@@ -1726,7 +1721,7 @@ CheckPointTwoPhase(XLogRecPtr redo_horizon)
for (i = 0; i < TwoPhaseState->numPrepXacts; i++)
{
/*
* Note that we are using gxact not pgxact so this works in recovery
* Note that we are using gxact not PGPROC so this works in recovery
* also
*/
GlobalTransaction gxact = TwoPhaseState->prepXacts[i];