database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-07-02 09:02:37 +03:00
Code Activity

A session that does not have any live snapshots does not have to be waited for

Browse Source 
when we are waiting for old snapshots to go away during a concurrent index
build.  In particular, this rule lets us avoid waiting for
idle-in-transaction sessions.

This logic could be improved further if we had some way to wake up when
the session we are currently waiting for goes idle-in-transaction.  However
that would be a significantly more complex/invasive patch, so it'll have to
wait for some other day.

Simon Riggs, with some improvements by Tom.
This commit is contained in:
Tom Lane
2009-04-04 17:40:36 +00:00
parent 1c2d408c01
commit c973051ae6
4 changed files with 101 additions and 32 deletions
Download Patch File Download Diff File Expand all files Collapse all files

53
src/backend/storage/ipc/procarray.c
View File

@ -23,7 +23,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.48 2009/03/31 05:18:33 heikki Exp $
* $PostgreSQL: pgsql/src/backend/storage/ipc/procarray.c,v 1.49 2009/04/04 17:40:36 tgl Exp $
*
*-------------------------------------------------------------------------
*/
@ -1022,25 +1022,42 @@ IsBackendPid(int pid)
/*
* GetCurrentVirtualXIDs -- returns an array of currently active VXIDs.
*
* The array is palloc'd and is terminated with an invalid VXID.
* The array is palloc'd. The number of valid entries is returned into *nvxids.
*
* If limitXmin is not InvalidTransactionId, we skip any backends
* with xmin >= limitXmin. If allDbs is false, we skip backends attached
* to other databases. If excludeVacuum isn't zero, we skip processes for
* which (excludeVacuum & vacuumFlags) is not zero. Also, our own process
* is always skipped.
* The arguments allow filtering the set of VXIDs returned. Our own process
* is always skipped. In addition:
* If limitXmin is not InvalidTransactionId, skip processes with
* xmin > limitXmin.
* If excludeXmin0 is true, skip processes with xmin = 0.
* If allDbs is false, skip processes attached to other databases.
* If excludeVacuum isn't zero, skip processes for which
* (vacuumFlags & excludeVacuum) is not zero.
*
* Note: the purpose of the limitXmin and excludeXmin0 parameters is to
* allow skipping backends whose oldest live snapshot is no older than
* some snapshot we have. Since we examine the procarray with only shared
* lock, there are race conditions: a backend could set its xmin just after
* we look. Indeed, on multiprocessors with weak memory ordering, the
* other backend could have set its xmin *before* we look. We know however
* that such a backend must have held shared ProcArrayLock overlapping our
* own hold of ProcArrayLock, else we would see its xmin update. Therefore,
* any snapshot the other backend is taking concurrently with our scan cannot
* consider any transactions as still running that we think are committed
* (since backends must hold ProcArrayLock exclusive to commit).
*/
VirtualTransactionId *
GetCurrentVirtualXIDs(TransactionId limitXmin, bool allDbs, int excludeVacuum)
GetCurrentVirtualXIDs(TransactionId limitXmin, bool excludeXmin0,
bool allDbs, int excludeVacuum,
int *nvxids)
{
VirtualTransactionId *vxids;
ProcArrayStruct *arrayP = procArray;
int count = 0;
int index;
/* allocate result space with room for a terminator */
/* allocate what's certainly enough result space */
vxids = (VirtualTransactionId *)
palloc(sizeof(VirtualTransactionId) * (arrayP->maxProcs + 1));
palloc(sizeof(VirtualTransactionId) * arrayP->maxProcs);
LWLockAcquire(ProcArrayLock, LW_SHARED);
@ -1056,15 +1073,18 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool allDbs, int excludeVacuum)
if (allDbs || proc->databaseId == MyDatabaseId)
{
/* Fetch xmin just once - might change on us? */
/* Fetch xmin just once - might change on us */
TransactionId pxmin = proc->xmin;
if (excludeXmin0 && !TransactionIdIsValid(pxmin))
continue;
/*
* Note that InvalidTransactionId precedes all other XIDs, so a
* proc that hasn't set xmin yet will always be included.
* InvalidTransactionId precedes all other XIDs, so a proc that
* hasn't set xmin yet will not be rejected by this test.
*/
if (!TransactionIdIsValid(limitXmin) ||
TransactionIdPrecedes(pxmin, limitXmin))
TransactionIdPrecedesOrEquals(pxmin, limitXmin))
{
VirtualTransactionId vxid;
@ -1077,10 +1097,7 @@ GetCurrentVirtualXIDs(TransactionId limitXmin, bool allDbs, int excludeVacuum)
LWLockRelease(ProcArrayLock);
/* add the terminator */
vxids[count].backendId = InvalidBackendId;
vxids[count].localTransactionId = InvalidLocalTransactionId;
*nvxids = count;
return vxids;
}