*
*
* IDENTIFICATION
- * $Header: /cvsroot/pgsql/src/backend/access/transam/varsup.c,v 1.28 2000/04/12 17:14:53 momjian Exp $
+ * $Header: /cvsroot/pgsql/src/backend/access/transam/varsup.c,v 1.29 2000/07/25 20:18:19 tgl Exp $
*
*-------------------------------------------------------------------------
*/
static void GetNewObjectIdBlock(Oid *oid_return, int oid_block_size);
static void VariableRelationGetNextOid(Oid *oid_return);
static void VariableRelationGetNextXid(TransactionId *xidP);
-static void VariableRelationPutNextOid(Oid *oidP);
+static void VariableRelationPutNextOid(Oid oid);
/* ---------------------
* spin lock for oid generation
*/
int OidGenLockId;
+/* ---------------------
+ * pointer to "variable cache" in shared memory (set up by shmem.c)
+ * ---------------------
+ */
VariableCache ShmemVariableCache = NULL;
+
/* ----------------------------------------------------------------
* variable relation query/update routines
* ----------------------------------------------------------------
VariableRelationContents var;
/* ----------------
- * We assume that a spinlock has been acquire to guarantee
+ * We assume that a spinlock has been acquired to guarantee
* exclusive access to the variable relation.
* ----------------
*/
var = (VariableRelationContents) BufferGetBlock(buf);
TransactionIdStore(var->nextXidData, xidP);
+
ReleaseBuffer(buf);
}
VariableRelationContents var;
/* ----------------
- * We assume that a spinlock has been acquire to guarantee
+ * We assume that a spinlock has been acquired to guarantee
* exclusive access to the variable relation.
* ----------------
*/
VariableRelationContents var;
/* ----------------
- * We assume that a spinlock has been acquire to guarantee
+ * We assume that a spinlock has been acquired to guarantee
* exclusive access to the variable relation.
* ----------------
*/
/* ----------------
* if the variable relation is not initialized, then we
* assume we are running at bootstrap time and so we return
- * an invalid object id -- during this time GetNextBootstrapObjectId
- * should be called instead..
+ * an invalid object id (this path should never be taken, probably).
* ----------------
*/
if (!RelationIsValid(VariableRelation))
{
- if (PointerIsValid(oid_return))
- (*oid_return) = InvalidOid;
+ (*oid_return) = InvalidOid;
return;
}
if (!BufferIsValid(buf))
{
SpinRelease(OidGenLockId);
- elog(ERROR, "VariableRelationGetNextXid: ReadBuffer failed");
+ elog(ERROR, "VariableRelationGetNextOid: ReadBuffer failed");
}
var = (VariableRelationContents) BufferGetBlock(buf);
- if (PointerIsValid(oid_return))
- {
-
- /* ----------------
- * nothing up my sleeve... what's going on here is that this code
- * is guaranteed never to be called until all files in data/base/
- * are created, and the template database exists. at that point,
- * we want to append a pg_database tuple. the first time we do
- * this, the oid stored in pg_variable will be bogus, so we use
- * a bootstrap value defined at the top of this file.
- *
- * this comment no longer holds true. This code is called before
- * all of the files in data/base are created and you can't rely
- * on system oid's to be less than BootstrapObjectIdData. mer 9/18/91
- * ----------------
- */
- if (OidIsValid(var->nextOid))
- (*oid_return) = var->nextOid;
- else
- (*oid_return) = BootstrapObjectIdData;
- }
+ (*oid_return) = var->nextOid;
ReleaseBuffer(buf);
}
* --------------------------------
*/
static void
-VariableRelationPutNextOid(Oid *oidP)
+VariableRelationPutNextOid(Oid oid)
{
Buffer buf;
VariableRelationContents var;
/* ----------------
- * We assume that a spinlock has been acquire to guarantee
+ * We assume that a spinlock has been acquired to guarantee
* exclusive access to the variable relation.
* ----------------
*/
if (!RelationIsValid(VariableRelation))
return;
- /* ----------------
- * sanity check
- * ----------------
- */
- if (!PointerIsValid(oidP))
- {
- SpinRelease(OidGenLockId);
- elog(ERROR, "VariableRelationPutNextOid: invalid oid pointer");
- }
-
/* ----------------
* read the variable page, update the nextXid field and
* write the page back out to disk.
var = (VariableRelationContents) BufferGetBlock(buf);
- var->nextOid = (*oidP);
+ var->nextOid = oid;
WriteBuffer(buf);
}
/* ----------------
* GetNewTransactionId
*
- * In the version 2 transaction system, transaction id's are
- * restricted in several ways.
- *
- * -- Old comments removed
+ * Transaction IDs are allocated via a cache in shared memory.
+ * Each time we need more IDs, we advance the "next XID" value
+ * in pg_variable by VAR_XID_PREFETCH and set the cache to
+ * show that many XIDs as available. Then, allocating those XIDs
+ * requires just a spinlock and not a buffer read/write cycle.
*
- * Second, since we may someday preform compression of the data
- * in the log and time relations, we cause the numbering of the
- * transaction ids to begin at 512. This means that some space
- * on the page of the log and time relations corresponding to
- * transaction id's 0 - 510 will never be used. This space is
- * in fact used to store the version number of the postgres
- * transaction log and will someday store compression information
- * about the log. -- this is also old comments...
+ * Since the cache is shared across all backends, cached but unused
+ * XIDs are not lost when a backend exits, only when the postmaster
+ * quits or forces shared memory reinit. So we can afford to have
+ * a pretty big value of VAR_XID_PREFETCH.
*
+ * This code does not worry about initializing the transaction counter
+ * (see transam.c's InitializeTransactionLog() for that). We also
+ * ignore the possibility that the counter could someday wrap around.
* ----------------
*/
* GetNewObjectIdBlock
*
* This support function is used to allocate a block of object ids
- * of the given size. applications wishing to do their own object
- * id assignments should use this
+ * of the given size.
* ----------------
*/
static void
-GetNewObjectIdBlock(Oid *oid_return, /* place to return the new object
- * id */
+GetNewObjectIdBlock(Oid *oid_return, /* place to return the first new
+ * object id */
int oid_block_size) /* number of oids desired */
{
+ Oid firstfreeoid;
Oid nextoid;
/* ----------------
- * SOMEDAY obtain exclusive access to the variable relation page
- * That someday is today -mer 6 Aug 1992
+ * Obtain exclusive access to the variable relation page
* ----------------
*/
SpinAcquire(OidGenLockId);
/* ----------------
* get the "next" oid from the variable relation
- * and give it to the caller.
* ----------------
*/
- VariableRelationGetNextOid(&nextoid);
- if (PointerIsValid(oid_return))
- (*oid_return) = nextoid;
+ VariableRelationGetNextOid(&firstfreeoid);
+
+ /* ----------------
+ * Allocate the range of OIDs to be returned to the caller.
+ *
+ * There are two things going on here.
+ *
+ * One: in a virgin database pg_variable will initially contain zeroes,
+ * so we will read out firstfreeoid = InvalidOid. We want to start
+ * allocating OIDs at BootstrapObjectIdData instead (OIDs below that
+ * are reserved for static assignment in the initial catalog data).
+ *
+ * Two: if a database is run long enough, the OID counter will wrap
+ * around. We must not generate an invalid OID when that happens,
+ * and it seems wise not to generate anything in the reserved range.
+ * Therefore we advance to BootstrapObjectIdData in this case too.
+ *
+ * The comparison here assumes that Oid is an unsigned type.
+ */
+ nextoid = firstfreeoid + oid_block_size;
+
+ if (! OidIsValid(firstfreeoid) || nextoid < firstfreeoid)
+ {
+ /* Initialization or wraparound time, force it up to safe range */
+ firstfreeoid = BootstrapObjectIdData;
+ nextoid = firstfreeoid + oid_block_size;
+ }
+
+ (*oid_return) = firstfreeoid;
/* ----------------
- * now increment the variable relation's next oid
- * field by the size of the oid block requested.
+ * Update the variable relation to show the block range as used.
* ----------------
*/
- nextoid += oid_block_size;
- VariableRelationPutNextOid(&nextoid);
+ VariableRelationPutNextOid(nextoid);
/* ----------------
- * SOMEDAY relinquish our lock on the variable relation page
- * That someday is today -mer 6 Apr 1992
+ * Relinquish our lock on the variable relation page
* ----------------
*/
SpinRelease(OidGenLockId);
* relation by 32 for each backend.
*
* Note: 32 has no special significance. We don't want the
- * number to be too large because if when the backend
+ * number to be too large because when the backend
* terminates, we lose the oids we cached.
*
+ * Question: couldn't we use a shared-memory cache just like XIDs?
+ * That would allow a larger interval between pg_variable updates
+ * without cache losses. Note, however, that we can assign an OID
+ * without even a spinlock from the backend-local OID cache.
+ * Maybe two levels of caching would be good.
* ----------------
*/
int oid_block_size = VAR_OID_PREFETCH;
/* ----------------
- * during bootstrap time, we want to allocate oids
- * one at a time. Otherwise there might be some
- * bootstrap oid's left in the block we prefetch which
- * would be passed out after the variable relation was
- * initialized. This would be bad.
+ * Make sure pg_variable is open.
* ----------------
*/
if (!RelationIsValid(VariableRelation))
void
CheckMaxObjectId(Oid assigned_oid)
{
- Oid pass_oid;
-
+ Oid temp_oid;
if (prefetched_oid_count == 0) /* make sure next/max is set, or
* reload */
- GetNewObjectId(&pass_oid);
+ GetNewObjectId(&temp_oid);
/* ----------------
* If we are below prefetched limits, do nothing
* If we are here, we are coming from a 'copy from' with oid's
*
* If we are in the prefetched oid range, just bump it up
- *
* ----------------
*/
* but we are loading oid's that we can not guarantee are unique
* anyway, so we must rely on the user
*
- *
* We now:
* set the variable relation with the new max oid
* force the backend to reload its oid cache
*
- * We use the oid cache so we don't have to update the variable
- * relation every time
- *
+ * By reloading the oid cache, we don't have to update the variable
+ * relation every time when sequential OIDs are being loaded by COPY.
* ----------------
*/
- pass_oid = assigned_oid;
- VariableRelationPutNextOid(&pass_oid); /* not modified */
- prefetched_oid_count = 0; /* force reload */
- pass_oid = assigned_oid;
- GetNewObjectId(&pass_oid); /* throw away returned oid */
+ SpinAcquire(OidGenLockId);
+ VariableRelationPutNextOid(assigned_oid);
+ SpinRelease(OidGenLockId);
+ prefetched_oid_count = 0; /* force reload */
+ GetNewObjectId(&temp_oid); /* cause target OID to be allocated */
}
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