When testing buffer pool logic, it is useful to be able to evict
arbitrary blocks. This function can be used in SQL queries over the
pg_buffercache view to set up a wide range of buffer pool states. Of
course, buffer mappings might change concurrently so you might evict a
block other than the one you had in mind, and another session might
bring it back in at any time. That's OK for the intended purpose of
setting up developer testing scenarios, and more complicated interlocking
schemes to give stronger guararantees about that would likely be less
flexible for actual testing work anyway. Superuser-only.
Author: Palak Chaturvedi
Author: Thomas Munro (docs, small tweaks)
Reviewed-by: Nitin Jadhav
Reviewed-by: Andres Freund
Reviewed-by: Cary Huang
Reviewed-by: Cédric Villemain
Reviewed-by: Jim Nasby
Reviewed-by: Maxim Orlov
Reviewed-by: Thomas Munro
Reviewed-by: Melanie Plageman
Discussion: https://postgr.es/m/CALfch19pW48ZwWzUoRSpsaV9hqt0UPyaBPC4bOZ4W+c7FF566A@mail.gmail.com
EXTENSION = pg_buffercache
DATA = pg_buffercache--1.2.sql pg_buffercache--1.2--1.3.sql \
pg_buffercache--1.1--1.2.sql pg_buffercache--1.0--1.1.sql \
- pg_buffercache--1.3--1.4.sql
+ pg_buffercache--1.3--1.4.sql pg_buffercache--1.4--1.5.sql
PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time"
REGRESS = pg_buffercache
'pg_buffercache--1.2--1.3.sql',
'pg_buffercache--1.2.sql',
'pg_buffercache--1.3--1.4.sql',
+ 'pg_buffercache--1.4--1.5.sql',
'pg_buffercache.control',
kwargs: contrib_data_args,
)
--- /dev/null
+\echo Use "ALTER EXTENSION pg_buffercache UPDATE TO '1.5'" to load this file. \quit
+
+CREATE FUNCTION pg_buffercache_evict(IN int)
+RETURNS bool
+AS 'MODULE_PATHNAME', 'pg_buffercache_evict'
+LANGUAGE C PARALLEL SAFE VOLATILE STRICT;
# pg_buffercache extension
comment = 'examine the shared buffer cache'
-default_version = '1.4'
+default_version = '1.5'
module_pathname = '$libdir/pg_buffercache'
relocatable = true
PG_FUNCTION_INFO_V1(pg_buffercache_pages);
PG_FUNCTION_INFO_V1(pg_buffercache_summary);
PG_FUNCTION_INFO_V1(pg_buffercache_usage_counts);
+PG_FUNCTION_INFO_V1(pg_buffercache_evict);
Datum
pg_buffercache_pages(PG_FUNCTION_ARGS)
return (Datum) 0;
}
+
+/*
+ * Try to evict a shared buffer.
+ */
+Datum
+pg_buffercache_evict(PG_FUNCTION_ARGS)
+{
+ Buffer buf = PG_GETARG_INT32(0);
+
+ if (!superuser())
+ ereport(ERROR,
+ (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+ errmsg("must be superuser to use pg_buffercache_evict function")));
+
+ if (buf < 1 || buf > NBuffers)
+ elog(ERROR, "bad buffer ID: %d", buf);
+
+ PG_RETURN_BOOL(EvictUnpinnedBuffer(buf));
+}
The pg_buffercache module provides a means for
examining what's happening in the shared buffer cache in real time.
+ It also offers a low-level way to evict data from it, for testing
+ purposes.
+
+
+
This module provides the pg_buffercache_pages()
function (wrapped in the pg_buffercache view),
- the pg_buffercache_summary() function, and the
- pg_buffercache_usage_counts() function.
+ the pg_buffercache_summary() function, the
+ pg_buffercache_usage_counts() function and
+ the pg_buffercache_evict() function.
- By default, use is restricted to superusers and roles with privileges of the
- pg_monitor role. Access may be granted to others
- using GRANT.
+ By default, use of the above functions is restricted to superusers and roles
+ with privileges of the pg_monitor role. Access may be
+ granted to others using GRANT.
+
+
+ The pg_buffercache_evict() function allows a block to
+ be evicted from the buffer pool given a buffer identifier. Use of this
+ function is restricted to superusers only.
-
+
+
The <structname>pg_buffercache_evict</structname> Function
+ The pg_buffercache_evict() function takes a buffer
+ identifier, as shown in the bufferid column of
+ the pg_buffercache view. It returns true on success,
+ and false if the buffer wasn't valid, if it couldn't be evicted because it
+ was pinned, or if it became dirty again after an attempt to write it out.
+ The result is immediately out of date upon return, as the buffer might
+ become valid again at any time due to concurrent activity. The function is
+ intended for developer testing only.
+
+
+
+
Sample Output
{
return DebugPrintBufferRefcount(DatumGetInt32(res));
}
+
+/*
+ * Try to evict the current block in a shared buffer.
+ *
+ * This function is intended for testing/development use only!
+ *
+ * To succeed, the buffer must not be pinned on entry, so if the caller had a
+ * particular block in mind, it might already have been replaced by some other
+ * block by the time this function runs. It's also unpinned on return, so the
+ * buffer might be occupied again by the time control is returned, potentially
+ * even by the same block. This inherent raciness without other interlocking
+ * makes the function unsuitable for non-testing usage.
+ *
+ * Returns true if the buffer was valid and it has now been made invalid.
+ * Returns false if it wasn't valid, if it couldn't be evicted due to a pin,
+ * or if the buffer becomes dirty again while we're trying to write it out.
+ */
+bool
+EvictUnpinnedBuffer(Buffer buf)
+{
+ BufferDesc *desc;
+ uint32 buf_state;
+ bool result;
+
+ /* Make sure we can pin the buffer. */
+ ResourceOwnerEnlarge(CurrentResourceOwner);
+ ReservePrivateRefCountEntry();
+
+ Assert(!BufferIsLocal(buf));
+ desc = GetBufferDescriptor(buf - 1);
+
+ /* Lock the header and check if it's valid. */
+ buf_state = LockBufHdr(desc);
+ if ((buf_state & BM_VALID) == 0)
+ {
+ UnlockBufHdr(desc, buf_state);
+ return false;
+ }
+
+ /* Check that it's not pinned already. */
+ if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
+ {
+ UnlockBufHdr(desc, buf_state);
+ return false;
+ }
+
+ PinBuffer_Locked(desc); /* releases spinlock */
+
+ /* If it was dirty, try to clean it once. */
+ if (buf_state & BM_DIRTY)
+ {
+ LWLockAcquire(BufferDescriptorGetContentLock(desc), LW_SHARED);
+ FlushBuffer(desc, NULL, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
+ LWLockRelease(BufferDescriptorGetContentLock(desc));
+ }
+
+ /* This will return false if it becomes dirty or someone else pins it. */
+ result = InvalidateVictimBuffer(desc);
+
+ UnpinBuffer(desc);
+
+ return result;
+}
extern void LimitAdditionalPins(uint32 *additional_pins);
extern void LimitAdditionalLocalPins(uint32 *additional_pins);
+extern bool EvictUnpinnedBuffer(Buffer buf);
+
/* in buf_init.c */
extern void InitBufferPool(void);
extern Size BufferShmemSize(void);
projects / postgresql.git / commitdiff