Collect statistics about SLRU caches

There's a number of SLRU caches used to access important data like clog,
commit timestamps, multixact, asynchronous notifications, etc. Until now
we had no easy way to monitor these shared caches, compute hit ratios,
number of reads/writes etc.

This commit extends the statistics collector to track this information
for a predefined list of SLRUs, and also introduces a new system view
pg_stat_slru displaying the data.

The list of built-in SLRUs is fixed, but additional SLRUs may be defined
in extensions. Unfortunately, there's no suitable registry of SLRUs, so
this patch simply defines a fixed list of SLRUs with entries for the
built-in ones and one entry for all additional SLRUs. Extensions adding
their own SLRU are fairly rare, so this seems acceptable.

This patch only allows monitoring of SLRUs, not tuning. The SLRU sizes
are still fixed (hard-coded in the code) and it's not entirely clear
which of the SLRUs might need a GUC to tune size. In a way, allowing us
to determine that is one of the goals of this patch.

Bump catversion as the patch introduces new functions and system view.

Author: Tomas Vondra
Reviewed-by: Alvaro Herrera
Discussion: https://www.postgresql.org/message-id/flat/20200119143707.gyinppnigokesjok@development

1 files changed, 91 insertions, 0 deletions

diff --git a/src/backend/utils/adt/pgstatfuncs.c b/src/backend/utils/adt/pgstatfuncs.c
index 6d66ff8b448..175f4fd26bb 100644
--- a/src/backend/utils/adt/pgstatfuncs.c
+++ b/src/backend/utils/adt/pgstatfuncs.c
@@ -1690,6 +1690,83 @@ pg_stat_get_buf_alloc(PG_FUNCTION_ARGS)
 	PG_RETURN_INT64(pgstat_fetch_global()->buf_alloc);
 }
 
+/*
+ * Returns statistics of SLRU caches.
+ */
+Datum
+pg_stat_get_slru(PG_FUNCTION_ARGS)
+{
+#define PG_STAT_GET_SLRU_COLS	9
+	ReturnSetInfo  *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
+	TupleDesc		tupdesc;
+	Tuplestorestate *tupstore;
+	MemoryContext 	per_query_ctx;
+	MemoryContext 	oldcontext;
+	int				i;
+	PgStat_SLRUStats *stats;
+
+	/* check to see if caller supports us returning a tuplestore */
+	if (rsinfo == NULL || !IsA(rsinfo, ReturnSetInfo))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("set-valued function called in context that cannot accept a set")));
+	if (!(rsinfo->allowedModes & SFRM_Materialize))
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("materialize mode required, but it is not allowed in this context")));
+
+	/* Build a tuple descriptor for our result type */
+	if (get_call_result_type(fcinfo, NULL, &tupdesc) != TYPEFUNC_COMPOSITE)
+		elog(ERROR, "return type must be a row type");
+
+	per_query_ctx = rsinfo->econtext->ecxt_per_query_memory;
+	oldcontext = MemoryContextSwitchTo(per_query_ctx);
+
+	tupstore = tuplestore_begin_heap(true, false, work_mem);
+	rsinfo->returnMode = SFRM_Materialize;
+	rsinfo->setResult = tupstore;
+	rsinfo->setDesc = tupdesc;
+
+	MemoryContextSwitchTo(oldcontext);
+
+	/* request SLRU stats from the stat collector */
+	stats = pgstat_fetch_slru();
+
+	for (i = 0; ; i++)
+	{
+		/* for each row */
+		Datum		values[PG_STAT_GET_SLRU_COLS];
+		bool		nulls[PG_STAT_GET_SLRU_COLS];
+		PgStat_SLRUStats	stat = stats[i];
+		char	   *name;
+
+		name = pgstat_slru_name(i);
+
+		if (!name)
+			break;
+
+		MemSet(values, 0, sizeof(values));
+		MemSet(nulls, 0, sizeof(nulls));
+
+		values[0] = PointerGetDatum(cstring_to_text(name));
+		values[1] = Int64GetDatum(stat.blocks_zeroed);
+		values[2] = Int64GetDatum(stat.blocks_hit);
+		values[3] = Int64GetDatum(stat.blocks_read);
+		values[4] = Int64GetDatum(stat.blocks_written);
+		values[5] = Int64GetDatum(stat.blocks_exists);
+		values[6] = Int64GetDatum(stat.flush);
+		values[7] = Int64GetDatum(stat.truncate);
+		values[8] = Int64GetDatum(stat.stat_reset_timestamp);
+
+		tuplestore_putvalues(tupstore, tupdesc, values, nulls);
+	}
+
+	/* clean up and return the tuplestore */
+	tuplestore_donestoring(tupstore);
+
+	return (Datum) 0;
+}
+
 Datum
 pg_stat_get_xact_numscans(PG_FUNCTION_ARGS)
 {
@@ -1935,6 +2012,20 @@ pg_stat_reset_single_function_counters(PG_FUNCTION_ARGS)
 	PG_RETURN_VOID();
 }
 
+/* Reset SLRU counters (a specific one or all of them). */
+Datum
+pg_stat_reset_slru(PG_FUNCTION_ARGS)
+{
+	char	   *target = NULL;
+
+	if (!PG_ARGISNULL(0))
+		target = text_to_cstring(PG_GETARG_TEXT_PP(0));
+
+	pgstat_reset_slru_counter(target);
+
+	PG_RETURN_VOID();
+}
+
 Datum
 pg_stat_get_archiver(PG_FUNCTION_ARGS)
 {