From 9caf0911bca9be5b630f8086befc861331e91c5a Mon Sep 17 00:00:00 2001 From: Melih Mutlu Date: Sat, 10 Sep 2022 02:08:24 +0300 Subject: [PATCH v15 1/2] pg_buffercache: Add pg_buffercache_summary() Using pg_buffercache_summary() is significantly cheaper than querying pg_buffercache and summarizing in SQL. Author: Melih Mutlu Reviewed-by: Andres Freund Reviewed-by: Aleksander Alekseev Reviewed-by: Zhang Mingli Discussion: https://postgr.es/m/CAGPVpCQAXYo54Q%3D8gqBsS%3Du0uk9qhnnq4%2B710BtUhUisX1XGEg%40mail.gmail.com --- contrib/pg_buffercache/Makefile | 3 +- .../expected/pg_buffercache.out | 9 ++ contrib/pg_buffercache/meson.build | 1 + .../pg_buffercache--1.3--1.4.sql | 13 ++ contrib/pg_buffercache/pg_buffercache.control | 2 +- contrib/pg_buffercache/pg_buffercache_pages.c | 84 +++++++++++++ contrib/pg_buffercache/sql/pg_buffercache.sql | 5 + doc/src/sgml/pgbuffercache.sgml | 111 +++++++++++++++++- 8 files changed, 221 insertions(+), 7 deletions(-) create mode 100644 contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql diff --git a/contrib/pg_buffercache/Makefile b/contrib/pg_buffercache/Makefile index d74b3e853c6..d6b58d4da94 100644 --- a/contrib/pg_buffercache/Makefile +++ b/contrib/pg_buffercache/Makefile @@ -7,7 +7,8 @@ OBJS = \ 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--1.1--1.2.sql pg_buffercache--1.0--1.1.sql \ + pg_buffercache--1.3--1.4.sql PGFILEDESC = "pg_buffercache - monitoring of shared buffer cache in real-time" REGRESS = pg_buffercache diff --git a/contrib/pg_buffercache/expected/pg_buffercache.out b/contrib/pg_buffercache/expected/pg_buffercache.out index 138556efc9f..6798eb74322 100644 --- a/contrib/pg_buffercache/expected/pg_buffercache.out +++ b/contrib/pg_buffercache/expected/pg_buffercache.out @@ -8,3 +8,12 @@ from pg_buffercache; t (1 row) +select buffers_used + buffers_unused > 0, + buffers_dirty <= buffers_used, + buffers_pinned <= buffers_used +from pg_buffercache_summary(); + ?column? | ?column? | ?column? +----------+----------+---------- + t | t | t +(1 row) + diff --git a/contrib/pg_buffercache/meson.build b/contrib/pg_buffercache/meson.build index dd9948e5f0b..ff7f9162cee 100644 --- a/contrib/pg_buffercache/meson.build +++ b/contrib/pg_buffercache/meson.build @@ -19,6 +19,7 @@ install_data( 'pg_buffercache--1.1--1.2.sql', 'pg_buffercache--1.2--1.3.sql', 'pg_buffercache--1.2.sql', + 'pg_buffercache--1.3--1.4.sql', 'pg_buffercache.control', kwargs: contrib_data_args, ) diff --git a/contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql b/contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql new file mode 100644 index 00000000000..77e250b430f --- /dev/null +++ b/contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql @@ -0,0 +1,13 @@ +/* contrib/pg_buffercache/pg_buffercache--1.3--1.4.sql */ + +-- complain if script is sourced in psql, rather than via ALTER EXTENSION +\echo Use "ALTER EXTENSION pg_buffercache UPDATE TO '1.4'" to load this file. \quit + +CREATE FUNCTION pg_buffercache_summary() +RETURNS TABLE (buffers_used int4, buffers_unused int4, buffers_dirty int4, + buffers_pinned int4, usagecount_avg real) +AS 'MODULE_PATHNAME', 'pg_buffercache_summary' +LANGUAGE C PARALLEL SAFE; + +-- Don't want these to be available to public. +REVOKE ALL ON FUNCTION pg_buffercache_summary() FROM PUBLIC; diff --git a/contrib/pg_buffercache/pg_buffercache.control b/contrib/pg_buffercache/pg_buffercache.control index 8c060ae9abf..a82ae5f9bb5 100644 --- a/contrib/pg_buffercache/pg_buffercache.control +++ b/contrib/pg_buffercache/pg_buffercache.control @@ -1,5 +1,5 @@ # pg_buffercache extension comment = 'examine the shared buffer cache' -default_version = '1.3' +default_version = '1.4' module_pathname = '$libdir/pg_buffercache' relocatable = true diff --git a/contrib/pg_buffercache/pg_buffercache_pages.c b/contrib/pg_buffercache/pg_buffercache_pages.c index c5754ea9fa5..a8f9750ac42 100644 --- a/contrib/pg_buffercache/pg_buffercache_pages.c +++ b/contrib/pg_buffercache/pg_buffercache_pages.c @@ -17,6 +17,7 @@ #define NUM_BUFFERCACHE_PAGES_MIN_ELEM 8 #define NUM_BUFFERCACHE_PAGES_ELEM 9 +#define NUM_BUFFERCACHE_SUMMARY_ELEM 5 PG_MODULE_MAGIC; @@ -59,6 +60,7 @@ typedef struct * relation node/tablespace/database/blocknum and dirty indicator. */ PG_FUNCTION_INFO_V1(pg_buffercache_pages); +PG_FUNCTION_INFO_V1(pg_buffercache_summary); Datum pg_buffercache_pages(PG_FUNCTION_ARGS) @@ -237,3 +239,85 @@ pg_buffercache_pages(PG_FUNCTION_ARGS) else SRF_RETURN_DONE(funcctx); } + +Datum +pg_buffercache_summary(PG_FUNCTION_ARGS) +{ + Datum result; + TupleDesc tupledesc; + HeapTuple tuple; + Datum values[NUM_BUFFERCACHE_SUMMARY_ELEM]; + bool nulls[NUM_BUFFERCACHE_SUMMARY_ELEM]; + + int32 buffers_used = 0; + int32 buffers_unused = 0; + int32 buffers_dirty = 0; + int32 buffers_pinned = 0; + float usagecount_avg = 0; + + /* Construct a tuple descriptor for the result rows. */ + tupledesc = CreateTemplateTupleDesc(NUM_BUFFERCACHE_SUMMARY_ELEM); + TupleDescInitEntry(tupledesc, (AttrNumber) 1, "buffers_used", + INT4OID, -1, 0); + TupleDescInitEntry(tupledesc, (AttrNumber) 2, "buffers_unused", + INT4OID, -1, 0); + TupleDescInitEntry(tupledesc, (AttrNumber) 3, "buffers_dirty", + INT4OID, -1, 0); + TupleDescInitEntry(tupledesc, (AttrNumber) 4, "buffers_pinned", + INT4OID, -1, 0); + TupleDescInitEntry(tupledesc, (AttrNumber) 5, "usagecount_avg", + FLOAT4OID, -1, 0); + + BlessTupleDesc(tupledesc); + + for (int i = 0; i < NBuffers; i++) + { + BufferDesc *bufHdr; + uint32 buf_state; + + /* + * This function summarizes the state of all headers. Locking the + * buffer headers wouldn't provide an improved result as the state of + * the buffer can still change after we release the lock and it'd + * noticeably increase the cost of the function. + */ + bufHdr = GetBufferDescriptor(i); + buf_state = pg_atomic_read_u32(&bufHdr->state); + + if (buf_state & BM_VALID) + { + buffers_used++; + usagecount_avg += BUF_STATE_GET_USAGECOUNT(buf_state); + + if (buf_state & BM_DIRTY) + buffers_dirty++; + } + else + buffers_unused++; + + if (BUF_STATE_GET_REFCOUNT(buf_state) > 0) + buffers_pinned++; + } + + memset(nulls, 0, sizeof(nulls)); + values[0] = Int32GetDatum(buffers_used); + values[1] = Int32GetDatum(buffers_unused); + values[2] = Int32GetDatum(buffers_dirty); + values[3] = Int32GetDatum(buffers_pinned); + + if (buffers_used != 0) + { + usagecount_avg = usagecount_avg / buffers_used; + values[4] = Float4GetDatum(usagecount_avg); + } + else + { + nulls[4] = true; + } + + /* Build and return the tuple. */ + tuple = heap_form_tuple(tupledesc, values, nulls); + result = HeapTupleGetDatum(tuple); + + PG_RETURN_DATUM(result); +} diff --git a/contrib/pg_buffercache/sql/pg_buffercache.sql b/contrib/pg_buffercache/sql/pg_buffercache.sql index e1ba6f7e8d4..897dfbc9245 100644 --- a/contrib/pg_buffercache/sql/pg_buffercache.sql +++ b/contrib/pg_buffercache/sql/pg_buffercache.sql @@ -4,3 +4,8 @@ select count(*) = (select setting::bigint from pg_settings where name = 'shared_buffers') from pg_buffercache; + +select buffers_used + buffers_unused > 0, + buffers_dirty <= buffers_used, + buffers_pinned <= buffers_used +from pg_buffercache_summary(); diff --git a/doc/src/sgml/pgbuffercache.sgml b/doc/src/sgml/pgbuffercache.sgml index a06fd3e26de..6c38ee5d5eb 100644 --- a/doc/src/sgml/pgbuffercache.sgml +++ b/doc/src/sgml/pgbuffercache.sgml @@ -9,7 +9,7 @@ The pg_buffercache module provides a means for - examining what's happening in the shared buffer cache in real time. + examining what's happening in the shared buffer in real time. @@ -17,10 +17,19 @@ - The module provides a C function pg_buffercache_pages - that returns a set of records, plus a view - pg_buffercache that wraps the function for - convenient use. + The module provides C functions pg_buffercache_pages + and pg_buffercache_summary. + + + + The pg_buffercache_pages function + returns a set of records, plus a view pg_buffercache that wraps the function for + convenient use is provided. + + + + The pg_buffercache_summary function returns a table with a single row + that contains summarized and aggregated information about shared buffer. @@ -164,6 +173,91 @@ + + The <structname>pg_buffercache_summary</structname> Function + + + The definitions of the columns exposed by the function are shown in . + + + + <structname>pg_buffercachesummary</structname> Columns + + + + + Column Type + + + Description + + + + + + + + buffers_used int4 + + + Number of shared buffers currently being used + + + + + + buffers_unused int4 + + + Number of shared buffers that not currently being used + + + + + + buffers_dirty int4 + + + Number of dirty shared buffers + + + + + + buffers_pinned int4 + + + Number of shared buffers that has a pinned backend + + + + + + usagecount_avg float + + + Average usagecount of used shared buffers + + + + +
+ + + There is a single row to show summarized information of all shared buffers. + pg_buffercache_summary is not interested + in the state of each shared buffer, only shows aggregated information. + + + + The pg_buffercache_summary doesn't provide a result + that is consistent across all buffers. This is intentional. The purpose + of this function is to provide a general idea about the state of shared + buffers as fast as possible. Additionally, pg_buffercache_summary + allocates much less memory. + + + Sample Output @@ -191,6 +285,13 @@ regression=# SELECT n.nspname, c.relname, count(*) AS buffers public | gin_test_tbl | 188 public | spgist_text_tbl | 182 (10 rows) + + +regression=# SELECT * FROM pg_buffercache_summary(); + buffers_used | buffers_unused | buffers_dirty | buffers_pinned | usagecount_avg +--------------+----------------+---------------+----------------+---------------- + 248 | 2096904 | 39 | 0 | 3.141129 +(1 row) -- 2.38.0