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help / color / mirror / Atom feedFrom: Palak Chaturvedi <[email protected]>
To: [email protected]
To: [email protected]
Subject: Extension Enhancement: Buffer Invalidation in pg_buffercache
Date: Fri, 30 Jun 2023 16:16:50 +0530
Message-ID: <CALfch19pW48ZwWzUoRSpsaV9hqt0UPyaBPC4bOZ4W+c7FF566A@mail.gmail.com> (raw)
I hope this email finds you well. I am excited to share that I have
extended the functionality of the `pg_buffercache` extension by
implementing buffer invalidation capability, as requested by some
PostgreSQL contributors for improved testing scenarios.
This marks my first time submitting a patch to pgsql-hackers, and I am
eager to receive your expert feedback on the changes made. Your
insights are invaluable, and any review or comments you provide will
be greatly appreciated.
The primary objective of this enhancement is to enable explicit buffer
invalidation within the `pg_buffercache` extension. By doing so, we
can simulate scenarios where buffers are invalidated and observe the
resulting behavior in PostgreSQL.
As part of this patch, a new function or mechanism has been introduced
to facilitate buffer invalidation. I would like to hear your thoughts
on whether this approach provides a good user interface for this
functionality. Additionally, I seek your evaluation of the buffer
locking protocol employed in the extension to ensure its correctness
and efficiency.
Please note that I plan to add comprehensive documentation once the
details of this enhancement are agreed upon. This documentation will
serve as a valuable resource for users and contributors alike. I
believe that your expertise will help uncover any potential issues and
opportunities for further improvement.
I have attached the patch file to this email for your convenience.
Your valuable time and consideration in reviewing this extension are
sincerely appreciated.
Thank you for your continued support and guidance. I am looking
forward to your feedback and collaboration in enhancing the PostgreSQL
ecosystem.
The working of the extension:
1. Creating the extension pg_buffercache and then call select query on
a table and note the buffer to be cleared.
pgbench=# create extension pg_buffercache;
CREATE EXTENSION
pgbench=# select count(*) from pgbench_accounts;
count
--------
100000
(1 row)
pgbench=# SELECT *
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
bufferid | relfilenode | reltablespace | reldatabase | relforknumber
| relblocknumber | isdirty | usagecount | pinning_backends
----------+-------------+---------------+-------------+---------------+----------------+---------+------------+------------------
233 | 16397 | 1663 | 16384 | 0
| 0 | f | 1 | 0
234 | 16397 | 1663 | 16384 | 0
| 1 | f | 1 | 0
235 | 16397 | 1663 | 16384 | 0
| 2 | f | 1 | 0
236 | 16397 | 1663 | 16384 | 0
| 3 | f | 1 | 0
237 | 16397 | 1663 | 16384 | 0
| 4 | f | 1 | 0
2. Clearing a single buffer by entering the bufferid.
pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1660
(1 row)
pgbench=# select pg_buffercache_invalidate(233);
pg_buffercache_invalidate
---------------------------
t
(1 row)
pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1659
(1 row)
3. Clearing the entire buffer for a relation using the function.
pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1659
(1 row)
pgbench=# select count(pg_buffercache_invalidate(bufferid)) from
pg_buffercache where relfilenode =
pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
1659
(1 row)
pgbench=# SELECT count(*)
FROM pg_buffercache
WHERE relfilenode = pg_relation_filenode('pgbench_accounts'::regclass);
count
-------
0
(1 row)
Best regards,
Palak
Attachments:
[application/octet-stream] v1-0001-Invalidate-Buffer-By-Bufnum.patch (5.4K, ../CALfch19pW48ZwWzUoRSpsaV9hqt0UPyaBPC4bOZ4W+c7FF566A@mail.gmail.com/2-v1-0001-Invalidate-Buffer-By-Bufnum.patch)
download | inline diff:
From 6bab3ba50b43c1bf3aa4ab28ec51b98bd50ac444 Mon Sep 17 00:00:00 2001
From: Palak <[email protected]>
Date: Fri, 30 Jun 2023 08:21:06 +0000
Subject: [PATCH v1] Invalidate Buffer By Bufnum
---
contrib/pg_buffercache/Makefile | 2 +-
.../pg_buffercache--1.4--1.5.sql | 6 ++
contrib/pg_buffercache/pg_buffercache.control | 2 +-
contrib/pg_buffercache/pg_buffercache_pages.c | 27 ++++++++
src/backend/storage/buffer/bufmgr.c | 64 +++++++++++++++++++
src/include/storage/bufmgr.h | 3 +
6 files changed, 102 insertions(+), 2 deletions(-)
create mode 100644 contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql
diff --git a/contrib/pg_buffercache/Makefile b/contrib/pg_buffercache/Makefile
index d6b58d4da9..eae65ead9e 100644
--- a/contrib/pg_buffercache/Makefile
+++ b/contrib/pg_buffercache/Makefile
@@ -8,7 +8,7 @@ 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--1.3--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
diff --git a/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql b/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql
new file mode 100644
index 0000000000..c7e47456d7
--- /dev/null
+++ b/contrib/pg_buffercache/pg_buffercache--1.4--1.5.sql
@@ -0,0 +1,6 @@
+\echo Use "ALTER EXTENSION pg_buffercache UPDATE TO '1.5'" to load this file. \quit
+
+CREATE FUNCTION pg_buffercache_invalidate(IN int)
+RETURNS bool
+AS 'MODULE_PATHNAME', 'pg_buffercache_invalidate'
+LANGUAGE C PARALLEL SAFE;
diff --git a/contrib/pg_buffercache/pg_buffercache.control b/contrib/pg_buffercache/pg_buffercache.control
index a82ae5f9bb..5ee875f77d 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 = '1.4'
+default_version = '1.5'
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 3316732365..6dd02fe6af 100644
--- a/contrib/pg_buffercache/pg_buffercache_pages.c
+++ b/contrib/pg_buffercache/pg_buffercache_pages.c
@@ -64,6 +64,33 @@ 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_invalidate);
+Datum
+pg_buffercache_invalidate(PG_FUNCTION_ARGS)
+{
+ Buffer bufnum;
+ bool result;
+
+ if (PG_ARGISNULL(0))
+ {
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("buffernum cannot be NULL")));
+ }
+
+ bufnum = PG_GETARG_INT32(0);
+ if (bufnum < 0 || bufnum > NBuffers)
+ {
+ ereport(ERROR,
+ (errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+ errmsg("buffernum is not valid")));
+
+ }
+
+ result = TryInvalidateBuffer(bufnum);
+ PG_RETURN_BOOL(result);
+}
+
Datum
pg_buffercache_pages(PG_FUNCTION_ARGS)
{
diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c
index 3c59bbd04e..376afcf996 100644
--- a/src/backend/storage/buffer/bufmgr.c
+++ b/src/backend/storage/buffer/bufmgr.c
@@ -5592,3 +5592,67 @@ TestForOldSnapshot_impl(Snapshot snapshot, Relation relation)
(errcode(ERRCODE_SNAPSHOT_TOO_OLD),
errmsg("snapshot too old")));
}
+
+/*
+Try Invalidating a buffer using bufnum.
+If the buffer is invalid, the function returns false.
+The function checks for dirty buffer and flushes the dirty buffer before invalidating.
+If the buffer is still dirty it returns false.
+*/
+bool
+TryInvalidateBuffer(Buffer bufnum)
+{
+ BufferDesc *bufHdr = GetBufferDescriptor(bufnum - 1);
+ uint32 buf_state;
+
+ ReservePrivateRefCountEntry();
+
+ buf_state = LockBufHdr(bufHdr);
+ if ((buf_state & BM_VALID) == BM_VALID)
+ {
+ /*
+ * The buffer is pinned therefore cannot invalidate.
+ */
+ if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
+ {
+ UnlockBufHdr(bufHdr, buf_state);
+ return false;
+ }
+ if ((buf_state & BM_DIRTY) == BM_DIRTY)
+ {
+ /*
+ * Try once to flush the dirty buffer.
+ */
+ PinBuffer_Locked(bufHdr);
+ LWLockAcquire(BufferDescriptorGetContentLock(bufHdr), LW_SHARED);
+ FlushBuffer(bufHdr, NULL, IOOBJECT_RELATION, IOCONTEXT_NORMAL);
+ LWLockRelease(BufferDescriptorGetContentLock(bufHdr));
+ UnpinBuffer(bufHdr);
+ buf_state = LockBufHdr(bufHdr);
+ if (BUF_STATE_GET_REFCOUNT(buf_state) > 0)
+ {
+ UnlockBufHdr(bufHdr, buf_state);
+ return false;
+ }
+
+ /*
+ * If its dirty again or not valid anymore give up.
+ */
+
+ if ((buf_state & (BM_DIRTY | BM_VALID)) != (BM_VALID))
+ {
+ UnlockBufHdr(bufHdr, buf_state);
+ return false;
+ }
+
+ }
+
+ InvalidateBuffer(bufHdr);
+ return true;
+ }
+ else
+ {
+ UnlockBufHdr(bufHdr, buf_state);
+ return false;
+ }
+}
diff --git a/src/include/storage/bufmgr.h b/src/include/storage/bufmgr.h
index 0f5fb6be00..4ba3d9089b 100644
--- a/src/include/storage/bufmgr.h
+++ b/src/include/storage/bufmgr.h
@@ -252,6 +252,9 @@ extern bool BgBufferSync(struct WritebackContext *wb_context);
extern void TestForOldSnapshot_impl(Snapshot snapshot, Relation relation);
+
+extern bool TryInvalidateBuffer(Buffer bufnum);
+
/* in buf_init.c */
extern void InitBufferPool(void);
extern Size BufferShmemSize(void);
--
2.25.1
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Subject: Re: Extension Enhancement: Buffer Invalidation in pg_buffercache
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