Hi hackers,
There is some places where we check that a struct is full of zeroes:
pgstat_report_bgwriter()
pgstat_report_checkpointer()
pgstat_relation_flush_cb()
Indeed that's the way we check if there is pending statistics to flush/report.
The current code is like (taking pgstat_relation_flush_cb() as an example):
"
static const PgStat_TableCounts all_zeroes;
.
.
if (memcmp(&lstats->counts, &all_zeroes,
sizeof(PgStat_TableCounts)) == 0)
.
.
"
The static declaration is not "really" related to the purpose of the function
it is declared in. It's there "only" to initialize a memory area with zeroes
and to use it in the memcmp.
I think it would make sense to "hide" all of this in a new macro, so please find
attached a patch proposal doing so (Andres suggested something along those lines
in [1] IIUC).
The macro is created in pgstat_internal.h as it looks like that "only" the
statistics related code would benefit of it currently (could be moved to other
header file later on if needed).
[1]:
https://www.postgresql.org/message-id/20230105002733.ealhzubjaiqis6ua%40awork3.anarazel.de
Looking forward to your feedback,
Regards,
--
Bertrand Drouvot
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com
>From 2e9071b54f7fd28027759de871b662a3eaa8e4a3 Mon Sep 17 00:00:00 2001
From: Bertrand Drouvot <[email protected]>
Date: Tue, 10 Sep 2024 01:22:02 +0000
Subject: [PATCH v1] define pg_structiszero(addr, s, r)
This new macro allows to test if a memory region (representing a struct "s")
starting at addr and of size sizeof(s) is full of zeroes.
---
src/backend/utils/activity/pgstat_bgwriter.c | 6 ++++--
src/backend/utils/activity/pgstat_checkpointer.c | 9 +++++----
src/backend/utils/activity/pgstat_relation.c | 9 ++++-----
src/include/utils/pgstat_internal.h | 11 +++++++++++
4 files changed, 24 insertions(+), 11 deletions(-)
69.1% src/backend/utils/activity/
30.8% src/include/utils/
diff --git a/src/backend/utils/activity/pgstat_bgwriter.c b/src/backend/utils/activity/pgstat_bgwriter.c
index 364a7a2024..61cbc27760 100644
--- a/src/backend/utils/activity/pgstat_bgwriter.c
+++ b/src/backend/utils/activity/pgstat_bgwriter.c
@@ -30,7 +30,7 @@ void
pgstat_report_bgwriter(void)
{
PgStatShared_BgWriter *stats_shmem = &pgStatLocal.shmem->bgwriter;
- static const PgStat_BgWriterStats all_zeroes;
+ bool is_all_zeroes;
Assert(!pgStatLocal.shmem->is_shutdown);
pgstat_assert_is_up();
@@ -39,7 +39,9 @@ pgstat_report_bgwriter(void)
* This function can be called even if nothing at all has happened. In
* this case, avoid unnecessarily modifying the stats entry.
*/
- if (memcmp(&PendingBgWriterStats, &all_zeroes, sizeof(all_zeroes)) == 0)
+ pg_structiszero(&PendingBgWriterStats, PgStat_BgWriterStats, is_all_zeroes);
+
+ if (is_all_zeroes)
return;
pgstat_begin_changecount_write(&stats_shmem->changecount);
diff --git a/src/backend/utils/activity/pgstat_checkpointer.c b/src/backend/utils/activity/pgstat_checkpointer.c
index bbfc9c7e18..3b9b688ae8 100644
--- a/src/backend/utils/activity/pgstat_checkpointer.c
+++ b/src/backend/utils/activity/pgstat_checkpointer.c
@@ -29,8 +29,7 @@ PgStat_CheckpointerStats PendingCheckpointerStats = {0};
void
pgstat_report_checkpointer(void)
{
- /* We assume this initializes to zeroes */
- static const PgStat_CheckpointerStats all_zeroes;
+ bool is_all_zeroes;
PgStatShared_Checkpointer *stats_shmem = &pgStatLocal.shmem->checkpointer;
Assert(!pgStatLocal.shmem->is_shutdown);
@@ -40,8 +39,10 @@ pgstat_report_checkpointer(void)
* This function can be called even if nothing at all has happened. In
* this case, avoid unnecessarily modifying the stats entry.
*/
- if (memcmp(&PendingCheckpointerStats, &all_zeroes,
- sizeof(all_zeroes)) == 0)
+ pg_structiszero(&PendingCheckpointerStats, PgStat_CheckpointerStats,
+ is_all_zeroes);
+
+ if (is_all_zeroes)
return;
pgstat_begin_changecount_write(&stats_shmem->changecount);
diff --git a/src/backend/utils/activity/pgstat_relation.c b/src/backend/utils/activity/pgstat_relation.c
index 8a3f7d434c..09e519a6ff 100644
--- a/src/backend/utils/activity/pgstat_relation.c
+++ b/src/backend/utils/activity/pgstat_relation.c
@@ -801,7 +801,7 @@ pgstat_twophase_postabort(TransactionId xid, uint16 info,
bool
pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
{
- static const PgStat_TableCounts all_zeroes;
+ bool is_all_zeroes;
Oid dboid;
PgStat_TableStatus *lstats; /* pending stats entry */
PgStatShared_Relation *shtabstats;
@@ -816,11 +816,10 @@ pgstat_relation_flush_cb(PgStat_EntryRef *entry_ref, bool nowait)
* Ignore entries that didn't accumulate any actual counts, such as
* indexes that were opened by the planner but not used.
*/
- if (memcmp(&lstats->counts, &all_zeroes,
- sizeof(PgStat_TableCounts)) == 0)
- {
+ pg_structiszero(&lstats->counts, PgStat_TableCounts, is_all_zeroes);
+
+ if (is_all_zeroes)
return true;
- }
if (!pgstat_lock_entry(entry_ref, nowait))
return false;
diff --git a/src/include/utils/pgstat_internal.h b/src/include/utils/pgstat_internal.h
index bba90e898d..1fd91874d9 100644
--- a/src/include/utils/pgstat_internal.h
+++ b/src/include/utils/pgstat_internal.h
@@ -22,6 +22,17 @@
#include "utils/dsa.h"
+/*
+ * Test if a memory region (representing a struct "s") starting
+ * at addr and of size sizeof(s) is full of zeroes.
+ */
+#define pg_structiszero(addr, s, r) \
+ do { \
+ /* We assume this initializes to zeroes */ \
+ static const s all_zeroes; \
+ r = (memcmp(addr, &all_zeroes, sizeof(all_zeroes)) == 0); \
+ } while (0)
+
/*
* Types related to shared memory storage of statistics.
*
--
2.34.1
