Optimize check for pending backend IO stats

This commit changes the backend stats code so as we rely on a single
boolean rather than a repeated check based on pg_memory_is_all_zeros()
in the code, making it cheaper should PgStat_PendingIO get bigger in
size.

The frequency of backend stats reports is not a bottleneck, but there is
no reason to not make that cheaper, and the logic is simple as the only
entry points updating backend IO stats are pgstat_count_backend_io_op()
and pgstat_count_backend_io_op_time().

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Xuneng Zhou <xunengzhou@gmail.com>
Discussion: https://postgr.es/m/Z8WYf1jyy4MwOveQ@ip-10-97-1-34.eu-west-3.compute.internal

diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index 5518a18e0607fb0ea6874b2dbc7d5ff730127b2e..187c5c76e1eda3b1c80b58e3eb571680df900d32 100644 (file)
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -37,6 +37,7 @@
  * memory allocation.
  */
 static PgStat_BackendPending PendingBackendStats;
+static bool backend_has_iostats = false;
 
 /*
  * WAL usage counters saved from pgWalUsage at the previous call to
@@ -63,6 +64,8 @@ pgstat_count_backend_io_op_time(IOObject io_object, IOContext io_context,
 
    INSTR_TIME_ADD(PendingBackendStats.pending_io.pending_times[io_object][io_context][io_op],
                   io_time);
+
+   backend_has_iostats = true;
 }
 
 void
@@ -76,6 +79,8 @@ pgstat_count_backend_io_op(IOObject io_object, IOContext io_context,
 
    PendingBackendStats.pending_io.counts[io_object][io_context][io_op] += cnt;
    PendingBackendStats.pending_io.bytes[io_object][io_context][io_op] += bytes;
+
+   backend_has_iostats = true;
 }
 
 /*
@@ -158,8 +163,7 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
     * statistics.  In this case, avoid unnecessarily modifying the stats
     * entry.
     */
-   if (pg_memory_is_all_zeros(&PendingBackendStats.pending_io,
-                              sizeof(struct PgStat_PendingIO)))
+   if (!backend_has_iostats)
        return;
 
    shbackendent = (PgStatShared_Backend *) entry_ref->shared_stats;
@@ -190,6 +194,8 @@ pgstat_flush_backend_entry_io(PgStat_EntryRef *entry_ref)
     * Clear out the statistics buffer, so it can be re-used.
     */
    MemSet(&PendingBackendStats.pending_io, 0, sizeof(PgStat_PendingIO));
+
+   backend_has_iostats = false;
 }
 
 /*
@@ -259,9 +265,7 @@ pgstat_flush_backend(bool nowait, bits32 flags)
        return false;
 
    /* Some IO data pending? */
-   if ((flags & PGSTAT_BACKEND_FLUSH_IO) &&
-       !pg_memory_is_all_zeros(&PendingBackendStats.pending_io,
-                               sizeof(struct PgStat_PendingIO)))
+   if ((flags & PGSTAT_BACKEND_FLUSH_IO) && backend_has_iostats)
        has_pending_data = true;
 
    /* Some WAL data pending? */
@@ -298,9 +302,7 @@ pgstat_backend_have_pending_cb(void)
    if (!pgstat_tracks_backend_bktype(MyBackendType))
        return false;
 
-   return (!pg_memory_is_all_zeros(&PendingBackendStats,
-                                   sizeof(struct PgStat_BackendPending)) ||
-           pgstat_backend_wal_have_pending());
+   return (backend_has_iostats || pgstat_backend_wal_have_pending());
 }
 
 /*
@@ -335,6 +337,7 @@ pgstat_create_backend(ProcNumber procnum)
    pgstat_unlock_entry(entry_ref);
 
    MemSet(&PendingBackendStats, 0, sizeof(PgStat_BackendPending));
+   backend_has_iostats = false;
 
    /*
     * Initialize prevBackendWalUsage with pgWalUsage so that