Re: pg_stat_bgwriter.buffers_backend is pretty meaningless (and more?)

On 2022-07-14 18:44:48 -0400, Melanie Plageman wrote:

> @@ -1427,8 +1445,10 @@ pgstat_read_statsfile(void)
>       FILE       *fpin;
>       int32           format_id;
>       bool            found;
> +     PgStat_BackendIOPathOps io_stats;
>       const char *statfile = PGSTAT_STAT_PERMANENT_FILENAME;
>       PgStat_ShmemControl *shmem = pgStatLocal.shmem;
> +     PgStatShared_BackendIOPathOps *io_stats_shmem = &shmem->io_ops;
> 
>       /* shouldn't be called from postmaster */
>       Assert(IsUnderPostmaster || !IsPostmasterEnvironment);
> @@ -1486,6 +1506,22 @@ pgstat_read_statsfile(void)
>       if (!read_chunk_s(fpin, &shmem->checkpointer.stats))
>               goto error;
> 
> +     /*
> +      * Read IO Operations stats struct
> +      */
> +     if (!read_chunk_s(fpin, &io_stats))
> +             goto error;
> +
> +     io_stats_shmem->stat_reset_timestamp = io_stats.stat_reset_timestamp;
> +
> +     for (int i = 0; i < BACKEND_NUM_TYPES; i++)
> +     {
> +             PgStat_IOPathOps *stats = &io_stats.stats[i];
> +             PgStatShared_IOPathOps *stats_shmem = &io_stats_shmem->stats[i];
> +
> +             memcpy(stats_shmem->data, stats->data, sizeof(stats->data));
> +     }

Why can't the data be read directly into shared memory?

> +void
> +pgstat_io_ops_snapshot_cb(void)
> +{
> +     PgStatShared_BackendIOPathOps *all_backend_stats_shmem = &pgStatLocal.shmem->io_ops;
> +     PgStat_BackendIOPathOps *all_backend_stats_snap = &pgStatLocal.snapshot.io_ops;
> +
> +     for (int i = 0; i < BACKEND_NUM_TYPES; i++)
> +     {
> +             PgStatShared_IOPathOps *stats_shmem = &all_backend_stats_shmem->stats[i];
> +             PgStat_IOPathOps *stats_snap = &all_backend_stats_snap->stats[i];
> +
> +             LWLockAcquire(&stats_shmem->lock, LW_EXCLUSIVE);

Why acquire the same lock repeatedly for each type, rather than once for
the whole?