Re: shared-memory based stats collector - v70

Hi,

On 8/18/22 1:30 AM, Andres Freund wrote:
> Hi,
>
> On 2022-08-17 15:46:42 -0400, Greg Stark wrote:
>> Isn't there also a local hash table used to find the entries to reduce
>> traffic on the shared hash table? Even if you don't take a snapshot
>> does it get entered there? There are definitely still parts of this
>> I'm working on a pretty vague understanding of :/
> Yes, there is. But it's more about code that generates stats, rather than
> reporting functions. While there's backend local pending stats we need to have
> a refcount on the shared stats item so that the stats item can't be dropped
> and then revived when those local stats are flushed.

What do you think about something along those lines for the reporting
part only?

Datum
pgstat_fetch_all_tables_stats(PG_FUNCTION_ARGS)
{
    int         dbid = PG_ARGISNULL(0) ? -1 : (int) PG_GETARG_OID(0);
    ReturnSetInfo *rsinfo = (ReturnSetInfo *) fcinfo->resultinfo;
    dshash_seq_status hstat;
    PgStatShared_HashEntry *p;
    PgStatShared_Common *stats_data;

    /* Only members of pg_read_all_stats can use this function */
    if (!has_privs_of_role(GetUserId(), ROLE_PG_READ_ALL_STATS))
    {
        aclcheck_error(ACLCHECK_NO_PRIV, OBJECT_FUNCTION,
"pgstat_fetch_all_tables_stats");
    }

    pgstat_assert_is_up();

    SetSingleFuncCall(fcinfo, 0);

    dshash_seq_init(&hstat, pgStatLocal.shared_hash, false);
    while ((p = dshash_seq_next(&hstat)) != NULL)
    {
        Datum           values[PG_STAT_GET_ALL_TABLES_STATS_COLS];
        bool            nulls[PG_STAT_GET_ALL_TABLES_STATS_COLS];
        PgStat_StatTabEntry * tabentry = NULL;

        MemSet(values, 0, sizeof(values));
        MemSet(nulls, false, sizeof(nulls));

        /* If looking for specific dbid, ignore all the others */
        if (dbid != -1 && p->key.dboid != (Oid) dbid)
            continue;

        /* If the entry is not of kind relation then ignore it */
        if (p->key.kind != PGSTAT_KIND_RELATION)
            continue;

        /* If the entry has been dropped then ignore it */
        if (p->dropped)
            continue;

        stats_data = dsa_get_address(pgStatLocal.dsa, p->body);
        LWLockAcquire(&stats_data->lock, LW_SHARED);
        tabentry = pgstat_get_entry_data(p->key.kind, stats_data);

        values[0] = ObjectIdGetDatum(p->key.dboid);
        values[1] = ObjectIdGetDatum(p->key.objoid);
        values[2]= DatumGetInt64(tabentry->tuples_inserted);
        values[3]= DatumGetInt64(tabentry->tuples_updated);
        values[4]= DatumGetInt64(tabentry->tuples_deleted);
        .
        .
        .
        tuplestore_putvalues(rsinfo->setResult, rsinfo->setDesc,
values, nulls);

        LWLockRelease(&stats_data->lock);
    }
    dshash_seq_term(&hstat);

    return (Datum) 0;
}

I also tried to make use of pgstat_get_entry_ref() but went into a
failed assertion: pgstat_get_entry_ref -> dshash_find ->
ASSERT_NO_PARTITION_LOCKS_HELD_BY_ME(hash_table) due to lock acquired by
dshash_seq_next().

Regards,

--

Bertrand Drouvot
Amazon Web Services: https://aws.amazon.com