Re: Declarative partitioning optimization for large amount of partitions

From: Aleksander Alekseev <a(dot)alekseev(at)postgrespro(dot)ru>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Amit Langote <Langote_Amit_f8(at)lab(dot)ntt(dot)co(dot)jp>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Declarative partitioning optimization for large amount of partitions
Date: 2017-03-09 16:02:53
Message-ID: 20170309160253.GB5216@e733.localdomain
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Hi, Andres

Thanks a lot for the review!

> Why are we keeping that list / the "batch" allocation pattern? It
> doesn't actually seem useful to me after these changes. Given that we
> don't shrink hash-tables, we could just use the hash-table memory for
> this, no?

I don't think we can do that. According to comments:

```
* NOTE: once allocated, TabStatusArray structures are never moved or deleted
[...]
* This allows relcache pgstat_info pointers to be treated as long-lived data,
* avoiding repeated searches in pgstat_initstats() when a relation is
* repeatedly opened during a transaction.
```

It is my understanding that dynahash can't give same guarantees.

> 'faster ... lookup' doesn't strike me as a useful comment, because it's
> only useful in relation of the current code to the new code.

Agree, fixed to 'O(1) ... lookup'.

> It's not really freed...

Right, it's actually zeroed. Fixed.

> Arguably it'd be better to HASH_ENTER directly here, instead of doing
> two lookups.

Good point. Fixed.

> Think it'd be better to move the hash creation into its own function.

Agree, fixed.

On Mon, Mar 06, 2017 at 12:01:50PM -0800, Andres Freund wrote:
> Hi,
>
> This issue has bothered me in non-partitioned use-cases recently, so
> thanks for taking it up.
>
>
> On 2017-03-06 18:22:17 +0300, Aleksander Alekseev wrote:
> > diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
> > index 2fb9a8bf58..fa906e7930 100644
> > --- a/src/backend/postmaster/pgstat.c
> > +++ b/src/backend/postmaster/pgstat.c
> > @@ -160,6 +160,16 @@ typedef struct TabStatusArray
> >
> > static TabStatusArray *pgStatTabList = NULL;
>
> Why are we keeping that list / the "batch" allocation pattern? It
> doesn't actually seem useful to me after these changes. Given that we
> don't shrink hash-tables, we could just use the hash-table memory for
> this, no?
>
> I think a separate list that only contains things that are "active" in
> the current transaction makes sense, because the current logic requires
> iterating over a potentially very large array at transaction commit.
>
>
> > +/* pgStatTabHash entry */
> > +typedef struct TabStatHashEntry
> > +{
> > + Oid t_id;
> > + PgStat_TableStatus* tsa_entry;
> > +} TabStatHashEntry;
> > +
> > +/* Hash table for faster t_id -> tsa_entry lookup */
> > +static HTAB *pgStatTabHash = NULL;
> > +
>
> 'faster ... lookup' doesn't strike me as a useful comment, because it's
> only useful in relation of the current code to the new code.
>
>
>
> > /*
> > * Backends store per-function info that's waiting to be sent to the collector
> > * in this hash table (indexed by function OID).
> > @@ -815,7 +825,13 @@ pgstat_report_stat(bool force)
> > pgstat_send_tabstat(this_msg);
> > this_msg->m_nentries = 0;
> > }
> > +
> > + /*
> > + * Entry will be freed soon so we need to remove it from the lookup table.
> > + */
> > + hash_search(pgStatTabHash, &entry->t_id, HASH_REMOVE, NULL);
> > }
>
> It's not really freed...
>
>
> > static PgStat_TableStatus *
> > get_tabstat_entry(Oid rel_id, bool isshared)
> > {
> > + TabStatHashEntry* hash_entry;
> > PgStat_TableStatus *entry;
> > TabStatusArray *tsa;
> > - TabStatusArray *prev_tsa;
> > - int i;
> > +
> > + /* Try to find an entry */
> > + entry = find_tabstat_entry(rel_id);
> > + if(entry != NULL)
> > + return entry;
>
> Arguably it'd be better to HASH_ENTER directly here, instead of doing
> two lookups.
>
>
> > /*
> > - * Search the already-used tabstat slots for this relation.
> > + * Entry doesn't exist - creating one.
> > + * First make sure there is a free space in a last element of pgStatTabList.
> > */
> > - prev_tsa = NULL;
> > - for (tsa = pgStatTabList; tsa != NULL; prev_tsa = tsa, tsa = tsa->tsa_next)
> > + if (!pgStatTabList)
> > {
> > - for (i = 0; i < tsa->tsa_used; i++)
> > - {
> > - entry = &tsa->tsa_entries[i];
> > - if (entry->t_id == rel_id)
> > - return entry;
> > - }
> > + HASHCTL ctl;
> >
> > - if (tsa->tsa_used < TABSTAT_QUANTUM)
> > + Assert(!pgStatTabHash);
> > +
> > + memset(&ctl, 0, sizeof(ctl));
> > + ctl.keysize = sizeof(Oid);
> > + ctl.entrysize = sizeof(TabStatHashEntry);
> > + ctl.hcxt = TopMemoryContext;
> > +
> > + pgStatTabHash = hash_create("pgstat t_id to tsa_entry lookup table",
> > + TABSTAT_QUANTUM, &ctl, HASH_ELEM | HASH_BLOBS | HASH_CONTEXT);
>
> Think it'd be better to move the hash creation into its own function.
>
>
> - Andres

--
Best regards,
Aleksander Alekseev

Attachment Content-Type Size
partitioning_bottleneck_fix_v2.patch text/x-diff 4.5 KB

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