Re: Reducing planning time on tables with many indexes

On 8/1/22 15:33, David Geier wrote:
> Hi Tom,
>
> On Wed, Jul 27, 2022 at 7:15 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> I wrote:
> > Unfortunately, as things stand today, the planner needs more
> than the
> > right to look at the indexes' schemas, because it makes physical
> accesses
> > to btree indexes to find out their tree height (and I think
> there are some
> > comparable behaviors in other AMs).  I've never particularly
> cared for
> > that implementation, and would be glad to rip out that behavior
> if we can
> > find another way.  Maybe we could persuade VACUUM or ANALYZE to
> store that
> > info in the index's pg_index row, or some such, and then the planner
> > could use it with no lock?
>
> It seems like _bt_getrootheight() first checks if the height is cached
> and only if it isn't it accesses index meta pages.
> If the index locks are only taken for the sake of _bt_getrootheight()
> accessing index meta pages in case they are not cached, maybe the
> index locks could be taken conditionally.
> However, postponing the call to where it is really needed sounds even
> better.
>
>
> A first step here could just be to postpone fetching
> _bt_getrootheight()
> until we actually need it during cost estimation.  That would
> avoid the
> need to do it at all for indexes that indxpath.c discards as
> irrelevant,
> which is a decision made on considerably more information than the
> proposed patch uses.
>
Hi Tom,

I gave the idea of moving _bt_getrootheight() into costsize.c and
filling IndexOptInfo in get_relation_info() via syscache instead of
relcache a try, but didn't get very far.
Moving out _bt_getrootheight() was straightforward, and we should do
nevertheless. However, it seems like get_relation_info() strongly
depends on the index's Relation for quite some stuff. A fair amount of
fields I could actually fill from syscache, but there are some that
either need data not stored in syscache (e.g. estimate_rel_size(),
Relation::rd_smgr needed by RelationGetNumberOfBlocksInFork()) or need
fields that are cached in the index's Relation and would have to be
recomputed otherwise (e.g. Relation::rd_indexprs filled by
RelationGetIndexExpressions(), Relation::rd_indpred filled by
RelationGetIndexPredicate()). Even if we could somehow obtain the
missing info from somewhere, recomputing the otherwise cached fields
from Relation would likely cause a significant slowdown in the serial case.

Beyond that I did some off-CPU profiling to precisely track down which
lock serializes execution. It turned out to be the MyProc::fpInfoLock
lightweight lock. This lock is used in the fast path of the heavyweight
lock. In the contenting case, fpInfoLock is acquired in LW_EXCLUSIVE
mode to (1) check if there is no other process holding a stronger lock,
and if not, to reserve a process local fast path lock slot and (2) to
return the fast path lock slots all in one go. To do so, the current
implementation always linearly iterates over all lock slots. The
corresponding call stacks are:

get_relation_info()               CommitTransaction()
  index_open()                      ResourceOwnerRelease()
    relation_open() ResourceOwnerReleaseInternal()
      LockRelationOid()                 ProcReleaseLocks()
        LockAcquireExtended()             LockReleaseAll() <-- called
twice from ProcReleaseLocks()
          LWLockAcquire()

On top of that there are only 16 fast path lock slots. One slot is
always taken up by the parent relation, leaving only 15 slots for the
indexes. As soon as a session process runs out of slots, it falls back
to the normal lock path which has to mess around with the lock table. To
do so it also acquires a lightweight lock in LW_EXCLUSIVE mode. This
lightweight lock however is partitioned and therefore does not content.
Hence, normal lock acquisition is slower but contents less.

To prove above findings I increased the number of fast path lock slots
per connection and optimized FastPathGrantRelationLock() and
FastPathUnGrantRelationLock(). With these changes the lock contention
disappeared and the workload scales linearly (the code I tested with
also included moving out _bt_getrootheight()):

| Parallel streams | TPS      | TPS / stream  |
|------------------|----------|---------------|
| 1                |   5,253  | 5,253         |
| 10               |  51,406  | 5,140         |
| 20               | 101,401  | 5,070         |
| 30               | 152,023  | 5,067         |
| 40               | 200,607  | 5,015         |
| 50               | 245,359  | 4,907         |
| 60               | 302,994  | 5,049         |

However, with the very same setup, the index filtering approach yields
486k TPS with 60 streams and 9,827 TPS with a single stream. The single
stream number shows that this is not because it scales even better, but
just because less work is spent during planning. A quick perf session
showed that a fair amount of time is spent to get the relation sizes in
blocks (RelationGetNumberOfBlocksInFork() -> lseek64()) and creating
index paths (pull_varattnos() -> bms_add_member(), surprisingly).

-   32.20%     1.58%  postgres  postgres            [.]
get_relation_info
   - 30.62% get_relation_info
      - 16.56% RelationGetNumberOfBlocksInFork
         - 16.42% smgrnblocks
            - 16.25% mdnblocks
               - 16.10% _mdnblocks
                  + 15.55% __libc_lseek64
      + 5.83% index_open
      + 2.71% estimate_rel_size
        1.56% build_index_tlist
      + 1.22% palloc
   + 1.57% __libc_start_main
-   23.02%     0.03%  postgres  postgres            [.]
make_one_rel
   - 22.99% make_one_rel
      - 22.01% set_base_rel_pathlists
         - 21.99% set_rel_pathlist
            - 21.89% set_plain_rel_pathlist
               - 21.53% create_index_paths
                  - 18.76% get_index_paths
                     - 18.33% build_index_paths
                        - 15.77% check_index_only
                           - 14.75% pull_varattnos
                              - 14.58% pull_varattnos_walker
                                 - 13.05% expression_tree_walker
                                    - 9.50% pull_varattnos_walker
                                         5.77% bms_add_member
                                      0.93% bms_add_member
                                      0.52% expression_tree_walker
                                   1.44% pull_varattnos_walker
                        + 1.79% create_index_path
                  + 0.90% match_restriction_clauses_to_index
      + 0.95% set_base_rel_sizes

Given the findings above, the two patches are actually complementary.
Optimizing the lock fast path not only helps when many indexes exist and
only a small subset is used, but whenever there are many locks used by a
query. The index filtering is another way to reduce lock contention, but
beyond that also greatly reduces the time spent on planning in the
serial case.

I have attached the patch to improve the heavyweight lock fast path. It
also for now contains moving out _bt_getrootheight(). For workloads
where the same set of locks is used over and over again, it only needs
on average a single loop iteration to find the relation (instead of a
linear scan before). This allows to increase the number of fast path
locks by a lot. In this patch I increased them from 16 to 64. The code
can be further improved for cases where to be locked relations change
frequently and therefore the chance of not finding a relation and
because of that having to linearly search the whole array is higher.

I would really appreciate your feedback Tom, also on the questions
around the approach of filtering out indexes, discussed in the last mails.

--
David Geier
(ServiceNow)