Re: POC: Parallel processing of indexes in autovacuum

Hi,
As I promised - meet parallel index autovacuum with bgworkers
(Parallel-index-autovacuum-with-bgworkers.patch). This is pretty
simple implementation :
1) Added new table option `parallel_idx_autovac_enabled` that must be
set to `true` if user wants autovacuum to process table in parallel.
2) Added new GUC variable `autovacuum_reserved_workers_num`. This is
number of parallel workers from bgworkers pool that can be used only
by autovacuum workers. The `autovacuum_reserved_workers_num` parameter
actually reserves a requested part of the processes, the total number
of which is equal to `max_worker_processes`.
3) When an autovacuum worker decides to process some table in
parallel, it just sets `VacuumParams->nworkers` to appropriate value
(> 0) and then the code is executed as if it were a regular VACUUM
PARALLEL.
4) I kept test/modules/autovacuum as sandbox where you can play with
parallel index autovacuum a bit.

What do you think about this implementation?

P.S.
I also improved "self-managed" parallel autovacuum implementation
(Self-managed-parallel-index-autovacuum.patch). For now it needs a lot
of refactoring, but all features are working good.
Both patches are targeting on master branch
(bc35adee8d7ad38e7bef40052f196be55decddec)

--
Best regards,
Daniil Davydov