Re: [HACKERS] [PATCH] Vacuum: Update FSM more frequently

On Sat, Jul 29, 2017 at 9:42 AM, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
> Introduce a tree pruning threshold to FreeSpaceMapVacuum that avoids
> recursing into branches that already contain enough free space, to
> avoid having to traverse the whole FSM and thus induce quadratic
> costs. Intermediate FSM vacuums are only supposed to make enough
> free space visible to avoid extension until the final (non-partial)
> FSM vacuum.

Hmm, I think this resolve a part of the issue. How about calling
AutoVacuumRequestWork() in PG_CATCH() if VACOPT_VACUUM is specified
and give the relid that we were vacuuming but could not complete as a
new autovacuum work-item? The new autovacuum work-item makes the
worker vacuum FSMs of the given relation and its indices. That way, we
can ensure that FSM gets vacuumed by the cancelled autovacuum process
or other autovacuum processes. Since a work-item can be handled by
other autovacuum process I think 256 work-item limit would not be a
problem. Or it might be better to make autovacuum launcher launch
worker process if there is pending work-item in a database even if
there is no table needs to be vacuumed/analyzed.

> For one, it would sometimes crash when adding the work item from
> inside autovacuum itself. I didn't find the cause of the crash, but I suspect
> AutoVacuumRequestWork was being called in a context where it
> was not safe.

Perhaps the cause of this might be that autovacuum work-item is
implemented using DSA, which has been changed before by commit
31ae1638ce35c23979f9bcbb92c6bb51744dbccb.

Regards,

--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center