RE: Some shared memory chunks are allocated even if related processes won't start

Dear Alvaro,

Thanks for giving comments!

> > I agreed it sounds good, but I don't think it can be implemented by
> > current interface. An interface for dynamically allocating memory is
> > GetNamedDSMSegment(), and it returns the same shared memory region if
> > input names are the same.  Therefore, there is no way to re-alloc the
> > shared memory.
> 
> Yeah, I was imagining something like this: the workitem-array becomes a
> struct, which has a name and a "next" pointer and a variable number of
> workitem slots; the AutoVacuumShmem struct has a pointer to the first
> workitem-struct and the last one; when a workitem is requested by
> brininsert, we initially allocate via GetNamedDSMSegment("workitem-0") a
> workitem-struct with a smallish number of elements; if we request
> another workitem and the array is full, we allocate another array via
> GetNamedDSMSegment("workitem-1") and store a pointer to it in workitem-0
> (so that the list can be followed by an autovacuum worker that's
> processing the database), and it's also set as the tail of the list in
> AutoVacuumShmem (so that we know where to store further work items).
> When all items in a workitem-struct are processed, we can free it
> (I guess via dsm_unpin_segment), and make AutoVacuumShmem->av_workitems
> point to the next one in the list.
> 
> This way, the "array" can grow arbitrarily.
>

Basically sounds good. My concerns are:

* GetNamedDSMSegment() does not returns a raw pointer to dsm_segment. This means
  that it may be difficult to do dsm_unpin_segment on the caller side.
* dynamic shared memory is recorded in dhash (dsm_registry_table) and the entry
  won't be deleted. The reference for the chunk might be remained.

Overall, it may be needed that dsm_registry may be also extended. I do not start
working yet, so will share results after trying them.

Best Regards,
Hayato Kuroda
FUJITSU LIMITED
https://www.fujitsu.com/