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

Hello Hayato,

On 2024-Mar-04, Hayato Kuroda (Fujitsu) wrote:

> OK, I understood that my initial proposal is not so valuable, so I can
> withdraw it.

Yeah, that's what it seems to me.

> About the suggetion, you imagined AutoVacuumRequestWork() and
> brininsert(), right?

Correct.

> 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.

--
Álvaro Herrera PostgreSQL Developer — https://www.EnterpriseDB.com/