Re: [PoC] Improve dead tuple storage for lazy vacuum

On Mon, Mar 6, 2023 at 1:28 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
wrote:

> > Since the block-level measurement is likely overestimating quite a bit,
I propose to simply reverse the order of the actions here, effectively
reporting progress for the *last page* and not the current one: First
update progress with the current memory usage, then add tids for this page.
If this allocated a new block, only a small bit of that will be written to.
If this block pushes it over the limit, we will detect that up at the top
of the loop. It's kind of like our earlier attempts at a "fudge factor",
but simpler and less brittle. And, as far as OS pages we have actually
written to, I think it'll effectively respect the memory limit, at least in
the local mem case. And the numbers will make sense.
> >
> > Thoughts?
>
> It looks to work but it still doesn't work in a case where a shared
> tidstore is created with a 64kB memory limit, right?
> TidStoreMemoryUsage() returns 1MB and TidStoreIsFull() returns true
> from the beginning.

I have two ideas:

1. Make it optional to track chunk memory space by a template parameter. It
might be tiny compared to everything else that vacuum does. That would
allow other users to avoid that overhead.
2. When context block usage exceeds the limit (rare), make the additional
effort to get the precise usage -- I'm not sure such a top-down facility
exists, and I'm not feeling well enough today to study this further.

--
John Naylor
EDB: http://www.enterprisedb.com