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

On Tue, Mar 7, 2023 at 1:01 AM John Naylor <john.naylor@enterprisedb.com> wrote:
>
> On Mon, Mar 6, 2023 at 1:28 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
>
> > > Since the block-level measurement is likely overestimating quite a bit, I propose to simply reverse the order of
theactions here, effectively reporting progress for the *last page* and not the current one: First update progress with
thecurrent memory usage, then add tids for this page. If this allocated a new block, only a small bit of that will be
writtento. 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
earlierattempts at a "fudge factor", but simpler and less brittle. And, as far as OS pages we have actually written to,
Ithink 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
thatvacuum 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
suresuch a top-down facility exists, and I'm not feeling well enough today to study this further. 

I prefer option (1) as it's straight forward. I mentioned a similar
idea before[1]. RT_MEMORY_USAGE() is defined only when the macro is
defined. It might be worth checking if there is visible overhead of
tracking chunk memory space. IIRC we've not evaluated it yet.

[1] https://www.postgresql.org/message-id/CAD21AoDK3gbX-jVxT6Pfso1Na0Krzr8Q15498Aj6tmXgzMFksA%40mail.gmail.com

Regards,

--
Masahiko Sawada
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