Re: [PoC] Improve dead tuple storage for lazy vacuum - Mailing list pgsql-hackers

On Fri, Feb 16, 2024 at 10:05 AM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> > v61-0007: Runtime-embeddable tids -- Optional for v17, but should
> > reduce memory regressions, so should be considered. Up to 3 tids can
> > be stored in the last level child pointer. It's not polished, but I'll
> > only proceed with that if we think we need this. "flags" iis called
> > that because it could hold tidbitmap.c booleans (recheck, lossy) in
> > the future, in addition to reserving space for the pointer tag. Note:
> > I hacked the tests to only have 2 offsets per block to demo, but of
> > course both paths should be tested.
>
> Interesting. I've run the same benchmark tests we did[1][2] (the
> median of 3 runs):
>
> monotonically ordered int column index:
>
> master: system usage: CPU: user: 14.91 s, system: 0.80 s, elapsed: 15.73 s
> v-59: system usage: CPU: user: 9.67 s, system: 0.81 s, elapsed: 10.50 s
> v-62: system usage: CPU: user: 1.94 s, system: 0.69 s, elapsed: 2.64 s

Hmm, that's strange -- this test is intended to delete all records
from the last 20% of the blocks, so I wouldn't expect any improvement
here, only in the sparse case. Maybe something is wrong. All the more
reason to put it off...

> I'm happy to see a huge improvement. While it's really fascinating to
> me, I'm concerned about the time left until the feature freeze. We
> need to polish both tidstore and vacuum integration patches in 5
> weeks. Personally I'd like to have it as a separate patch for now, and
> focus on completing the main three patches since we might face some
> issues after pushing these patches. I think with 0007 patch it's a big
> win but it's still a win even without 0007 patch.

Agreed to not consider it for initial commit. I'll hold on to it for
some future time.

> > 2. Management of memory contexts. It's pretty verbose and messy. I
> > think the abstraction could be better:
> > A: tidstore currently passes CurrentMemoryContext to RT_CREATE, so we
> > can't destroy or reset it. That means we have to do a lot of manual
> > work.
> > B: Passing "max_bytes" to the radix tree was my idea, I believe, but
> > it seems the wrong responsibility. Not all uses will have a
> > work_mem-type limit, I'm guessing. We only use it for limiting the max
> > block size, and aset's default 8MB is already plenty small for
> > vacuum's large limit anyway. tidbitmap.c's limit is work_mem, so
> > smaller, and there it makes sense to limit the max blocksize this way.
> > C: The context for values has complex #ifdefs based on the value
> > length/varlen, but it's both too much and not enough. If we get a bump
> > context, how would we shoehorn that in for values for vacuum but not
> > for tidbitmap?
> >
> > Here's an idea: Have vacuum (or tidbitmap etc.) pass a context to
> > TidStoreCreate(), and then to RT_CREATE. That context will contain the
> > values (for local mem), and the node slabs will be children of the
> > value context. That way, measuring memory usage and free-ing can just
> > call with this parent context, and let recursion handle the rest.
> > Perhaps the passed context can also hold the radix-tree struct, but
> > I'm not sure since I haven't tried it. What do you think?
>
> If I understand your idea correctly, RT_CREATE() creates the context
> for values as a child of the passed context and the node slabs as
> children of the value context. That way, measuring memory usage can
> just call with the value context. It sounds like a good idea. But it
> was not clear to me how to address point B and C.

For B & C, vacuum would create a context to pass to TidStoreCreate,
and it wouldn't need to bother changing max block size. RT_CREATE
would use that directly for leaves (if any), and would only create
child slab contexts under it. It would not need to know about
max_bytes. Modifyng your diagram a bit, something like:

- caller-supplied radix tree memory context (the 3 structs -- and
leaves, if any) (aset (or future bump?))
    - node slab contexts

This might only be workable with aset, if we need to individually free
the structs. (I haven't studied this, it was a recent idea)
It's simpler, because with small fixed length values, we don't need to
detect that and avoid creating a leaf context. All leaves would live
in the same context as the structs.

> Another variant of this idea would be that RT_CREATE() creates the
> parent context of the value context to store radix-tree struct. That
> is, the hierarchy would be like:
>
> A MemoryContext (passed by vacuum through tidstore)
>     - radix tree memory context (store radx-tree struct, control
> struct, and iterator)
>         - value context (aset, slab, or bump)
>             - node slab contexts

The template handling the value context here is complex, and is what I
meant by 'C' above. Most fixed length allocations in all of the
backend are aset, so it seems fine to use it always.

> Freeing can just call with the radix tree memory context. And perhaps
> it works even if tidstore passes CurrentMemoryContex to RT_CREATE()?

Seems like it would, but would keep some complexity, as I mentioned.