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

On Wed, Mar 27, 2024 at 9:25 AM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
>
> On Mon, Mar 25, 2024 at 8:07 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Mon, Mar 25, 2024 at 3:25 PM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
> > >
> > > On Fri, Mar 22, 2024 at 12:20 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
>
> > > - * remaining LP_DEAD line pointers on the page in the dead_items
> > > - * array. These dead items include those pruned by lazy_scan_prune()
> > > - * as well we line pointers previously marked LP_DEAD.
> > > + * remaining LP_DEAD line pointers on the page in the dead_items.
> > > + * These dead items include those pruned by lazy_scan_prune() as well
> > > + * we line pointers previously marked LP_DEAD.
> > >
> > > Here maybe "into dead_items".
>
> - * remaining LP_DEAD line pointers on the page in the dead_items.
> + * remaining LP_DEAD line pointers on the page into the dead_items.
>
> Let me explain. It used to be "in the dead_items array." It is not an
> array anymore, so it was changed to "in the dead_items". dead_items is
> a variable name, and names don't take "the". "into dead_items" seems
> most natural to me, but there are other possible phrasings.

Thanks for the explanation. I was distracted. Fixed in the latest patch.

>
> > > > > Did you try it with 1MB m_w_m?
> > > >
> > > > I've incorporated the above comments and test results look good to me.
> > >
> > > Could you be more specific about what the test was?
> > > Does it work with 1MB m_w_m?
> >
> > If m_w_m is 1MB, both the initial and maximum segment sizes are 256kB.
> >
> > FYI other test cases I tested were:
> >
> > * m_w_m = 2199023254528 (maximum value)
> > initial: 1MB
> > max: 128GB
> >
> > * m_w_m = 64MB (default)
> > initial: 1MB
> > max: 8MB
>
> If the test was a vacuum, how big a table was needed to hit 128GB?

I just checked how TIdStoreCreateLocal() calculated the initial and
max segment sizes while changing m_w_m, so didn't check how big
segments are actually allocated in the maximum value test case.

>
> > > The existing comment slipped past my radar, but max_bytes is not a
> > > limit, it's a hint. Come to think of it, it never was a limit in the
> > > normal sense, but in earlier patches it was the criteria for reporting
> > > "I'm full" when asked.
> >
> > Updated the comment.
>
> + * max_bytes is not a limit; it's used to choose the memory block sizes of
> + * a memory context for TID storage in order for the total memory consumption
> + * not to be overshot a lot. The caller can use the max_bytes as the criteria
> + * for reporting whether it's full or not.
>
> This is good information. I suggest this edit:
>
> "max_bytes" is not an internally-enforced limit; it is used only as a
> hint to cap the memory block size of the memory context for TID
> storage. This reduces space wastage due to over-allocation. If the
> caller wants to monitor memory usage, it must compare its limit with
> the value reported by TidStoreMemoryUsage().
>
> Other comments:

Thanks for the suggestion!

>
> v79-0002 looks good to me.
>
> v79-0003:
>
> "With this commit, when creating a shared TidStore, a dedicated DSA
> area is created for TID storage instead of using the provided DSA
> area."
>
> This is very subtle, but "the provided..." implies there still is one.
> -> "a provided..."
>
> + * Similar to TidStoreCreateLocal() but create a shared TidStore on a
> + * DSA area. The TID storage will live in the DSA area, and a memory
> + * context rt_context will have only meta data of the radix tree.
>
> -> "the memory context"

Fixed in the latest patch.

>
> I think you can go ahead and commit 0002 and 0003/4.

I've pushed the 0002 (dsa init and max segment size) patch, and will
push the attached 0001 patch next.

>
> v79-0005:
>
> - bypass = (vacrel->lpdead_item_pages < threshold &&
> - vacrel->lpdead_items < MAXDEADITEMS(32L * 1024L * 1024L));
> + bypass = (vacrel->lpdead_item_pages < threshold) &&
> + TidStoreMemoryUsage(vacrel->dead_items) < (32L * 1024L * 1024L);
>
> The parentheses look strange, and the first line shouldn't change
> without a good reason.

Fixed.

>
> - /* Set dead_items space */
> - dead_items = (VacDeadItems *) shm_toc_lookup(toc,
> - PARALLEL_VACUUM_KEY_DEAD_ITEMS,
> - false);
> + /* Set dead items */
> + dead_items = TidStoreAttach(shared->dead_items_dsa_handle,
> + shared->dead_items_handle);
>
> I feel ambivalent about this comment change. The original is not very
> descriptive to begin with. If we need to change at all, maybe "find
> dead_items in shared memory"?

Agreed.

>
> v79-0005: As I said earlier, Dilip Kumar reviewed an earlier version.
>
> v79-0006:
>
> vac_work_mem should also go back to being an int.

Fixed.

I've attached the latest patches.

Regards,

--
Masahiko Sawada
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