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

On Thu, Mar 21, 2024 at 12:40 PM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
>
> On Thu, Mar 21, 2024 at 9:37 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> > On Wed, Mar 20, 2024 at 11:19 PM John Naylor <johncnaylorls(at)gmail(dot)com> wrote:
> > > Are they (the blocks to be precise) really out of order? The VALUES
> > > statement is ordered, but after inserting it does not output that way.
> > > I wondered if this is platform independent, but CI and our dev
> > > machines haven't failed this test, and I haven't looked into what
> > > determines the order. It's easy enough to hide the blocks if we ever
> > > need to, as we do elsewhere...
> >
> > It seems not necessary as such a test is already covered by
> > test_radixtree. I've changed the query to hide the output blocks.
>
> Okay.
>
> > The buildfarm has been all-green so far.
>
> Great!
>
> > I've attached the latest vacuum improvement patch.
> >
> > I just remembered that the tidstore cannot still be used for parallel
> > vacuum with minimum maintenance_work_mem. Even when the shared
> > tidstore is empty, its memory usage reports 1056768 bytes, a bit above
> > 1MB (1048576 bytes). We need something discussed on another thread[1]
> > in order to make it work.
>
> For exactly this reason, we used to have a clamp on max_bytes when it
> was internal to tidstore, so that it never reported full when first
> created, so I guess that got thrown away when we got rid of the
> control object in shared memory. Forcing callers to clamp their own
> limits seems pretty unfriendly, though.

Or we can have a new function for dsa.c to set the initial and max
segment size (or either one) to the existing DSA area so that
TidStoreCreate() can specify them at creation. In shared TidStore
cases, since all memory required by shared radix tree is allocated in
the passed-in DSA area and the memory usage is the total segment size
allocated in the DSA area, the user will have to prepare a DSA area
only for the shared tidstore. So we might be able to expect that the
DSA passed-in to TidStoreCreate() is empty and its segment sizes can
be adjustable.

>
> The proposals in that thread are pretty simple. If those don't move
> forward soon, a hackish workaround would be to round down the number
> we get from dsa_get_total_size to the nearest megabyte. Then
> controlling min/max segment size would be a nice-to-have for PG17, not
> a prerequisite.

Interesting idea.

Regards,

--
Masahiko Sawada
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