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

On Fri, Feb 24, 2023 at 12:50 PM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com>
wrote:
>
> On Wed, Feb 22, 2023 at 6:55 PM John Naylor
> <john(dot)naylor(at)enterprisedb(dot)com> wrote:
> >
> > That doesn't seem useful to me. If we've done enough testing to
reassure us the new way always gives the same answer, the old way is not
needed at commit time. If there is any doubt it will always give the same
answer, then the whole patchset won't be committed.

> My idea is to make the bug investigation easier but on
> reflection, it seems not the best idea given this purpose.

My concern with TIDSTORE_DEBUG is that it adds new code that mimics the old
tid array. As I've said, that doesn't seem like a good thing to carry
forward forevermore, in any form. Plus, comparing new code with new code is
not the same thing as comparing existing code with new code. That was my
idea upthread.

Maybe the effort my idea requires is too much vs. the likelihood of finding
a problem. In any case, it's clear that if I want that level of paranoia,
I'm going to have to do it myself.

> What do you think
> about the attached patch? Please note that it also includes the
> changes for minimum memory requirement.

Most of the asserts look logical, or at least harmless.

- int max_off; /* the maximum offset number */
+ OffsetNumber max_off; /* the maximum offset number */

I agree with using the specific type for offsets here, but I'm not sure why
this change belongs in this patch. If we decided against the new asserts,
this would be easy to lose.

This change, however, defies common sense:

+/*
+ * The minimum amount of memory required by TidStore is 2MB, the current
minimum
+ * valid value for the maintenance_work_mem GUC. This is required to
allocate the
+ * DSA initial segment, 1MB, and some meta data. This number is applied
also to
+ * the local TidStore cases for simplicity.
+ */
+#define TIDSTORE_MIN_MEMORY (2 * 1024 * 1024L) /* 2MB */

+ /* Sanity check for the max_bytes */
+ if (max_bytes < TIDSTORE_MIN_MEMORY)
+ elog(ERROR, "memory for TidStore must be at least %ld, but %zu provided",
+ TIDSTORE_MIN_MEMORY, max_bytes);

Aside from the fact that this elog's something that would never get past
development, the #define just adds a hard-coded copy of something that is
already hard-coded somewhere else, whose size depends on an implementation
detail in a third place.

This also assumes that all users of tid store are limited by
maintenance_work_mem. Andres thought of an example of some day unifying
with tidbitmap.c, and maybe other applications will be limited by work_mem.

But now that I'm looking at the guc tables, I am reminded that work_mem's
minimum is 64kB, so this highlights a design problem: There is obviously no
requirement that the minimum work_mem has to be >= a single DSA segment,
even though operations like parallel hash and parallel bitmap heap scan are
limited by work_mem. It would be nice to find out what happens with these
parallel features when work_mem is tiny (maybe parallelism is not even
considered?).

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