Re: LogwrtResult contended spinlock

On Mon, Mar 4, 2024 at 9:15 PM Bharath Rupireddy
<bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
>
> > 0003:
> >
> > * We need to maintain the invariant that Copy >= Write >= Flush. I
> > believe that's always satisfied, because the
> > XLogWaitInsertionsToFinish() is always called before XLogWrite(). But
> > we should add an assert or runtime check of this invariant somewhere.
>
> Yes, that invariant is already maintained by the server. Although, I'm
> not fully agree, I added an assertion to WaitXLogInsertionsToFinish
> after updating XLogCtl->LogwrtResult.Copy. CF bot is happy with it -
> https://github.com/BRupireddy2/postgres/tree/atomic_LogwrtResult_v13.

I've now separated these invariants out into the 0004 patch.

With the assertions placed in WaitXLogInsertionsToFinish after
updating Copy ptr, I observed the assertion failing in one of the CF
bot machines - https://cirrus-ci.com/build/6202112288227328. I could
reproduce it locally with [1]. I guess the reason is that the Write
and Flush ptrs are now updated independently and atomically without
lock, they might drift and become out-of-order for a while if
concurrently they are accessed in WaitXLogInsertionsToFinish. So, I
guess the right place to verify the invariant Copy >= Write >= Flush
is in XLogWrite once Write and Flush ptrs in shared memory are updated
(note that only one process at a time can do this). Accordingly, I've
moved the assertions to XLogWrite in the attached v14-0004 patch.

> Please see the attached v13 patch set for further review.

Earlier versions of the patches removed a piece of code ensuring
shared WAL 'request' values did not fall beading the 'result' values.
There's a good reason for us to have it. So, I restored it.

- /*
- * Update shared-memory status
- *
- * We make sure that the shared 'request' values do not fall behind the
- * 'result' values. This is not absolutely essential, but it saves some
- * code in a couple of places.
- */

Please see the attached v14 patch set.

[1] for i in {1..100}; do make check
PROVE_TESTS="t/027_stream_regress.pl"; if [ $? -ne 0 ]; then echo "The
command failed on iteration $i"; break; fi; done

--
Bharath Rupireddy
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com