Re: Session WAL activity

On 11.12.2019 7:26, Kyotaro Horiguchi wrote:
>
> Still I'm not sure non-atomic write is acceptable, but I agree on the
> necessity of updating it during a transaction. Couldn't we update
> shared stats every n bytes (XLOG_BLCKSZ or such) or every command end?
>
> I think we should refrain from inserting an instruction within the
> WALInsertLock section, but I'm not sure which is better between "var
> += var" within the section and "if (inserted) var += var;" outside. If
> we can ignore the possitbility of the case where xlogswitch is
> omitted, the "if (inserted)" is not needed.

I think that 32-bit Postgres installations are really exotic, but I
agree that showing incorrect result (even with very small probability)
is not acceptable behavior in this case. I attached new versoin of the
patch with use pg_atomic_write_u64 for updating walWritten field.
As far as at 64-bit systems, pg_atomic_write_u64and pg_atomic_read_u64
are translated to ordinary memory access, them should not have some
negative
impact on performance.

--
Konstantin Knizhnik
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The Russian Postgres Company