Re: Improve WALRead() to suck data directly from WAL buffers when possible

On Wed, Mar 1, 2023 at 12:06 AM Kuntal Ghosh <kuntalghosh(dot)2007(at)gmail(dot)com> wrote:
>
> On Tue, Feb 28, 2023 at 10:38 AM Bharath Rupireddy
> <bharath(dot)rupireddyforpostgres(at)gmail(dot)com> wrote:
> >
> +/*
> + * Guts of XLogReadFromBuffers().
> + *
> + * Read 'count' bytes into 'buf', starting at location 'ptr', from WAL
> + * fetched WAL buffers on timeline 'tli' and return the read bytes.
> + */
> s/fetched WAL buffers/fetched from WAL buffers

Modified that comment a bit and moved it to the XLogReadFromBuffers.

> + else if (nread < nbytes)
> + {
> + /*
> + * We read some of the requested bytes. Continue to read remaining
> + * bytes.
> + */
> + ptr += nread;
> + nbytes -= nread;
> + dst += nread;
> + *read_bytes += nread;
> + }
>
> The 'if' condition should always be true. You can replace the same
> with an assertion instead.

Yeah, added an assert and changed that else if (nread < nbytes) to
else only condition.

> s/Continue to read remaining/Continue to read the remaining

Done.

> The good thing about this patch is that it reduces read IO calls
> without impacting the write performance (at least not that
> noticeable). It also takes us one step forward towards the
> enhancements mentioned in the thread.

Right.

> If performance is a concern, we
> can introduce a GUC to enable/disable this feature.

I didn't see any performance issues from my testing so far with 3
different pgbench cases
https://www.postgresql.org/message-id/CALj2ACWXHP6Ha1BfDB14txm%3DXP272wCbOV00mcPg9c6EXbnp5A%40mail.gmail.com.

While adding a GUC to enable/disable a feature sounds useful, IMHO it
isn't good for the user. Because we already have too many GUCs for the
user and we may not want all features to be defensive and add their
own GUCs. If at all, any bugs arise due to some corner-case we missed
to count in, we can surely help fix them. Having said this, I'm open
to suggestions here.

Please find the attached v5 patch set for further review.

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