Re: Why is pq_begintypsend so slow?

On Sat, Jan 11, 2020 at 03:19:37PM -0500, Tom Lane wrote:
> Jeff Janes <jeff(dot)janes(at)gmail(dot)com> writes:
> > I saw that the hotspot was pq_begintypsend at 20%, which was twice the
> > percentage as the next place winner (AllocSetAlloc).
>
> Weird.
>
> > Why is this such a bottleneck?
>
> Not sure, but it seems like a pretty dumb way to push the stringinfo's
> len forward. We're reading/updating the len word in each line, and
> if your perf measurements are to be believed, it's the re-fetches of
> the len values that are bottlenecked --- maybe your CPU isn't too
> bright about that? The bytes of the string value are getting written
> twice too, thanks to uselessly setting up a terminating nul each time.
>
> I'd be inclined to replace the appendStringInfoCharMacro calls with
> appendStringInfoSpaces(buf, 4) --- I don't think we care exactly what
> is inserted into those bytes at this point. And maybe
> appendStringInfoSpaces could stand some micro-optimization, too.
> Use a memset and a single len adjustment, perhaps?

Please find attached a patch that does it both of the things you
suggested.

Best,
David.
--
David Fetter <david(at)fetter(dot)org> http://fetter.org/
Phone: +1 415 235 3778

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