Re: Performance improvements for src/port/snprintf.c

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: pgsql-hackers(at)lists(dot)postgresql(dot)org, Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com>, Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, Alexander Kuzmenkov <a(dot)kuzmenkov(at)postgrespro(dot)ru>
Subject: Re: Performance improvements for src/port/snprintf.c
Date: 2018-10-03 16:43:26
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Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-10-03 11:59:27 -0400, Tom Lane wrote:
>> I experimented with adding an initial check for "format is exactly %s"
>> at the top of dopr(), and couldn't get excited about that. Instrumenting
>> things showed that the optimization fired in only 1.8% of the calls
>> during a run of our core regression tests. Now, that might not count
>> as a really representative workload, but it doesn't make me think that
>> the case is worth optimizing for us.

> Seems right. I also have a hard time to believe that any of those "%s"
> printfs are performance critical - we'd hopefully just have avoided the
> sprintf in that case.

Yup, that's probably a good chunk of the reason why there aren't very
many. But we *do* use %s a lot to assemble multiple strings or combine
them with fixed text, which is why the other form of the optimization
is useful.

>> But then it occurred to me that there's more than one way to skin this
>> cat. We could, for an even cheaper extra test, detect that any one
>> format specifier is just "%s", and use the same kind of fast-path
>> within the loop. With the same sort of instrumentation, I found that
>> a full 45% of the format specs executed in the core regression tests
>> are just %s. That makes me think that a patch along the lines of the
>> attached is a good win for our use-cases. Comparing to Fedora 28's
>> glibc, this gets us to

> Hm, especially if we special case the float->string conversions directly
> at the hot callsites, that seems reasonable. I kinda wish we could just
> easily move the format string processing to compile-time, but given
> translatability that won't be widely possible even if it were otherwise
> feasible.

Yeah, there's a mighty big pile of infrastructure that depends on the
way *printf works. I agree that one way or another we're going to be
special-casing float8out and float4out.

BTW, I poked around in the related glibc sources the other day, and
it seemed like they are doing some sort of quasi-compilation of format
strings. I couldn't figure out how they made it pay, though --- without
some way to avoid re-compiling the same format string over and over,
seems like it couldn't net out as a win. But if they are avoiding
that, I didn't find where.

regards, tom lane

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