Re: WIP: Make timestamptz_out less slow.

On 2015-09-03 16:28:40 +1200, David Rowley wrote:
> > Wouldn't it be better to just normalize fsec to an integer in that case?
> > Afaics that's the only remaining reason for the alternative path?
> >
> > A special case would need to exist somewhere, unless we just modified
> timestamp2tm() to multiply fsec by 1 million when HAVE_INT64_TIMESTAMP is
> not defined, but that changes the function signature.

Sure, but that's a one line #ifdef?

> > We could alternatively handle this by special-casing INT_MIN, probably
> > resulting in a bit less duplicated code.
> >
>
> Those special cases seem to do some weird stuff to the performance
> characteristics of those functions. I find my method to handle negative
> numbers to produce much faster code.

That's a bit odd.

> I experimented with finding the fastest way to convert an int to a string
> at the weekend, I did happen to be testing with int64's but likely int32
> will be the same.
>
> This is the time taken to convert 30 into "30" 2 billion times.
>
> The closest implementation I'm using in the patch is pg_int64tostr() one.
> The pg_int64tostr_memcpy() only performs better with bigger numbers /
> longer strings.
>
> david(at)ubuntu:~/C$ gcc5.2 tostring.c -o tostring -O2
> david(at)ubuntu:~/C$ ./tostring
> pg_int64tostr_memcpy in 13.653252 seconds
> pg_int64tostr in 2.042616 seconds
> pg_int64tostr_new in 2.056688 seconds
> pg_lltoa in 13.604653 seconds
>
> david(at)ubuntu:~/C$ clang tostring.c -o tostring_clang -O2
> david(at)ubuntu:~/C$ ./tostring_clang
> pg_int64tostr_memcpy in 0.000004 seconds
> pg_int64tostr in 2.063335 seconds
> pg_int64tostr_new in 2.102068 seconds
> pg_lltoa in 3.424630 seconds

Are you sure this isn't an optimization artifact where the actual work
is optimized away? Doing 2 billion conversions in 2s kinda implies that
the string conversion is done in ~4 cycles. That seems unrealistically
fast, even for a pipelined cpu.

Greetings,

Andres Freund