On 2015-09-03 16:28:40 +1200, David Rowley wrote: > 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@ubuntu:~/C$ gcc5.2 tostring.c -o tostring -O2 > david@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@ubuntu:~/C$ clang tostring.c -o tostring_clang -O2 > david@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.
I think you're right.
If I change the NUM_TO_STRING in my tostring.c to
#define NUM_TO_PRINT i%100
I just wanted a way to keep the strings 2 digits long, so that it matches what will happen when building timestamps.
It looks a bit different, but it's still a bit faster than pg_lltoa()
