(2014/01/23 12:00), Andrew Dunstan wrote:
On 01/22/2014 08:28 PM, KONDO Mitsumasa wrote:
(2014/01/22 22:26), Robert Haas wrote:
On Wed, Jan 22, 2014 at 3:32 AM, KONDO Mitsumasa
<kondo.mitsum...@lab.ntt.co.jp> wrote:
OK, Kondo, please demonstrate benchmarks that show we have <1% impact
from this change. Otherwise we may need a config parameter to allow
the calculation.
OK, testing DBT-2 now. However, error range of benchmark might be 1% higher.
So I show you detail HTML results.
To see any impact from spinlock contention, I think you're pretty much
going to need a machine with >32 cores, I think, and lots of
concurrency. pgbench -S is probably a better test than DBT-2, because
it leaves out all the writing, so percentage-wise more time will be
spent doing things like updating the pgss hash table.
Oh, thanks to inform me. I think essential problem of my patch has bottle neck
in sqrt() function and other division caluculation. I will replcace sqrt()
function in math.h to more faster algorithm. And moving unneccessary part of
caluculation in LWlocks or other locks. It might take time to improvement, so
please wait for a while.
Umm, I have not read the patch, but are you not using Welford's method? Its
per-statement overhead should be absolutely tiny (and should not compute a
square
root at all per statement - the square root should only be computed when the
standard deviation is actually wanted, e.g. when a user examines
pg_stat_statements) See for example
<http://www.johndcook.com/standard_deviation.html>
Thanks for your advice. I read your example roughly, however, I think calculating
variance is not so heavy in my patch. Double based sqrt caluculation is most
heavily in my mind. And I find fast square root algorithm that is used in 3D games.
http://en.wikipedia.org/wiki/Fast_inverse_square_root
This page shows inverse square root algorithm, but it can caluculate normal
square root, and it is faster algorithm at the price of precision than general
algorithm. I think we want to fast algorithm, so it will be suitable.
Regards,
--
Mitsumasa KONDO
NTT Open Source Software Center
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