On 01/22/2014 11:33 PM, KONDO Mitsumasa wrote:
(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
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.

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.

According to the link I gave above:

   The most obvious way to compute variance then would be to have two
   sums: one to accumulate the sum of the x's and another to accumulate
   the sums of the squares of the x's. If the x's are large and the
   differences between them small, direct evaluation of the equation
   above would require computing a small number as the difference of
   two large numbers, a red flag for numerical computing. The loss of
   precision can be so bad that the expression above evaluates to a
/negative/ number even though variance is always positive.
As I read your patch that's what it seems to be doing.

What is more, if the square root calculation is affecting your benchmarks, I suspect you are benchmarking the wrong thing. The benchmarks should not call for a single square root calculation. What we really want to know is what is the overhead from keeping these stats. But your total runtime code (i.e. code NOT from calling pg_stat_statements()) for stddev appears to be this:

   e->counters.total_sqtime += total_time * total_time;



Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org)
To make changes to your subscription:

Reply via email to