Re: Improvement of checkpoint IO scheduler for stable transaction responses

Hi,

I took results of my separate patches and original PG.

* Result of DBT-2
| TPS 90%tile Average Maximum
------------------------------------------------------
original_0.7 | 3474.62 18.348328 5.739 36.977713
original_1.0 | 3469.03 18.637865 5.842 41.754421
fsync | 3525.03 13.872711 5.382 28.062947
write | 3465.96 19.653667 5.804 40.664066
fsync + write | 3564.94 16.31922 5.1 34.530766

- 'original_*' indicates checkpoint_completion_target in PG 9.2.4.
- In other patched postgres, checkpoint_completion_target sets 0.7.
- 'write' is applied write patch, and 'fsync' is applied fsync patch.
- 'fsync + write' is applied both patches.

* Investigation of result
- Large value of checkpoint_completion_target in original and the patch in
write become slow latency in benchmark transactions. Because slow write pages are
caused long time fsync IO in final checkpoint.
- The patch in fsync has an effect latency in each file fsync. Continued
fsyncsin each files are caused slow latency. Therefore, it is good for latency
that fsync stage in checkpoint has sleeping time after slow fsync IO.
- The patches of fsync + write were seemed to improve TPS. I think that write
patch does not disturb transactions which are in full-page-write WAL write than
original(plain) PG.

I will send you more detail investigation and result next week. And I will also
take result in pgbench. If you mind other part of benchmark result or parameter
of postgres, please tell me.

Best Regards,
--
Mitsumasa KONDO
NTT Open Source Software Center