Re: Improvement of checkpoint IO scheduler for stable transaction responses

Hi,

I understand why my patch is faster than original, by executing Heikki's patch.
His patch execute write() and fsync() in each relation files in write-phase in
checkpoint. Therefore, I expected that write-phase would be slow, and fsync-phase
would be fast. Because disk-write had executed in write-phase. But fsync time in
postgresql with his patch is almost same time as original. It's very mysterious!

I checked /proc/meminfo in executing benchmark and other resources. As a result,
this was caused by separating checkpointer process and writer process. In 9.1 or
older, checkpoint and background-write are executed in writer process by serial
schedule. But in 9.2 or later, it is executed by parallel schedule, regardless
executing checkpoint. Therefore, less fsync and long-term fsync schedule method
which likes my patch are so faster. Because waste disk-write was descend by
thease method. In worst case his patch, same peges disk-write are executed twice
in one checkpoint, moreover it might be random disk-write.

By the way, when dirty buffers which have always under dirty_background_ratio *
physical memory / 100, write-phase does not disk-write at all. Therefore, in
fsync-phase disk-write all of dirty buffer. So when this case, write-schedule is
not making sense. It's very heavy and waste, but it might not change by OS and
postgres parameters. I set small dirty_backjground_ratio, but the result was very
miserable...

Now, I am confirming my theory by dbt-2 benchmark in lru_max_pages = 0. And I
will be told about OS background-writing mechanism by my colleague who is kernel
hacker next week.

What do you think?

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