Message-ID: <51F0F9C1.9080207@lab.ntt.co.jp>
Date: Thu, 25 Jul 2013 19:11:13 +0900
From: KONDO Mitsumasa <kondo.mitsumasa@lab.ntt.co.jp>
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To: Greg Smith <greg@2ndQuadrant.com>, Robert Haas <robertmhaas@gmail.com>,
 Heikki Linnakangas <hlinnakangas@vmware.com>, didier <did447@gmail.com>
CC: PostgreSQL-development <pgsql-hackers@postgresql.org>
Subject: Re: Improvement of checkpoint IO scheduler for stable transaction
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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


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