| From: | Peter Geoghegan <pg(at)heroku(dot)com> |
|---|---|
| To: | Kevin Grittner <kgrittn(at)ymail(dot)com> |
| Cc: | Robert Haas <robertmhaas(at)gmail(dot)com>, Martijn van Oosterhout <kleptog(at)svana(dot)org>, Ants Aasma <ants(at)cybertec(dot)at>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: Vacuum, Freeze and Analyze: the big picture |
| Date: | 2013-06-03 23:41:32 |
| Message-ID: | CAM3SWZRzVB0gTY1LNh4L+MBHythC_ez5isF6ARmnD7jfJHJX_A@mail.gmail.com |
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| Lists: | pgsql-hackers |
On Mon, Jun 3, 2013 at 3:16 PM, Kevin Grittner <kgrittn(at)ymail(dot)com> wrote:
>> But it seems like the kernel is disposed to cache large amounts
>> of dirty data for an unbounded period of time even if the I/O
>> system is completely idle,
>
> It's not unbounded time. Last I heard, the default was 30 seconds.
I'm pretty sure it is unbounded. The VM documentation is a bit vague
on what dirty_expire_centisecs actually means, which is I presume
where this number comes from. It says:
"This tunable is used to define when dirty data is old enough to be eligible
for writeout by the pdflush daemons. It is expressed in 100'ths of a second.
Data which has been dirty in-memory for longer than this interval will be
written out next time a pdflush daemon wakes up."
So I think the a pdflush daemon won't necessarily wake up until
dirty_background_bytes or dirty_background_ratio have been exceeded,
regardless of this threshold. Am I mistaken?
https://www.kernel.org/doc/Documentation/sysctl/vm.txt
--
Peter Geoghegan
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