On Mon, Jun 3, 2013 at 3:16 PM, Kevin Grittner <kgrittn@ymail.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