Re: [PATCH 0/3] VM throttling: avoid blocking occasional writers

From: Bill Davidsen
Date: Sun Mar 18 2007 - 10:56:51 EST


Andrew Morton wrote:
On Wed, 14 Mar 2007 21:42:46 +0900 Tomoki Sekiyama <tomoki.sekiyama.qu@xxxxxxxxxxx> wrote:

...


-Solution:

I consider that all of the dirty pages for the disk have been written
back and that the disk is clean if a process cannot write 'write_chunk'
pages in balance_dirty_pages().

To avoid using up the free memory with dirty pages by passing blocking,
this patchset adds a new threshold named vm.dirty_limit_ratio to sysctl.

It modifies balance_dirty_pages() not to block when the amount of
Dirty+Writeback is less than vm.dirty_limit_ratio percent of the memory.
In the other cases, writers are throttled as current Linux does.


In this patchset, vm.dirty_limit_ratio, instead of vm.dirty_ratio, is
used as the clamping level of Dirty+Writeback. And, vm.dirty_ratio is
used as the level at which a writers will itself start writeback of the
dirty pages.

Might be a reasonable solution - let's see what Peter comes up with too.

Comments on the patch:

- Please don't VM_DIRTY_LIMIT_RATIO: just use CTL_UNNUMBERED and leave
sysctl.h alone.

- The 40% default is already too high. Let's set this new upper limit to
40% and decrease he non-blocking ratio.

- Please update the procfs documentation in ./Docmentation/

- I wonder if dirty_limit_ratio is the best name we could choose. vm_dirty_blocking_ratio, perhaps? Dunno.

I don't like it, but I dislike it less than "dirty_limit_ratio" I guess. It would probably break things to change it now, including my sysctl.conf on a number of systems :-(

--
Bill Davidsen <davidsen@xxxxxxx>
"We have more to fear from the bungling of the incompetent than from
the machinations of the wicked." - from Slashdot
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