Re: [RFC] [PATCH -mm 0/2] memcg: per cgroup dirty_ratio

KOSAKI Motohiro wrote:
Currently the problem we are hitting is that we cannot specify pdflush
to have background limits less than 1% of memory. I am currently
finishing up a patch right now that adds a dirty_ratio_millis
interface. I hope to submit the patch to LKML by the end of the week.

The idea is that we don't want to break backwards compatibility and we
also don't want to have two conflicting knobs in the sysctl or
/proc/sys/vm/ space. I thought adding a new knob for those who want to
specify finer grained functionality was a compromise. So the patch has
a vm_dirty_ratio and a vm_dirty_ratio_millis interface. The first to
specify 0-100% and the second to specify .0 to .999%.

So to represent 0.125% of RAM we set
vm_dirty_ratio = 0
vm_dirty_ratio_millis = 125

The same for the background_ratio.

Why vm_dirty_ratio = 0.125 is wrong?
it is hardly for parser maker, but it have nicer user experience.

I would also prefer using a bytes interface but I am not sure how to
offer that without either removing the legacy interface of the ratios
or by offering a concurrent interface that might be confusing such as
when users are looking at the old one and not aware of a new one.

Any feedback?

Sure.
We don't have any motivation of its interface change.

The more I think about this and the more I would prefer to have an
interface in KB (or pages) that automatically adjusts the old int percentage
in dirty_ratio (the same for dirty_background_ratio).

The parser issue for writing decimal values doesn't seem to be a big
problem, but if the user expects to read an int from vm_dirty_ratio and
instead receives something like 0.125, well... this could break
something. So, IMHO also in this way we're changing the kernel-userspace
interface.

-Andrea
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