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

[Andrea Righi <righi.andrea@gmail.com>]

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