Re: [RFC PATCH v3 04/14] mm: use part per 1000 for bdi ratios.

[Stefan Roesch <shr@devkernel.io>]

Andrew Morton <akpm@linux-foundation.org> writes:

> On Mon, 24 Oct 2022 12:05:53 -0700 Stefan Roesch <shr@devkernel.io> wrote:
>
>> To get finer granularity for ratio calculations use part per 1000
>> instead of percentiles. This is especially important if we want to
>> automatically convert byte values to ratios. Otherwise the values that
>> are actually used can be quite different. This is also important for
>> machines with more main memory (1% of 256GB is already 2.5GB).
>>
>> ...
>>
>
> This changes an existing userspace interface, doesn't it?
> /sys/class/bdi/<bdi>/min_ratio.  Can't do that!
>

It does not change the user interface. It maintains the percent values
in the min_ratio and max_ratio knobs.

For instance:

-BDI_SHOW(min_ratio, bdi->min_ratio)
+BDI_SHOW(min_ratio, bdi->min_ratio / BDI_RATIO_SCALE)

> We could add a new interace to the same thing, I guess.
> /sys/class/bdi/<bdi>/min_ratio_fine or whatever.
>
> We might want to go for more than 100->1000, too.  Otherwise in a few
> years we'll be adding /sys/class/bdi/<bdi>/min_ratio_even_finer.
>

The next version of the patch series will also add min_ratio_fine and
max_ratio_fine. This makes sure that also ratio values can be specified
with a finer granularity.

> Also, this patch forgot to update
> Documentation/ABI/testing/sysfs-class-bdi.

The exisiting user behavior is not changed, only the internal
calculation.