Re: [PATCH] backing_dev_info: introduce min_bw/max_bw limits

From: Michael Stapelberg
Date: Mon Jun 21 2021 - 05:20:32 EST

Hey Miklos

On Fri, 18 Jun 2021 at 16:42, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Fri, 18 Jun 2021 at 10:31, Michael Stapelberg
> <stapelberg+linux@xxxxxxxxxx> wrote:
> > Maybe, but I don’t have the expertise, motivation or time to
> > investigate this any further, let alone commit to get it done.
> > During our previous discussion I got the impression that nobody else
> > had any cycles for this either:
> >
> >
> > Have you had a look at the China LSF report at
> >
> > The author of the heuristic has spent significant effort and time
> > coming up with what we currently have in the kernel:
> >
> > """
> > Fengguang said he draw more than 10K performance graphs and read even
> > more in the past year.
> > """
> >
> > This implies that making changes to the heuristic will not be a quick fix.
> Having a piece of kernel code sitting there that nobody is willing to
> fix is certainly not a great situation to be in.


> And introducing band aids is not going improve the above situation,
> more likely it will prolong it even further.

Sounds like “Perfect is the enemy of good” to me: you’re looking for a
perfect hypothetical solution,
whereas we have a known-working low risk fix for a real problem.

Could we find a solution where medium-/long-term, the code in question
is improved,
perhaps via a Summer Of Code project or similar community efforts,
but until then, we apply the patch at hand?

As I mentioned, I think adding min/max limits can be useful regardless
of how the heuristic itself changes.

If that turns out to be incorrect or undesired, we can still turn the
knobs into a no-op, if removal isn’t an option.

Best regards