linux-kernel - Re: [PATCH] backing_dev_info: introduce min_bw/max

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Message-ID: <CAH9Oa-YxeZ25Vbto3NyUw=RK5vQWv_v7xp3vHS9667iJJ8XV_A@mail.gmail.com>
Date:   Mon, 21 Jun 2021 11:20:10 +0200
From:   Michael Stapelberg <stapelberg+linux@...gle.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>,
        linux-fsdevel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Dennis Zhou <dennis@...nel.org>, Jens Axboe <axboe@...nel.dk>,
        Roman Gushchin <guro@...com>,
        Johannes Thumshirn <johannes.thumshirn@....com>,
        Jan Kara <jack@...e.cz>, Song Liu <song@...nel.org>,
        David Sterba <dsterba@...e.com>
Subject: Re: [PATCH] backing_dev_info: introduce min_bw/max_bw limits

Hey Miklos

On Fri, 18 Jun 2021 at 16:42, Miklos Szeredi <miklos@...redi.hu> wrote:
>
> On Fri, 18 Jun 2021 at 10:31, Michael Stapelberg
> <stapelberg+linux@...gle.com> 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:
> > https://lore.kernel.org/linux-fsdevel/CANnVG6n=ySfe1gOr=0ituQidp56idGARDKHzP0hv=ERedeMrMA@mail.gmail.com/
> >
> > Have you had a look at the China LSF report at
> > http://bardofschool.blogspot.com/2011/?
> > 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.

Agreed.

>
> 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.

Thanks
Best regards
Michael