linux-kernel - Re: [PATCH v5 0/6] Add hardware prefetch control driver for A64FX and x86

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Date:   Mon, 27 Jun 2022 11:36:11 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     "tarumizu.kohei@...itsu.com" <tarumizu.kohei@...itsu.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
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Subject: Re: [PATCH v5 0/6] Add hardware prefetch control driver for A64FX and x86

On Fri, Jun 17, 2022 at 11:21 AM tarumizu.kohei@...itsu.com
<tarumizu.kohei@...itsu.com> wrote:

Jumping in here.

> Hi Greg,
>
> > That's not ok.  Linux is a "general purpose" operating system and needs to
> > work well for all applications.  Doing application-specific-tuning based on the
> > specific hardware like this is a nightmare for users,
>
> Hardware prefetch behavior is enabled by default in x86 and A64FX.
> Many applications can perform well without changing the register
> setting. Use this feature for some applications that want to be
> improved performance.

The right way to solve this is to make the Linux kernel contain the necessary
heuristics to identify which tasks and thus cores need this to improve
efficiency and then apply it automatically.

Putting it in userspace is making a human do a machines job which isn't
sustainable.

By putting the heuristics in kernelspace Linux will improve performance also
on workloads the human operator didn't think of as the machine will
detect them from
statictical or other behaviour patterns.

Yours,
Linus Walleij