linux-kernel - Re: Running an Ivy Bridge cpu at fixed frequency

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Message-ID: <CALW4P++GCR9Vha8Ef25rAmPZWNDxXxqebH+Vw1WXyLm9NZ+3pQ@mail.gmail.com>
Date:   Fri, 6 Dec 2019 14:47:13 +0000
From:   Alexey Klimov <klimov.linux@...il.com>
To:     David Laight <David.Laight@...lab.com>
Cc:     "x86@...nel.org" <x86@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Running an Ivy Bridge cpu at fixed frequency

On Wed, Dec 4, 2019 at 5:32 PM David Laight <David.Laight@...lab.com> wrote:
>
> Is there any way to persuade the intel_pstate driver to make an Ivy bridge (i7-3770)
> cpu run at a fixed frequency?
> It is really difficult to compare code execution times when the cpu clock speed
> keeps changing.
> I thought I'd managed by setting the 'scaling_max_freq' to 1.7GHz, but even that
> doesn't seem to be working now.
> It would also be nice to run a little faster than that - but without it 'randomly'
> going to 'turbo' frequencies (which it is doing even after I've set no_turbo to 1).
>
> An alternative would be a variable frequency TSC - might give more consistent values.

Have you tried intel_pstate=passive parameter in cmdline?
You'll be able to fix the frequency using governors or sysfs.
Not sure that this is what you're looking for. I personally also don't
know that 'passive' mode will work on Ivy Bridge.

Best regards,
Alexey