Date: Mon, 25 May 2026 17:33:28 +0200 From: Olivier Certner <olce@freebsd.org> To: Warner Losh <imp@bsdimp.com> Cc: Chris Torek <chris.torek@gmail.com>, Adrian Chadd <adrian@freebsd.org>, freebsd-current <freebsd-current@freebsd.org>, ShengYi Hung <aokblast@freebsd.org> Subject: Re: AMD, CPPC, etc Message-ID: <29674359.QdLYigECfs@ravel> In-Reply-To: <CANCZdfoyqdEr_1%2Bkmi5_0TTG0VaAZMjXgGOubWt5nVK8aqe9DQ@mail.gmail.com> References: <CAPx1GvfaDdmj0vkzzinyhGm2mQRKgmOh3fpNXo-cgznoujVY-Q@mail.gmail.com> <1978836.vR5SVPPSqJ@ravel> <CANCZdfoyqdEr_1%2Bkmi5_0TTG0VaAZMjXgGOubWt5nVK8aqe9DQ@mail.gmail.com>
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[-- Attachment #1 --] > At work, we run powerd on all our video streaming servers to save power by > ramping the cpu frequency up in times of high demand, and then back down > for low demand times.... it saves a lot of power. Yes, I'm aware (see our discussion with Andrew). Unless I'm mistaken, you're not using vanilla powerd(8) though. It's a very interesting use case, and something we would like to do in base FreeBSD coupled with CPPC. > But to interpret our performance data, we have to record the settings since 90% cpu at 800MHz is > way different than 90% at 3GHz... whatever we do here has to be observeable. This is where problems may come with CPPC, despite its parameters being observable. I remember reading somewhere (ACPI spec probably; don't have time to check right now) that the performance levels are to be *roughly* linear (they may have meant affine instead). As long as AMD continues to use only frequency levels to implement some CPPC performance level, affine regression could be perfect (but even that should be tested). If/when that changes, you will find yourself having to calibrate some model of CPPC levels to be able to compare 90% CPU at perf' level 127 and let's say 85% CPU at perf' level 128, and may never be able to do so with sufficient precision (depending on the exact hardware mechanisms used to implement the levels). Regards. -- Olivier Certner [-- Attachment #2 --] -----BEGIN PGP SIGNATURE----- iQIzBAABCQAdFiEEmNCxHjkosai0LYIujKEwQJceJicFAmoUa8gACgkQjKEwQJce JiezSg//Z5lGD5Or1eyw5bzvCL7lnLFzaAtWP3CBuEqZC4kPiMV0BmMTbdDEAgL9 /Oh2IcWxeuTo4N0j4fiyDooh9fbdQjN9edaDMaugAdHL65DQUkV5Dl9fK+pmaHCB XFKVhg4L579p0SijMNzi/qmCnbJklmZlFYbgONvelRBcu+c7U+65LqyTjPS7axgQ adfoHcCelDFDbzOhlgNXfSjrlpyPSIqJFa+tOBjoLaeDOOXZP65lWzFq+h/DCaib a2gNNTuLDhEyppFZNw8iZmjdiOAdD4/L1bj5Cm0zeUOelBKNdXYhy6cOCFRv9DxD LAf27yY0qcOUKhJgCPdu9C25P9bRF6ZZhaWb5gUzBC/+i0e1wUKCe9Ad3qNRqcUG WfvGpvO2CDCiGTr1RzB7R2b0rDPybP9bdboJ/DU0F2UzR91q/GxWr6/czFkS1AqH n7Hcd3nxiFJJd6h48vsGD7dIW/ajbWtOy1mTTpKWA/YMiCv6QawjbcIZBd6dM8r5 pDUcxPyM4eFiImPl56f6pga2Hp0HjIevgaPdy5bMod+MscCMwhUwuq6aqSudwPTq cj91K9k0xnObEnQdLdblSKadtpOe2ZShsju2LaiC9U9xn3twFpOVhNASWnua5Wzz 6Ta0GZ8mcK89Ax3WOmjlMOIdFDfkqJx0NzrPx3ovSH7xixP5Deo= =pfSB -----END PGP SIGNATURE-----home | help
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