From mboxrd@z Thu Jan 1 00:00:00 1970 From: David C Niemi Subject: Re: Improving High-Load Performance with the Ondemand Governor Date: Mon, 13 Sep 2010 18:02:17 -0400 Message-ID: <4C8E9F69.3020502@verisign.com> References: <4C88EF04.1030908@verisign.com> <87r5h2t5b5.fsf@basil.nowhere.org> <4C8E872B.2020503@verisign.com> <20100913225456.13cbbaef@basil.nowhere.org> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20100913225456.13cbbaef@basil.nowhere.org> Sender: cpufreq-owner@vger.kernel.org List-ID: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: Andi Kleen Cc: cpufreq@vger.kernel.org Andi Kleen wrote: > On Mon, 13 Sep 2010 16:18:51 -0400 > David C Niemi wrote: > > >> I have looked at the latest kernels too, and the changes in the >> ondemand governor between that and RHEL 6's 2.6.32 kernel are quite >> modest. I mention 2.6.18 just because it's what's been out in the >> field a while. >> > > Most of the interesting changes were post 2.6.32 (2.6.32 is ancient > too for mainline) > I did see a few changes in cpufreq_ondemand.c between 2.6.32 and the git version I grabbed last week, but not really relevant to what I was trying to do. >>> FWIW when you're truly idle you typically don't need ondemand, >>> the idle states on modern CPUs go to the lowest frequency by >>> themselves or simply turn off the frequency completely. >>> >>> >> I do see c-states getting used on Intel hardware to save power, and >> > > ondemand has nothing to do with c-states, c-states are handled > by the menu governor. > We're using the standard cpuidle on the newer (RHEL 6 beta-based) kernels. If you think there are compelling improvements in it after 2.6.32 I'll certainly take a look. >> in some cases these are quite effective. On AMD hardware lowering >> frequency tends to be very important to saving power. >> > > AFAIK modern AMD doesn't need this either in c-states. > It makes a dramatic difference in power consumption whether you use a p-state governor on the 2-year-old AMD hardware that matters to me. On both old (Woodcrest) and new (Nehalem) Intel hardware the difference is much smaller, as c-states are the dominant form of power saving, but using a p-state governor still makes a measurable difference. On the plus side the power-saving c-states we are using don't measurably hurt performance on our workloads, so cpuidle is doing a pretty good job; whereas the stock ondemand p-state governor does in a big way. >> On moderate load I might agree, but on the servers I care about it is >> a workload that's a bit like war -- long periods of boredom >> punctuated by sudden bursts of sheer terror. >> > > In this case on modern hardware you don't need a p-state > governor at all except for "performance" > > -Andi > No doubt true in the long run, but see above. DCN