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Date: Fri, 13 Jun 2014 20:39:27 +0300 From: Stratos Karafotis <stratosk@...aphore.gr> To: Doug Smythies <dsmythies@...us.net> CC: "'Rafael J. Wysocki'" <rjw@...ysocki.net>, viresh.kumar@...aro.org, dirk.j.brandewie@...el.com, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] cpufreq: intel_pstate: Fix rounding of core_pct On 13/06/2014 09:49 πμ, Doug Smythies wrote: > On 2014.06.12 13:03 Rafael J. Wysocki wrote: >> On Thursday, June 12, 2014 05:35:59 PM Stratos Karafotis wrote: >>> On 12/06/2014 12:15 πμ, Doug Smythies wrote: >>>> >>>> >>>> On 2014.06.11 13:20 Stratos Karafotis wrote: >>>>> On 11/06/2014 06:02 μμ, Doug Smythies wrote: >>>>>> >>>>>> On 2104.06.11 07:08 Stratos Karafotis wrote: >>>>>>> On 11/06/2014 04:41 μμ, Doug Smythies wrote: >>>>>>> >>>>>>> No. >>>>> >>>>>>> The intent was only ever to round properly the pseudo floating point result of the divide. >>>>>>> It was much more important (ugh, well 4 times more) when FRACBITS was still 6, which also got changed to 8 in a recent patch. >>>>>>> >>>>>> >>>>>> Are you sure? >>>>>> >>>>>> Yes. >>>>>> >>>>>>> This rounding was very recently added. >>>>>>> As far as I can understand, I don't see the meaning of this rounding, as is. >>>>>>> Even if FRAC_BITS was 6, I think it would have almost no improvement in >>>>>>> calculations. >>>>>> >>>>>> Note: I had not seen this e-mail when I wrote a few minutes ago: >>>>>> >>>>>> You may be correct. >>>>>> If Dirk agrees, I will re-analyse the entire driver for rounding effects soon. >>>>>> When FRACBITS was 6 there were subtle cases where the driver would get stuck, and not make a final pstate change, with the default PID gains. >>>>>> Other things have changed, and the analysis needs to be re-done. >>>>>> >>>> >>>>> Could you please elaborate a little bit more what we need these 2 lines below? >>>>> >>>>> if ((rem << 1) >= int_tofp(sample->mperf)) >>>>> core_pct += 1; >>>>> >>>>> Because nothing is mentioned for them in commit's changelog. >>>>> Do we need to round core_pct or not? >>>>> Because if we try to round it, I think this patch should work. >>>> >>>> As mentioned originally, they are there just to round the pseudo floating number, not the integer portion only. >>>> Let us bring back the very numbers you originally gave and work through it. >>>> >>>> aperf = 5024 >>>> mperf = 10619 >>>> >>>> core_pct = 47.31142292% >>>> or 47 and 79.724267 256ths >>>> or to the closest kept fractional part 47 and 80 256ths >>>> or 12112 as a pseudo float. >>>> The maximum error with this rounding will be 1 part in 512 and symmetric instead of the 1 part in 256 always in one direction without. >>>> >>>> Now if FRACBITS was still 6: >>>> core_pct = 47.31142292% >>>> or 47 and 19.931 64ths >>>> or to the closest kept fractional part 47 and 20 64ths >>>> or 3028 as a pseudo float. >>>> The maximum error with this rounding will be 1 part in 128 and symmetric instead of the 1 part in 64 (1.6% !!!) always in one direction without. >>>> >>>> Hope this helps. >>>> >>> >>> Yes, it helps. Thanks a lot! >>> >>> But please note that the maximum error without this rounding will be 1.6% _only_ >>> in fractional part. In the real number it will be much smaller: > > Fair comment. Thanks. > >>> >>> 47.19 instead of 47.20 >>> >>> And using FRAC_BITS 8: >>> >>> 47.79 instead of 47.80 >>> > > I really wouldn't write it that way, as I find it misleading. It is really 47 and 19 256ths... > Anyway, I think we all understand. > >>> This is a 0.0002% difference. I can't see how this is can affect the calculations >>> even with FRAC_BITS 6. > > O.K. The solution is overkill and div_u64 could have been used instead of div_u64_rem. > On my list, it is the lowest of priorities. > >>> >>> Another thing is that this algorithm generally is used to round to the >>> nearest integer. I'm not sure if it's valid to apply it for the rounding of >>> the fractional part of fixed point number. > > I'm not sure how to reply, a pseudo floating point number is just an integer. > >> Depending on the original reason, it may or may not be. > > The original reason for that overall code patch was to address the possible overflow of the math, which (as far I know and have tested) it does. > I think we have gone down a bit of rat hole here in terms of the detail. > Hi Doug, I'm sorry if I pushed it too far. But sometimes many small details could make the difference. At least we finally agreed to something! :-) Thanks for your time and for your comments! Stratos P.S. Talking about small details and with a sense of humor (I hope) I present some roughly estimates about the tiny change (the 2-3 lines removal). Let's assume that this code needs 100 CPU cycles to run. In a full active core, at a sampling rate 10ms, the code runs 8,640,000 times/day and if we suppose that the core will be 90% of the day inactive, it needs 86,400,000 CPU cycles/day. If the CPU runs in a typical 2GHz freq the code will need 0.0432 secs to be executed (per day). With a 15W avg power consumption we need 0,648 Joules/day per core. In a typical quad core PC we need 2.592 Joules/day or 946,08 Joules/year. I read that there are 2 billion PCs in the planet. Assuming that 1.5% of them running Linux and 50% of them will use this driver, the code will run on 30,000,000 PCs. So, we need: 14,191,200,000 Joules/year = 3,942 KWh And: 3,942 KWh * 2.3 = 9,066 lb CO2 = 4,112 Kg CO2 Thus, removing these 2 lines we will reduce the global CO2 emissions by 4,112 Kg! :-) -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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