Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754521Ab1D1V2Y (ORCPT ); Thu, 28 Apr 2011 17:28:24 -0400 Received: from smtp-out.google.com ([216.239.44.51]:46651 "EHLO smtp-out.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751245Ab1D1V2W convert rfc822-to-8bit (ORCPT ); Thu, 28 Apr 2011 17:28:22 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=google.com; s=beta; h=mime-version:in-reply-to:references:from:date:message-id:subject:to :cc:content-type:content-transfer-encoding; b=qAyEFmQgSmL7hdy+5WiXjIDOU+g1tHmvM9tdhxie45x+DZxDQxhNVvSWZYJOKW2sdi yH5JdbyzfJnOZXjtQz9Q== MIME-Version: 1.0 In-Reply-To: References: <1303332697-16426-1-git-send-email-ncrao@google.com> <20110428121244.GA26851@linux.vnet.ibm.com> From: Nikhil Rao Date: Thu, 28 Apr 2011 14:27:59 -0700 Message-ID: Subject: Re: [RFC][PATCH 00/18] Increase resolution of load weights To: Paul Turner Cc: vatsa@linux.vnet.ibm.com, "Nikunj A. Dadhania" , Ingo Molnar , Peter Zijlstra , Mike Galbraith , "linux-kernel@vger.kernel.org" , Bharata B Rao Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT X-System-Of-Record: true Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2721 Lines: 60 On Thu, Apr 28, 2011 at 11:51 AM, Paul Turner wrote: > On Thu, Apr 28, 2011 at 11:33 AM, Nikhil Rao wrote: >> On Thu, Apr 28, 2011 at 5:12 AM, Srivatsa Vaddagiri >> wrote: >>> On Thu, Apr 28, 2011 at 05:18:27PM +0530, Nikunj A. Dadhania wrote: >>>> --- kernel/sched.c.orig       2011-04-28 16:34:24.000000000 +0530 >>>> +++ kernel/sched.c    2011-04-28 16:36:29.000000000 +0530 >>>> @@ -1336,7 +1336,7 @@ calc_delta_mine(unsigned long delta_exec >>>>                       lw->inv_weight = 1 + (WMULT_CONST - w/2) / (w + 1); >>>>       } >>>> >>>> -     tmp = (u64)delta_exec * weight; >>>> +     tmp = (u64)delta_exec * (weight >> SCHED_LOAD_RESOLUTION); >>> >>> Should we be fixing inv_weight rather to account for SCHED_LOAD_RESOLUTION here? >>> >> >> Yes, I have been looking into fixing inv_weight and calc_delta_mine() >> calculations based on the assumption that we have u64 weights. IMO the >> function is complicated because the return value needs to be >> calculated to fit into unsigned long. I would like to update users of >> calc_delta_mine() to use u64 instead of unsigned longs and I think >> this can be easily done (quick inspection of the code shows two call >> sites that need to be updated - update_curr() and wakeup_gran()). >> Without the restriction to fit into unsigned long, I think we can make >> calc_delta_mine() and the inv_weight calculations simpler. >> > > I don't think you have much room to maneuver here, the calculations in > c_d_m() are already u64 based, even on 32bit.  Changing the external > load factors to 64 bit doesn't change this. > > We lose fairness in cdm beyond 32 bits, at the old LOAD_SCALE=10 > you've got 22 bits with which you can maintain fairness. This gives > total accuracy in total curr on any delta <= ~4ms (for a NICE_0 task). >  If you bump this up (and don't downshift before computing the inverse > as you are) then you start introducing rounding errors beyond ~4us. > > This would also be further exacerbated in sched_period() since that's > using the total cfs_rq weight. > Hmm... yeah, I think you have nicely explained the problem at hand. As you said, we can't bump up inv_weight without losing accuracy. We want to keep the 32-bit bound because it avoids the division. I was exploring other optimizations in this functions, but I think what Paul pointed out is key here. -Thanks Nikhil >> -Thanks, >> Nikhil >> > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/