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[209.132.180.67]) by mx.google.com with ESMTP id p6si13834227plo.4.2019.03.25.03.38.28; Mon, 25 Mar 2019 03:38:43 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730668AbfCYKgc (ORCPT + 99 others); Mon, 25 Mar 2019 06:36:32 -0400 Received: from smtp.nue.novell.com ([195.135.221.5]:58000 "EHLO smtp.nue.novell.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730484AbfCYKgb (ORCPT ); Mon, 25 Mar 2019 06:36:31 -0400 Received: from emea4-mta.ukb.novell.com ([10.120.13.87]) by smtp.nue.novell.com with ESMTP (TLS encrypted); Mon, 25 Mar 2019 11:36:30 +0100 Received: from [192.168.0.30] (nwb-a10-snat.microfocus.com [10.120.13.201]) by emea4-mta.ukb.novell.com with ESMTP (TLS encrypted); Mon, 25 Mar 2019 10:36:02 +0000 Message-ID: <8eec0949bac18d8ee6f151027ff68de9827f143d.camel@suse.com> Subject: Re: [PATCH] x86/xen: Add "xen_timer_slop" command line option From: Dario Faggioli To: Boris Ostrovsky , Ryan Thibodeaux Cc: xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, oleksandr_andrushchenko@epam.com, tglx@linutronix.de, jgross@suse.com, ryan.thibodeaux@starlab.io, "luca.abeni" Date: Mon, 25 Mar 2019 11:36:00 +0100 In-Reply-To: <20190324180724.GA4400@nat-wireless-guest-reg-153-50.bu.edu> References: <1553279397-130201-1-git-send-email-ryan.thibodeaux@starlab.io> <20190323120052.GB11100@centos-dev.localdomain> <20190324180724.GA4400@nat-wireless-guest-reg-153-50.bu.edu> Organization: SUSE Content-Type: multipart/signed; micalg="pgp-sha256"; protocol="application/pgp-signature"; boundary="=-2Aj+Mypawqc5P5Mv2XTx" User-Agent: Evolution 3.30.5 MIME-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org --=-2Aj+Mypawqc5P5Mv2XTx Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Sun, 2019-03-24 at 14:07 -0400, Boris Ostrovsky wrote: > On Sat, Mar 23, 2019 at 08:00:52AM -0400, Ryan Thibodeaux wrote: > > This test system was configured to use a TSC clocksource, disabled > > C states, and lowered the timer slop. I am not claiming the timer > > slop change was solely responsible for the best results. >=20 > How can we then be sure that the proposed change will indeed provide > some sort of benefit? >=20 > Were there any other changes between your tests to think that slop > time modification may not be responsible for better results? >=20 FWIW, in mine and Luca's experiments, changing lowering both timer_slop in Xen and TIMER_SLOP in Linux, improved latency dramatically, without any other change. We also tried _only_ playing with the Xen tunable (as there's a Xen boot parameter for it already) but that wasn't enough. It was only when we also tuned TIMER_SLOP in Linux's Xen clockevent device that we got decent numbers (i.e., comparable to KVM ones). Reason why we had not share these results yet was that we were still "polishing" them, and because we also found a couple of other issues, and we were trying to understand them better, before sending anything out. But those other issues were --although still about achieving low latencies-- orthogonal from this, and lowering the default slop is absolute prerequisite for even talking about having a reasonable vcpu response time. A patch like this one, was something we were thinking to submit ourself sooner or later (backed up by our results). Personally, in addition to making the value tunable, which I think is a good thing, I also would lower the default. Regards, Dario --=20 <> (Raistlin Majere) ----------------------------------------------------------------- Dario Faggioli, Ph.D, http://about.me/dario.faggioli Software Engineer @ SUSE https://www.suse.com/ --=-2Aj+Mypawqc5P5Mv2XTx Content-Type: application/pgp-signature; name="signature.asc" Content-Description: This is a digitally signed message part Content-Transfer-Encoding: 7bit -----BEGIN PGP SIGNATURE----- iQIzBAABCAAdFiEES5ssOj3Vhr0WPnOLFkJ4iaW4c+4FAlyYrxAACgkQFkJ4iaW4 c+4RBw/+Jb25t+D3NqB7raaBYvuDFoFlEdJJEBtsPLe8HNDFU3aD8p3Mr+v9m0KU hWiWBcAfqISbCk3gsb4Qfw5met5E1hg9EsA1qJVhsKdPiwPUrDnRfwxOHRgLSw/6 FPZ+MKo0REE9VvXPkuhcebmM7Ec98Q4iiqXS9POF0iTlDnu1NqiQ2e1II9Eu2/L7 +Bdaq6kbSsTLRuI5HBNdOCcoFMJ74NcI+p6xSvVZOkHTyPFlNiXfhc/tXVXvkPJP jdJPyqGvVNUtawG/AtEDDwIUxhrEbr+iq4tFRGpVqEQBlYfqI1DbJMCRInEEGA+v n2bMWJGGSYTteIe1pyxnanCNrVr4q7IdIip0AIxnur6mwx2TDTdzTFI9kBjWZtGb VfzzXt6YXYiCAtcOIYNdd81xtVJukYsmuWLy8kG2/SCDLTSK1EaBidmUJzcydmfY pkpN2rS/4SyIw/uiAFxLuC3DaDNumVmiB+kJSJd0sOw+381YDmKr9O0YtiFD8E8K sgHSaaev73etzvIDawbZJ/6zwVp4wGzzll+lnLEqenzNaq7oPVc1AXag3JWU5dwU hT11iJseaqIcDhDZHJjQug//9enDk6gBDTcsCX/pGNi3wqrCm1BSSrD8fFqfCJWO Gc69tHtjXCUjJpOBign0HbJuni4P6LJcklcKyejceSCBwcaUsi8= =0s/V -----END PGP SIGNATURE----- --=-2Aj+Mypawqc5P5Mv2XTx--