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[46.139.12.213]) by smtp.gmail.com with ESMTPSA id 204sm30841621wmc.1.2019.04.26.02.45.47 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Fri, 26 Apr 2019 02:45:48 -0700 (PDT) Date: Fri, 26 Apr 2019 11:45:45 +0200 From: Ingo Molnar To: Mel Gorman Cc: Aubrey Li , Julien Desfossez , Vineeth Remanan Pillai , Nishanth Aravamudan , Peter Zijlstra , Tim Chen , Thomas Gleixner , Paul Turner , Linus Torvalds , Linux List Kernel Mailing , Subhra Mazumdar , Fr?d?ric Weisbecker , Kees Cook , Greg Kerr , Phil Auld , Aaron Lu , Valentin Schneider , Pawan Gupta , Paolo Bonzini , Jiri Kosina Subject: Re: [RFC PATCH v2 00/17] Core scheduling v2 Message-ID: <20190426094545.GD126896@gmail.com> References: <20190424140013.GA14594@sinkpad> <20190425095508.GA8387@gmail.com> <20190425144619.GX18914@techsingularity.net> <20190425185343.GA122353@gmail.com> <20190425213145.GY18914@techsingularity.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190425213145.GY18914@techsingularity.net> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Mel Gorman wrote: > > > I can show a comparison with equal levels of parallelisation but with > > > HT off, it is a completely broken configuration and I do not think a > > > comparison like that makes any sense. > > > > I would still be interested in that comparison, because I'd like > > to learn whether there's any true *inherent* performance advantage to > > HyperThreading for that particular workload, for exactly tuned > > parallelism. > > > > It really isn't a fair comparison. MPI seems to behave very differently > when a machine is saturated. It's documented as changing its behaviour > as it tries to avoid the worst consequences of saturation. > > Curiously, the results on the 2-socket machine were not as bad as I > feared when the HT configuration is running with twice the number of > threads as there are CPUs > > Amean bt 771.15 ( 0.00%) 1086.74 * -40.93%* > Amean cg 445.92 ( 0.00%) 543.41 * -21.86%* > Amean ep 70.01 ( 0.00%) 96.29 * -37.53%* > Amean is 16.75 ( 0.00%) 21.19 * -26.51%* > Amean lu 882.84 ( 0.00%) 595.14 * 32.59%* > Amean mg 84.10 ( 0.00%) 80.02 * 4.84%* > Amean sp 1353.88 ( 0.00%) 1384.10 * -2.23%* Yeah, so what I wanted to suggest is a parallel numeric throughput test with few inter-process data dependencies, and see whether HT actually improves total throughput versus the no-HT case. No over-saturation - but exactly as many threads as logical CPUs. I.e. with 20 physical cores and 40 logical CPUs the numbers to compare would be a 'nosmt' benchmark running 20 threads, versus a SMT test running 40 threads. I.e. how much does SMT improve total throughput when the workload's parallelism is tuned to utilize 100% of the available CPUs? Does this make sense? Thanks, Ingo