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[82.29.237.198]) by smtp.gmail.com with ESMTPSA id g138sm43125753wmg.32.2021.07.26.05.07.51 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 26 Jul 2021 05:07:52 -0700 (PDT) Date: Mon, 26 Jul 2021 13:07:50 +0100 From: "Dr. David Alan Gilbert" To: Philippe =?iso-8859-1?Q?Mathieu-Daud=E9?= Subject: Re: Prefetches in buffer_zero_* Message-ID: References: <092f9b8b-4a14-d059-49be-010b760828aa@linaro.org> MIME-Version: 1.0 In-Reply-To: User-Agent: Mutt/2.0.7 (2021-05-04) Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=dgilbert@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=216.205.24.124; envelope-from=dgilbert@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -34 X-Spam_score: -3.5 X-Spam_bar: --- X-Spam_report: (-3.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.717, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_LOW=-0.7, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: =?utf-8?B?THVrw6HFoQ==?= Doktor , Richard Henderson , qemu-devel@nongnu.org, Joe Mario Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" * Philippe Mathieu-Daudé (philmd@redhat.com) wrote: > +Lukáš > > On 7/26/21 10:47 AM, Dr. David Alan Gilbert wrote: > > * Joe Mario (jmario@redhat.com) wrote: > >> On Thu, Jul 22, 2021 at 3:14 PM Dr. David Alan Gilbert > >> wrote: > >> > >>> * Richard Henderson (richard.henderson@linaro.org) wrote: > >>>> On 7/22/21 12:02 AM, Dr. David Alan Gilbert wrote: > >>>>> Hi Richard, > >>>>> I think you were the last person to fiddle with the prefetching > >>>>> in buffer_zero_avx2 and friends; Joe (cc'd) wondered if explicit > >>>>> prefetching still made sense on modern CPUs, and that their hardware > >>>>> generally figures stuff out better on simple increments. > >>>>> > >>>>> What was your thinking on this, and did you actually measure > >>>>> any improvement? > >>>> > >>>> Ah, well, that was 5 years ago so I have no particular memory of this. > >>> It > >>>> wouldn't surprise me if you can't measure any improvement on modern > >>>> hardware. > >>>> > >>>> Do you now measure an improvement with the prefetches gone? > >>> > >>> Not tried, it just came from Joe's suggestion that it was generally a > >>> bad idea these days; I do remember that the behaviour of those functions > >>> is quite tricky because there performance is VERY data dependent - many > >>> VMs actually have pages that are quite dirty so you never iterate the > >>> loop, but then you hit others with big zero pages and you spend your > >>> entire life in the loop. > >>> > >>> > >> Dave, Richard: > >> My curiosity got the best of me. So I created a small test program that > >> used the buffer_zero_avx2() routine from qemu's bufferiszero.c. > > > > Thanks for testing, > > > >> When I run it on an Intel Cascade Lake processor, the cost of calling > >> "__builtin_prefetch(p)" is in the noise range . It's always "just > >> slightly" slower. I doubt it could ever be measured in qemu. > >> > >> Ironically, when I disabled the hardware prefetchers, the program slowed > >> down over 33%. And the call to "__builtin_prefetch(p)" actually hurt > >> performance by over 3%. > > > > Yeh that's a bit odd. > > > >> My results are below, (only with the hardware prefetchers enabled). The > >> program is attached. > >> Joe > >> > >> # gcc -mavx buffer_zero_avx.c -O -DDO_PREFETCH ; for i in {1..5}; do > >> ./a.out; done > >> TSC 356144 Kcycles. > >> TSC 356714 Kcycles. > >> TSC 356707 Kcycles. > >> TSC 356565 Kcycles. > >> TSC 356853 Kcycles. > >> # gcc -mavx buffer_zero_avx.c -O ; for i in {1..5}; do ./a.out; done > >> TSC 355520 Kcycles. > >> TSC 355961 Kcycles. > >> TSC 355872 Kcycles. > >> TSC 355948 Kcycles. > >> TSC 355918 Kcycles. > > > > This basically agrees with the machines I've just tried your test on - > > *except* AMD EPYC 7302P's - that really like the prefetch: > > > > [root@virtlab720 ~]# gcc -mavx buffer_zero_avx.c -O -DDO_PREFETCH ; for i in {1..5}; do ./a.out; done > > TSC 322162 Kcycles. > > TSC 321861 Kcycles. > > TSC 322212 Kcycles. > > TSC 321957 Kcycles. > > TSC 322085 Kcycles. > > > > [root@virtlab720 ~]# gcc -mavx buffer_zero_avx.c -O ; for i in {1..5}; do ./a.out; done > > TSC 377988 Kcycles. > > TSC 380125 Kcycles. > > TSC 379440 Kcycles. > > TSC 379689 Kcycles. > > TSC 379571 Kcycles. > > > > The 1st gen doesn't seem to see much difference with/without it. > > > > Probably best to leave this code as is! > > Regardless the decision of changing the code or not, it would be > nice to have this test committed in the repository to run > performance regression testing from time to time. It could be, although this is a slightly odd microtest for that; it's a bit specific (the avx2 variant, and only really testing the all zero case). Dave > >> /* > >> * Simple program to test if a prefetch helps or hurts buffer_zero_avx2. > >> * > >> * Compile with either: > >> * gcc -mavx buffer_zero_avx.c -O > >> * or > >> * gcc -mavx buffer_zero_avx.c -O -DDO_PREFETCH > >> */ > >> > [...] > -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK