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Wed, 10 Sep 2025 05:09:05 -0700 (PDT) Received: from draig.lan ([185.126.160.19]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-3e75223898csm7031588f8f.39.2025.09.10.05.09.04 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 10 Sep 2025 05:09:04 -0700 (PDT) Received: from draig (localhost [IPv6:::1]) by draig.lan (Postfix) with ESMTP id 758355F82D; Wed, 10 Sep 2025 13:09:03 +0100 (BST) From: =?utf-8?Q?Alex_Benn=C3=A9e?= To: "Julian Ganz" Cc: "BALATON Zoltan" , qemu-devel@nongnu.org Subject: Re: [PATCH v6 00/25] tcg-plugins: add hooks for discontinuities In-Reply-To: <71a535f9154ddc269c0a948053e39053550bd8c0@nut.email> (Julian Ganz's message of "Wed, 10 Sep 2025 11:41:15 +0000") References: <3c8d0c3733edc984937ac33a417488f92dccb148@nut.email> <819c9f3b-13f4-2ef2-e651-5cfec52f2e12@eik.bme.hu> <71a535f9154ddc269c0a948053e39053550bd8c0@nut.email> User-Agent: mu4e 1.12.12; emacs 30.1 Date: Wed, 10 Sep 2025 13:09:03 +0100 Message-ID: <87ldmm5x28.fsf@draig.linaro.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Received-SPF: pass client-ip=2a00:1450:4864:20::42b; envelope-from=alex.bennee@linaro.org; helo=mail-wr1-x42b.google.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, 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: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org "Julian Ganz" writes: > September 10, 2025 at 12:06 PM, "BALATON Zoltan" wrote: >> On Tue, 9 Sep 2025, Julian Ganz wrote: >> > I ran streamPPCpowerpcO3 on qemu with these patches: >> >=20 >> > ------------------------------------------------------------- >> > Function Best Rate MB/s Avg time Min time Max time >> > Copy: 2867.6 0.056828 0.055795 0.061792 >> > Scale: 1057.5 0.153282 0.151305 0.158115 >> > Add: 1308.8 0.187095 0.183380 0.193672 >> > Triad: 1111.6 0.220863 0.215902 0.230440 >> > ------------------------------------------------------------- >> >=20 >> > After doing a clean build, with the fans still audible: >> >=20 >> > ------------------------------------------------------------- >> > Function Best Rate MB/s Avg time Min time Max time >> > Copy: 2932.9 0.055131 0.054554 0.055667 >> > Scale: 1067.9 0.151520 0.149832 0.155000 >> > Add: 1324.9 0.184807 0.181150 0.191386 >> > Triad: 1122.0 0.220080 0.213896 0.229302 >> > ------------------------------------------------------------- >> >=20 >> What was different between the above two runs? I guess maybe one is with= plugins disabled but it's not clear from the description. > > The difference is nothing but a a clean rebuild of qemu. As you see > there are fluctuations already. Plugins are enabled for both cases. > >> > On qemu (6a9fa5ef3230a7d51e0d953a59ee9ef10af705b8) without these >> > patches, but plugins enabled: >> >=20 >> > ------------------------------------------------------------- >> > Function Best Rate MB/s Avg time Min time Max time >> > Copy: 2972.1 0.054407 0.053834 0.054675 >> > Scale: 1068.6 0.151503 0.149726 0.154594 >> > Add: 1327.6 0.185160 0.180784 0.193181 >> > Triad: 1127.2 0.219249 0.212915 0.229230 >> > ------------------------------------------------------------- >> >=20 >> > And on qemu (6a9fa5ef3230a7d51e0d953a59ee9ef10af705b8) without these >> > patches, with plugins disabled: >> >=20 >> > ------------------------------------------------------------- >> > Function Best Rate MB/s Avg time Min time Max time >> > Copy: 2983.4 0.055141 0.053630 0.060013 >> > Scale: 1058.3 0.152353 0.151186 0.155072 >> > Add: 1323.9 0.184707 0.181279 0.188868 >> > Triad: 1128.2 0.218674 0.212734 0.230314 >> > ------------------------------------------------------------- >> >=20 >> > I fail to see any significant indication that these patches, or >> > plugins in general, would result in a degredation of performance. >> >=20 >> With worst case Copy test it seems to be about 3.5% (and about 1.7% >> with plugins disabled?) and should be less than that normally so it >> does not add much more overhead to plugins than there is already so >> this should be acceptable. It may still be interesting to see if the >> overhead with plugins disabled can be avoided with a similar way as >> logging does it. > > The thing is: that's probably just usual fluctuations. As you have seen > with the first two measurements the values fluctuate quite a bit between > runs of the test on the very same qemu (assuming that a clean build did > not incur any _other_ relevant change). For example, the best rate for > scale shown with plugins enabled is one percent faster than with plugins > disabled. Is this significant? Probably not. Or at least it doesn't make > much sense. I wouldn't spend too much time chasing this down. As you say this fluctuation is well within the noise range. I can recommend hyperfine as a runner: https://github.com/sharkdp/hyperfine as it does some work on how many times you need to run a test before the results are statistically relevant. > I may do some more tests this week, with runtimes longer than a few > seconds if I can find the motivation to set up everything I'd need to > compile your benchmark. In the mean-time, you are welcome to make your > own measurements if you want to. The patches are also availible at [1] > if you don't want to apply them to your local tree yourself. Balton, I don't think worries about performance impact are justified and Julian has certainly done enough due diligence here. If you can come up with a repeatable test that shows a measurable impact then please do so. > > Regards, > Julian > > [1]: https://github.com/patchew-project/qemu/tree/patchew/cover.175701862= 6.git.neither@nut.email --=20 Alex Benn=C3=A9e Virtualisation Tech Lead @ Linaro