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Wed, 15 Jul 2020 07:59:53 -0400 (EDT) From: Thomas Monjalon To: "Medvedkin, Vladimir" Cc: "Kinsella, Ray" , Stephen Hemminger , dev@dpdk.org, david.marchand@redhat.com, jerinj@marvell.com, konstantin.ananyev@intel.com, bruce.richardson@intel.com Date: Wed, 15 Jul 2020 13:59:51 +0200 Message-ID: <4060360.4fru8zSAKM@thomas> In-Reply-To: <07b5fd0a-25b7-3bb3-f268-458b2266c93e@intel.com> References: <1758703.2oNzlfH3Ak@thomas> <07b5fd0a-25b7-3bb3-f268-458b2266c93e@intel.com> MIME-Version: 1.0 Content-Transfer-Encoding: 7Bit Content-Type: text/plain; charset="us-ascii" Subject: Re: [dpdk-dev] [PATCH v6 0/8] fib: implement AVX512 vector lookup X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" 15/07/2020 12:35, Medvedkin, Vladimir: > On 15/07/2020 10:47, Thomas Monjalon wrote: > > 14/07/2020 16:38, Stephen Hemminger: > >> "Kinsella, Ray" wrote: > >>> On 13/07/2020 23:19, Stephen Hemminger wrote: > >>>> Did anyone else see the recent AVX512 discussion from Linus: > >>>> "I hope AVX512 dies a painful death, and that Intel starts fixing real problems > >>>> instead of trying to create magic instructions to then create benchmarks that they can look good on. > >>> > >>> Yup - I saw this one. > >>> Sweeping statements like these are good to provoke debate, the truth is generally more nuanced. > >>> If you continue to read the post, Linus appears to be mostly questioning microprocessor design decisions. > >>> > >>> That is an interesting discussion, however the reality is that the technology does exists and may be beneficial for Packet Processing. > >>> > >>> I would suggest, we continue to apply the same logic governing adoption of any technology by DPDK. > >>> When the technology is present and a clear benefit is shown, we use it with caution. > >>> > >>> In the case of Vladimir's patch, > >>> the user has to explicitly switch on the AVX512 lookup with RTE_FIB_DIR24_8_VECTOR_AVX512. > >> > >> Using what is available makes sense in DPDK. > > > > Why does it require explicit enabling in application? > > AVX512 is not reliable enough to be automatically used when available? > > It is reliable enough. User have to explicitly trigger to avx512 lookup > because using avx512 instructions can reduce the frequency of your > cores. The user knows their environment better. So the scalar version is > used so as not to affect the frequency. So the user must know which micro-optimization is better for a code they don't know. Reminder: an user is not a developper. I understand we have no better solution though. Can we improve the user experience with some recommendations, numbers, etc?