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[204.195.96.226]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-4830512e61dsm6787895e9.8.2026.02.02.10.43.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 02 Feb 2026 10:43:34 -0800 (PST) Date: Mon, 2 Feb 2026 10:43:30 -0800 From: Stephen Hemminger To: Morten =?UTF-8?B?QnLDuHJ1cA==?= Cc: , "John W. Linville" Subject: Re: [RFC 4/4] net/af_packet: add VPP-style prefetching to receive path Message-ID: <20260202104330.27156f31@phoenix.local> In-Reply-To: <98CBD80474FA8B44BF855DF32C47DC35F656C7@smartserver.smartshare.dk> References: <20260128173138.151837-1-stephen@networkplumber.org> <20260128173138.151837-5-stephen@networkplumber.org> <20260128170612.648cd657@phoenix.local> <98CBD80474FA8B44BF855DF32C47DC35F656C7@smartserver.smartshare.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org On Thu, 29 Jan 2026 10:00:15 +0100 Morten Br=C3=B8rup wrote: > > This and previous proposal to prefetch have no impact on performance. > > Rolled a simple perf test and all three versions come out the same. =20 >=20 > Please be aware that many test cases are inadvertently designed in a way = where mbufs unintendedly are hot in the cache, so prefetching does not prov= ide the expected performance gain. > E.g. when working on a newly allocated mbuf, the mbuf should be cold. > But if it came from the mempool cache, and was recently worked on and the= n freed into the mempool cache, then it will be hot. >=20 > > The bottleneck is not here, probably at system call and copies now. =20 >=20 > The most important bottleneck might be elsewhere. > But this optimization might not be as irrelevant as the test results indi= cate. >=20 > Anyway, I agree that dropping the patch (for now) makes sense. I doubt pre-fetch will matter much in a driver like this because: - on tx the data is still in cache since just setup by caller - on rx the data is still in cache since kernel just copied it into the buffer.=20