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[204.210.126.223]) by smtp.gmail.com with ESMTPSA id s195sm6544155pfs.119.2021.07.22.11.53.20 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 22 Jul 2021 11:53:20 -0700 (PDT) Subject: Re: Prefetches in buffer_zero_* To: "Dr. David Alan Gilbert" References: From: Richard Henderson Message-ID: <092f9b8b-4a14-d059-49be-010b760828aa@linaro.org> Date: Thu, 22 Jul 2021 08:53:17 -1000 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.11.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=2607:f8b0:4864:20::634; envelope-from=richard.henderson@linaro.org; helo=mail-pl1-x634.google.com X-Spam_score_int: -22 X-Spam_score: -2.3 X-Spam_bar: -- X-Spam_report: (-2.3 / 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, NICE_REPLY_A=-0.203, 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: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: qemu-devel@nongnu.org, jmario@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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? r~