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[62.219.42.90]) by smtp.gmail.com with ESMTPSA id o3-20020a5d4743000000b00225307f43fbsm7155649wrs.44.2022.09.29.09.25.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 29 Sep 2022 09:25:50 -0700 (PDT) Message-ID: <91ebc84d-c0e3-b792-4f92-79612271eb91@grimberg.me> Date: Thu, 29 Sep 2022 19:25:48 +0300 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.11.0 From: Sagi Grimberg Subject: Re: [PATCH rfc] nvme: support io stats on the mpath device To: Jens Axboe , Max Gurtovoy , linux-nvme@lists.infradead.org Cc: Christoph Hellwig , Keith Busch , Chaitanya Kulkarni , linux-block@vger.kernel.org, Hannes Reinecke References: <20220928195510.165062-1-sagi@grimberg.me> <20220928195510.165062-2-sagi@grimberg.me> <760a7129-945c-35fa-6bd6-aa315d717bc5@nvidia.com> <04b39974-6b55-7aca-70de-4a567f2eac8f@kernel.dk> Content-Language: en-US In-Reply-To: <04b39974-6b55-7aca-70de-4a567f2eac8f@kernel.dk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20220929_092554_268242_40FF9F3C X-CRM114-Status: GOOD ( 14.53 ) X-BeenThere: linux-nvme@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "Linux-nvme" Errors-To: linux-nvme-bounces+linux-nvme=archiver.kernel.org@lists.infradead.org >>> 3. Do you have some performance numbers (we're touching the fast path here) ? >> >> This is pretty light-weight, accounting is per-cpu and only wrapped by >> preemption disable. This is a very small price to pay for what we gain. > > Is it? Enabling IO stats for normal devices has a very noticeable impact > on performance at the higher end of the scale. Interesting, I didn't think this would be that noticeable. How much would you quantify the impact in terms of %? I don't have any insight on this for blk-mq, probably because I've never seen any user turn IO stats off (or at least don't remember). My (very limited) testing did not show any noticeable differences for nvme-loop. All I'm saying that we need to have IO stats for the mpath device node. If there is a clever way to collect this from the hidden devices just for nvme, great, but we need to expose these stats. > So much so that I've contemplated how we can make this less expensive than it currently is. Then nvme-mpath would benefit from that as well.