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Wed, 19 Nov 2025 00:59:47 -0800 (PST) Received: from [192.168.88.32] ([212.105.155.41]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-42b53f0b617sm36254438f8f.31.2025.11.19.00.59.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 19 Nov 2025 00:59:46 -0800 (PST) Message-ID: <9e1011a8-70bd-468d-96b2-a306039b97f9@redhat.com> Date: Wed, 19 Nov 2025 09:59:45 +0100 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net-next 2/2] tcp: add net.ipv4.tcp_rtt_threshold sysctl To: Eric Dumazet Cc: "David S . Miller" , Jakub Kicinski , Simon Horman , Neal Cardwell , Kuniyuki Iwashima , netdev@vger.kernel.org, eric.dumazet@gmail.com References: <20251117132802.2083206-1-edumazet@google.com> <20251117132802.2083206-3-edumazet@google.com> Content-Language: en-US From: Paolo Abeni In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 11/18/25 10:22 PM, Eric Dumazet wrote: > I would perhaps use 8 senders, and force all receivers on one cpu (cpu > 4 in the following run) > > for i in {1..8} > do > netperf -H host -T,4 -l 100 & > done > > This would I think show what can happen when receivers can not keep up. Thanks for the suggestion. I should have understood the receiver needs to be under stress in the relevant scenario. With the above setup, on vanilla kernel, the rcvbuf I see is: min 2134391 max 33554432 avg 12085941 with multiple connections hitting tcp_rmem[2] with the patched kernel: min 1192472 max 33554432 avg 4247351 there is a single outlier hitting tcp_rmem[2], and in that case the connection observes for some samples a rtt just above tcp_rtt_threshold sysctl/tcp_rcvbuf_low_rtt. FWIW I guess you can add: Tested-by: Paolo Abeni Thanks, Paolo