From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 54288221FB4 for ; Fri, 28 Nov 2025 09:51:32 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764323492; cv=none; b=X7anjiGJXk0gPdhMgwO0JJEL+wEUB7g4q5r5cmKW9aZTAkqTcGSj+VsbiVIoA8H3ODTeIoVWZ2faWRo+W+WhZWi5kSHS9hmwuhL/+ZwrTp+TInSs7zn6woB54EuhsdkJZfloEjSZBD/kZnQ91LybuzrI+bj7Jp+WVZr9rdDnhik= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1764323492; c=relaxed/simple; bh=uKBh60/GqNkJteSjJShNXKTpemOyTAekTu0swAmL3JM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=YR/E+8lcz8hJoNZVebpyJ9BDjd7Ps6UO692XvCUz1vwzgLpbS93IwfEEQp+zBPy0Q2ODrmBzi0gknEWzARRiKY9wG24T9BM6kq2ue0Df9QGMEcukMHjU8GEGocrmLR5Mc/pW0fg1coeMvXesvCRnACLzrdbOWVNquSs3GB+ZJbc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=dXJsSA8V; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="dXJsSA8V" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1845AC4CEF1; Fri, 28 Nov 2025 09:51:30 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1764323491; bh=uKBh60/GqNkJteSjJShNXKTpemOyTAekTu0swAmL3JM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=dXJsSA8VnhRb5oENApyPSMeKls2Us2VXH/h2Df+esZXJYKn2EuFpdOHUmLqpWbR/0 F1uMbJ+BPqJ14Fp6llDMoz8VwkW7HVZZlbDu6weviio0cNi9OpzDDXD1yb5huJrBp+ dt6Ai0Zm/VaOnsGP3Zp5SBWMt7Jb5HzM0IWUR9c5ZWj/hd18LG5ifZQX40Ma9jhDaK 2ZWGtEX2Td9zMg9qYJm8OsF/v1wj0x53pa4QvMTo5GR6d+qHyEHBAUC0m4l1GSLjUb znQMwkPyDDIfigrNpJ39aZS1TjjMS55MG3Sbnv8WX6g45l00vGFuznkfC0OJFeMofE Qdi2X5iZW74vA== Message-ID: Date: Fri, 28 Nov 2025 10:51:28 +0100 Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Beta Subject: Re: [PATCH v7 mptcp-next 5/6] mptcp: better mptcp-level RTT estimator Content-Language: en-GB, fr-BE To: Paolo Abeni , martineau@kernel.org Cc: mptcp@lists.linux.dev References: <6ffc28b03d35f1b5698ff9ed6b22bd3e82fc81be.1763625391.git.pabeni@redhat.com> <08ca5b33-8592-4060-9211-ecbef30cda0e@kernel.org> <7e85ed1a-3072-4230-a14f-b7eecdf2d4d2@redhat.com> From: Matthieu Baerts Autocrypt: addr=matttbe@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit Hi Paolo, On 28/11/2025 09:47, Paolo Abeni wrote: > On 11/27/25 7:13 PM, Matthieu Baerts wrote: >> On 20/11/2025 09:39, Paolo Abeni wrote: >>> The current MPTCP-level RTT estimator has several issues. On high speed >>> links, the MPTCP-level receive buffer auto-tuning happens with a frequency >>> well above the TCP-level's one. That in turn can cause excessive/unneeded >>> receive buffer increase. >>> >>> On such links, the initial rtt_us value is considerably higher >>> than the actual delay, and the current mptcp_rcv_space_adjust() updates >>> msk->rcvq_space.rtt_us with a period equal to the such field previous >>> value. If the initial rtt_us is 40ms, its first update will happen after >>> 40ms, even if the subflows see actual RTT orders of magnitude lower. >>> >>> Additionally: >>> - setting the msk rtt to the maximum among all the subflows RTTs makes DRS >>> constantly overshooting the rcvbuf size when a subflow has considerable >>> higher latency than the other(s). >>> >>> - during unidirectional bulk transfers with multiple active subflows, the >>> TCP-level RTT estimator occasionally sees considerably higher value than >>> the real link delay, i.e. when the packet scheduler reacts to an incoming >>> ack on given subflow pushing data on a different subflow. >>> >>> - currently inactive but still open subflows (i.e. switched to backup mode) >>> are always considered when computing the msk-level rtt. >>> >>> Address the all the issues above with a more accurate RTT estimation >>> strategy: the MPTCP-level RTT is set to the minimum of all the subflows >>> actually feeding data into the MPTCP receive buffer, using a small sliding >>> window. >> >> (...) >> >>> diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h >>> index ee0dbd6dbacf..b392d7855928 100644 >>> --- a/net/mptcp/protocol.h >>> +++ b/net/mptcp/protocol.h >>> @@ -269,6 +269,13 @@ struct mptcp_data_frag { >>> struct page *page; >>> }; >>> >>> +/* Arbitrary compromise between as low as possible to react timely to subflow >>> + * close event and as big as possible to avoid being fouled by biased large >>> + * samples due to peer sending data on a different subflow WRT to the incoming >>> + * ack. >>> + */ >>> +#define MPTCP_RTT_SAMPLES 5 >>> + >>> /* MPTCP connection sock */ >>> struct mptcp_sock { >>> /* inet_connection_sock must be the first member */ >>> @@ -340,11 +347,17 @@ struct mptcp_sock { >>> */ >>> struct mptcp_pm_data pm; >>> struct mptcp_sched_ops *sched; >>> + >>> + /* Most recent rtt_us observed by in use incoming subflows. */ >>> + struct { >>> + u32 samples[MPTCP_RTT_SAMPLES]; >>> + u32 next_sample; >>> + } rcv_rtt_est; >> >> I'm sorry to react only now, I didn't manage to follow this in details, >> but I have one question: why not using a smooth RTT [1]? Is it because >> the goal is to mix data from the active/recently used subflows and only >> to take the minimum, and not "combining" RTT from different subflows? >> >> [1] https://datatracker.ietf.org/doc/rfc6298/ >> srtt = old * (1-alpha) + new * alpha # alpha is 1/8 in RFC6298 > > TCP already use EWMA for rtt; the values seen by MPTCP on each subflow > went already into such processing. > > If there is a single subflow, doing again EWMA should only cause slower > reactions. > > If there are multiple subflows, we really want the min() not the > smoothed average, because: > > - there are high spikes caused by the mptcp packet scheduler: we want to > entirely filter them out, while EWMA will make them contributing the the > estimate and results are very visible (negatively) in my experiments. > > - different subflows can have very different rtt (say 1ms vs 100ms). We > really want the minimum, otherwise DRS will be fouled/the rcvbuf will > "explode" Thank you for having taken the time to explain me all this. That's much clearer, and I just realised I was mixing up acronyms! :) Cheers, Matt -- Sponsored by the NGI0 Core fund.