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Date:   Sat, 4 May 2019 02:35:40 +0000
From:   Lawrence Brakmo <brakmo@...com>
To:     Eric Dumazet <eric.dumazet@...il.com>,
        Neal Cardwell <ncardwell@...gle.com>
CC:     netdev <netdev@...r.kernel.org>, Martin Lau <kafai@...com>,
        "Alexei Starovoitov" <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        Yuchung Cheng <ycheng@...gle.com>
Subject: Re: [PATCH v2 bpf-next 5/7] bpf: sysctl for probe_on_drop



´╗┐On 4/8/19, 10:07 AM, "Eric Dumazet" <eric.dumazet@...il.com> wrote:

    
    
    On 04/08/2019 09:16 AM, Neal Cardwell wrote:
    > On Wed, Apr 3, 2019 at 8:13 PM brakmo <brakmo@...com> wrote:
    >>
    >> When a packet is dropped when calling queue_xmit in  __tcp_transmit_skb
    >> and packets_out is 0, it is beneficial to set a small probe timer.
    >> Otherwise, the throughput for the flow can suffer because it may need to
    >> depend on the probe timer to start sending again. The default value for
    >> the probe timer is at least 200ms, this patch sets it to 20ms when a
    >> packet is dropped and there are no other packets in flight.
    >>
    >> This patch introduces a new sysctl, sysctl_tcp_probe_on_drop_ms, that is
    >> used to specify the duration of the probe timer for the case described
    >> earlier. The allowed values are between 0 and TCP_RTO_MIN. A value of 0
    >> disables setting the probe timer with a small value.
    
    This seems to contradict our recent work ?
    
    See recent Yuchung patch series :
    
    c1d5674f8313b9f8e683c265f1c00a2582cf5fc5 tcp: less aggressive window probing on local congestion
    590d2026d62418bb27de9ca87526e9131c1f48af tcp: retry more conservatively on local congestion
    9721e709fa68ef9b860c322b474cfbd1f8285b0f tcp: simplify window probe aborting on USER_TIMEOUT
    01a523b071618abbc634d1958229fe3bd2dfa5fa tcp: create a helper to model exponential backoff
    c7d13c8faa74f4e8ef191f88a252cefab6805b38 tcp: properly track retry time on passive Fast Open
    7ae189759cc48cf8b54beebff566e9fd2d4e7d7c tcp: always set retrans_stamp on recovery
    7f12422c4873e9b274bc151ea59cb0cdf9415cf1 tcp: always timestamp on every skb transmission
    
Thank you for pointing this out. I'm taking this patch out of the patchset and will reconsider it.
    

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