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  • Eric Dumazet's avatar
    tcp: do not pace pure ack packets · 98781965
    Eric Dumazet authored
    When we added pacing to TCP, we decided to let sch_fq take care
    of actual pacing.
    All TCP had to do was to compute sk->pacing_rate using simple formula:
    sk->pacing_rate = 2 * cwnd * mss / rtt
    It works well for senders (bulk flows), but not very well for receivers
    or even RPC :
    cwnd on the receiver can be less than 10, rtt can be around 100ms, so we
    can end up pacing ACK packets, slowing down the sender.
    Really, only the sender should pace, according to its own logic.
    Instead of adding a new bit in skb, or call yet another flow
    dissection, we tweak skb->truesize to a small value (2), and
    we instruct sch_fq to use new helper and not pace pure ack.
    Note this also helps TCP small queue, as ack packets present
    in qdisc/NIC do not prevent sending a data packet (RPC workload)
    This helps to reduce tx completion overhead, ack packets can use regular
    sock_wfree() instead of tcp_wfree() which is a bit more expensive.
    This has no impact in the case packets are sent to loopback interface,
    as we do not coalesce ack packets (were we would detect skb->truesize
    In case netem (with a delay) is used, skb_orphan_partial() also sets
    skb->truesize to 1.
    This patch is a combination of two patches we used for about one year at
    Signed-off-by: default avatarEric Dumazet <>
    Signed-off-by: default avatarDavid S. Miller <>