• Eric Dumazet's avatar
    tcp: TCP Small Queues · 46d3ceab
    Eric Dumazet authored
    This introduce TSQ (TCP Small Queues)
    
    TSQ goal is to reduce number of TCP packets in xmit queues (qdisc &
    device queues), to reduce RTT and cwnd bias, part of the bufferbloat
    problem.
    
    sk->sk_wmem_alloc not allowed to grow above a given limit,
    allowing no more than ~128KB [1] per tcp socket in qdisc/dev layers at a
    given time.
    
    TSO packets are sized/capped to half the limit, so that we have two
    TSO packets in flight, allowing better bandwidth use.
    
    As a side effect, setting the limit to 40000 automatically reduces the
    standard gso max limit (65536) to 40000/2 : It can help to reduce
    latencies of high prio packets, having smaller TSO packets.
    
    This means we divert sock_wfree() to a tcp_wfree() handler, to
    queue/send following frames when skb_orphan() [2] is called for the
    already queued skbs.
    
    Results on my dev machines (tg3/ixgbe nics) are really impressive,
    using standard pfifo_fast, and with or without TSO/GSO.
    
    Without reduction of nominal bandwidth, we have reduction of buffering
    per bulk sender :
    < 1ms on Gbit (instead of 50ms with TSO)
    < 8ms on 100Mbit (instead of 132 ms)
    
    I no longer have 4 MBytes backlogged in qdisc by a single netperf
    session, and both side socket autotuning no longer use 4 Mbytes.
    
    As skb destructor cannot restart xmit itself ( as qdisc lock might be
    taken at this point ), we delegate the work to a tasklet. We use one
    tasklest per cpu for performance reasons.
    
    If tasklet finds a socket owned by the user, it sets TSQ_OWNED flag.
    This flag is tested in a new protocol method called from release_sock(),
    to eventually send new segments.
    
    [1] New /proc/sys/net/ipv4/tcp_limit_output_bytes tunable
    [2] skb_orphan() is usually called at TX completion time,
      but some drivers call it in their start_xmit() handler.
      These drivers should at least use BQL, or else a single TCP
      session can still fill the whole NIC TX ring, since TSQ will
      have no effect.
    Signed-off-by: default avatarEric Dumazet <edumazet@google.com>
    Cc: Dave Taht <dave.taht@bufferbloat.net>
    Cc: Tom Herbert <therbert@google.com>
    Cc: Matt Mathis <mattmathis@google.com>
    Cc: Yuchung Cheng <ycheng@google.com>
    Cc: Nandita Dukkipati <nanditad@google.com>
    Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
    46d3ceab
tcp_ipv4.c 66.8 KB