Comment by Ono-Sendai

10 years ago

This is my proposed solution to this kind of problem: Sockets should have a flushHint() API call: http://www.forwardscattering.org/post/3

8 comments

Ono-Sendai

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Animats  10 years ago

Look up the history of the PUSH bit in TCP.

  • Ono-Sendai  10 years ago

    Ok, and is there a portable way to set this PUSH bit in the sockets API? the semantics seem a little different as well since the PUSH bit seems to do something on the receiver side as well.

daurnimator  10 years ago

Sounds exactly like what TCP_NODELAY does on linux.

Note the final sentence from tcp(7):

    TCP_NODELAY
              If  set,  disable the Nagle algorithm.  This means that segments are always sent as soon as possible, even if there is
              only a small amount of data.  When not set, data is buffered until there is a sufficient amount to send  out,  thereby
              avoiding  the  frequent  sending  of  small packets, which results in poor utilization of the network.  This option is
              overridden by TCP_CORK; however, setting this option forces an explicit flush of pending output, even if  TCP_CORK  is
              currently set.

  • Ono-Sendai  10 years ago

    There is some overlap, yes. TCP_CORK is a mode however. It's silly to introduce the complexity of extra state when a single method call (flushHint()) would suffice.

    My proposed flushHint() is also quite different to TCP_NODELAY. Let's say you do 100 writes of 1 byte to a socket. If TCP_NODELAY is set, 100 packets would be sent. However if you do 100 writes to the socket, then one flushHint() call, only one packet would be sent.

    • daurnimator  10 years ago

      > There is some overlap, yes. TCP_CORK is a mode however. It's silly to introduce the complexity of extra state when a single method call (flushHint()) would suffice.

      It is a single call. Note that last sentence from the man page entry: "setting this option forces an explicit flush of pending output, even if TCP_CORK is currently set."

      When TCP_CORK is on (turn it on once at socket creation time), the following code is the implementation of your flushHint function:

          int flushHint(int fd) {
        return setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, &(int){ 1 }, sizeof(int))
    }

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