How does setting TCP_NODELAY behave when TCP_CORK is set?

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setting
90%

According to the manpage, TCP_CORK prevents sending partials, unsetting it flushes partials. Having TCP_CORK set and setting TCP_NODELAY also flushes partial data. Sounds similar, where exactly lies the difference?,Can TCP_NODELAY be set repeatedly on a TCP_CORK connection to flush out the pending data? (or would it need to be unset first?),As you can see, TCP_NODELAY can be used to flush the data repeatedly. (Question number 2 is therefore answered),Bonus: When looking at the packets on the wire, we can see that when setting TCP_NODELAY, the PSH flag is set on the packet. Thus the data is possibly also flushed on the receiver end. (Compare Difference between push and urgent flags in TCP)

Client:

    optval = 1;
    setsockopt(sockfd, IPPROTO_TCP, TCP_CORK, & optval, sizeof(int));
    write(sockfd, "foo\n", 4);
    setsockopt(sockfd, IPPROTO_TCP, TCP_NODELAY, & optval, sizeof(int));
    usleep(70000);
    write(sockfd, "bar\n", 4);
    setsockopt(sockfd, IPPROTO_TCP, TCP_NODELAY, & optval, sizeof(int));
    usleep(70000);
    write(sockfd, "hello\n", 6);
    usleep(70000);
    write(sockfd, "world\n", 6);
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88%

There are two socket options that are relevant to Red Hat Enterprise Linux for Real Time applications: TCP_NODELAY and TCP_CORK. , There are no special options or restrictions to using sockets on a Red Hat Enterprise Linux for Real Time system. , In the first variation, neither TCP_NODELAY nor TCP_CORK are in use. This is a baseline measurement. TCP coalesces writes and has to wait to check if the application has more data than can optimally fit in the network packet: , This example demonstrates the performance impact that TCP_NODELAY and TCP_CORK can have on an application.

One example of heuristic behavior in TCP is that small buffers are delayed. This allows them to be sent as one network packet. This generally works well, but it can also create latencies. For Red Hat Enterprise Linux for Real Time applications, TCP_NODELAY is a socket option that can be used to turn this behavior off. It can be enabled through the setsockopt sockets API, with the following function:

int one = 1;
setsockopt(descriptor, SOL_TCP, TCP_NODELAY, & one, sizeof(one));

The TCP_CORK option can can be enabled by using the following function:

int one = 1;
setsockopt(descriptor, SOL_TCP, TCP_CORK, & one, sizeof(one));

In a situation where the kernel is not able to identify when to remove the cork, it can be manually removed with the function:

int zero = 0;
setsockopt(descriptor, SOL_TCP, TCP_CORK, & zero, sizeof(zero));

The server waits for packets of 30 bytes and then sends a 2 byte packet in response. To start with, define the TCP port and the number of packets it should process. In this example, it is 10,000 packets:

$. / tcp_nodelay_server 5001 10000

In the first variation, neither TCP_NODELAY nor TCP_CORK are in use. This is a baseline measurement. TCP coalesces writes and has to wait to check if the application has more data than can optimally fit in the network packet:

$. / tcp_nodelay_client localhost 5001 10000
10000 packets of 30 bytes sent in 400129.781250 ms: 0.749757 bytes / ms

The second variation uses TCP_NODELAY only. TCP is instructed not to coalesce small packets, but to send buffers immediately. This improves performance significantly, but creates a large number of network packets for each logical packet:

$. / tcp_nodelay_client localhost 5001 10000 no_delay
10000 packets of 30 bytes sent in 1649.771240 ms: 181.843399 bytes / ms using TCP_NODELAY

The third variation uses TCP_CORK only. It halves the time required to the send the same number of logical packets. This is because TCP coalesces full logical packets in its buffers, and sends fewer overall network packets:

$. / tcp_nodelay_client localhost 5001 10000 cork
10000 packets of 30 bytes sent in 850.796448 ms: 352.610779 bytes / ms using TCP_CORK
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72%

Any errors defined for ip(7) or the generic socket layer may also be returned for TCP. ,Some applications require a quicker error notification. This can be enabled with the IPPROTO_IP level IP_RECVERR socket option. When this option is enabled, all incoming errors are immediately passed to the user program. Use this option with care -- it makes TCP less tolerant to routing changes and other normal network conditions. ,tcp_socket = socket(AF_INET, SOCK_STREAM, 0);

int value;
error = ioctl(tcp_socket, ioctl_type, & value);
65%

The TCP_CORK switch only affects the TCP_NAGLE_CORK option. When the nagle test is closed (TCP_NAGLE_OFF is set through TCP_NODELAY), TCP_NAGLE_PUSH will be setAnd TCP_NODELAY switches nagle by setting TCP_NAGLE_OFF.TCP_NAGLE_PUSH is a one-time option value, every time a new skb is created and put into the sending queue, TCP_NAGLE_PUSH will be cleared (skb_entail function),Related issues of Nagle algorithm-TCP_NODELAY detailed explanation,Starting TCP_NODELAY means that Nagle algorithm is disabled http server is generally disabled     ,Use the TCP socket option TCP_NODELAY to turn off the socket option;

#define TCP_NAGLE_OFF 1 /* Nagle's algo is disabled */
#define TCP_NAGLE_CORK 2 /* Socket is corked        */
#define TCP_NAGLE_PUSH 4 /* Cork is overridden for already queued data */
case TCP_CORK:
   /* When set indicates to always queue non-full frames.
    * Later the user clears this option and we transmit
    * any pending partial frames in the queue.  This is
    * meant to be used alongside sendfile() to get properly
    * filled frames when the user (for example) must write
    * out headers with a write() call first and then use
    * sendfile to send out the data parts.
    *
    * TCP_CORK can be set together with TCP_NODELAY and it is
    * stronger than TCP_NODELAY.
    */
   if (val) {
      tp - > nonagle |= TCP_NAGLE_CORK;
   } else {
      tp - > nonagle &= ~TCP_NAGLE_CORK;
      if (tp - > nonagle & TCP_NAGLE_OFF)
         tp - > nonagle |= TCP_NAGLE_PUSH;
      tcp_push_pending_frames(sk);
   }
break;
case TCP_NODELAY:
   if (val) {
      /* TCP_NODELAY is weaker than TCP_CORK, so that
       * this option on corked socket is remembered, but
       * it is not activated until cork is cleared.
       *
       * However, when TCP_NODELAY is set we make
       * an explicit push, which overrides even TCP_CORK
       * for currently queued segments.
       */
      tp - > nonagle |= TCP_NAGLE_OFF | TCP_NAGLE_PUSH;
      tcp_push_pending_frames(sk);
   } else {
      tp - > nonagle &= ~TCP_NAGLE_OFF;
   }
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