Re: sendfile

[Mark Mielke <mark@mark.mielke.cc>]

On Thu, May 01, 2003 at 12:34:32AM +0200, Pål Halvorsen wrote:
> On Wed, 30 Apr 2003, Mark Mielke wrote:
> > To some degree, couldn't sendto() fit this description? (Assuming the
> > kernel implemented 'zero-copy' on sendto()) The benefit of sendfile()
> > is that data isn't coming from a memory location. It is coming from disk,
> > meaning that your process doesn't have to become active in order for work
> > to be done. In the case of UDP packets, you almost always want a layer on
> > top that either times the UDP packet output, or sends output in response
> > to input, mostly defeating the purpose of sendfile()...
> Maybe, but then I'll have two system calls...

As I mentioned before, the real benefit to sendfile(), as I understand it, is
that sendfile() makes it unnecessary for the OS to fully activate the calling
process in order to do work for the calling process. Unless you can point out
some other benefit provided by sendfile(), I fail to see how you will do:

    while (1) {
        send_frame_over_udp();
        sleep();
    }

Without two system calls. Whether send_frame_over_udp() uses sendfile() as
you seem to want it to, or whether it just calls sendto(), doesn't make a
difference. Because one of your requirements is that you need to provide a
smooth feed, the primary benefit of sendfile(), that of not having to activate
your process, becomes invalid.

I haven't done timings, or looked deeply at this part of linux-2.5.x,
however, I fail to see why the following code should not meet your
requirements:

    void *p = mmap(0, length_of_file, PROT_READ, MAP_SHARED, fd, 0);
    off_t offset = 0;

    while (offset < length_of_file)
      {
        int packet_size = max(512, length_of_file - offset);
        send(socket, &p[offset], packet_size, 0);
        offset += packet_size;
        usleep(packets_size * 1000000 / packets_per_second);
      }

In theory, send() should be able to provide the zero copy benefits you
are requesting. In practice, it might be a little harder, but in this
case, from my perspective, send() and sendfile() should both provide
equivalent performance. Why would sendfile() perform better than send()?

mark

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