Date: Tue, 27 May 1997 16:47:22 -0400 (EDT) From: Christopher Sedore <cmsedore@mailbox.syr.edu> To: "Ron G. Minnich" <rminnich@Sarnoff.COM> Cc: FreeBSD-Hackers@freebsd.org Subject: Re: async socket stuff Message-ID: <Pine.SOL.3.95.970527163325.11761A-100000@rodan.syr.edu> In-Reply-To: <Pine.SUN.3.91.970527141129.23490O-100000@terra>
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On Tue, 27 May 1997, Ron G. Minnich wrote:
> > It sure could, but you end up with many more system calls, and it is not
> > async. The real advantage to a call like TransmitFile() is that you can
> > send an entire file (or a range of a file) with a single system call, and
> > you can do it async. This means that you can more efficiently implement
> ^^^^^^^^^^^^^^^^^^^^^^^^^^
> > things like FTP servers, Web servers, pop servers, etc.
>
> And the measurements which show this are to be found ... where? I'm not
> convinced. You've got to pay the cost for this somewhere.
> "complexity is conserved" -- D. Jensen
Well, you can do some math which might demonstrate it:
To transfer a 10MB file now, you might implement something like:
open(...)
while (!eof) {
read(file,buf,64k)
write(sock,buf,64k)
}
close(file)
with a transmitfile-like facility, you would:
open(...)
transmitfile(file,sock,...)
close(file)
Now, in the case of a 10MB file, you've essentially saved 20MB worth of
memory bandwidth/time for transfer (since the data has to be copied into
user space on read, and back again on write), plus 160*2=320-1=319 system
calls avoided.
I do acknowledge that you could of course do:
open(...)
mmap(...)
write(...)
munmap(...)
close(...)
but this has another set of efficiency concerns with setting up the map.
I think you still end up with a user to kernel copy you might otherwise
avoid (corrections welcome). You also have restrictions on how much space
you can map into a process (a problem I currently experience with an NT
app which hangs around 1.5GB of reserved address space with a 2GB
OS limit).
What's the cost? another few hundred bytes of kernel code. Worth it to
me, esp since it would probably be duplicated many times over in
user code.
-Chris
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