Although I don't have much to add to the conversation, I just wanted to say
that this is one of the most absolutely technically enlightening posts I've
read on the mod_perl list in a while. It's really interesting to finally
clarify this once and for all.
Smells like a mod_perl guide addition. :)
At 07:21 PM 11/2/2000 +0100, Roger Espel Llima wrote:
>Ask Bjoern Hansen <[EMAIL PROTECTED]> wrote:
> > Mr. Llima must do something I don't, because with real world
> > requests I see a 15-20 to 1 ratio of mod_proxy/mod_perl processes at
> > "my" site. And that is serving <500byte stuff.
>
>and Michael Blakeley <[EMAIL PROTECTED]> later replied:
> > Solaris lets a user-level application close() a socket immediately
> > and go on to do other work. The sockets layer (the TCP/IP stack) will
> > continue to keep that socket open while it delivers any buffered
> > sends - but the user application doesn't need to know this [...]
> > Anyway, since the socket is closed from the mod_perl point of view,
> > the heavyweight mod_perl process is no longer tied up. I don't know
> > if this holds true for Linux as well, but if it doesn't, there's
> > always the source code.
>
>This is exactly it. I did some tests with a real-world server, and the
>conclusion was that, as long as the total write() size is less than the
>kernel's max write buffer, then write() followed by close() doesn't
>block. That was using Linux, where the kernel buffer size can be set by
>echo'ing numbers into /proc/sys/net/core/wmem_{default,max}.
>
>However, Apache doesn't use a plain close(), but instead calls a
>function called lingering_close, which tries to make sure that the other
>side has received all the data. That is done by select()ing on the
>socket in 2 second increments, until the other side either closes the
>connection too, or times out. And *THIS* is the reason why front-end
>servers are good. An apache process spends an average of 0.8 seconds
>(in my measurements) per request doing lingering close. This is
>consistent with Ask's ratio of 15-20 to 1 frontend to backend servers.
>
>Now, there's no reason in principle why lingering_close() couldn't be
>done in the kernel, freeing the user process from the waiting job, and
>making frontend servers unnecessary. There's even an interface for it,
>namely SO_LINGER, and Apache knows how to use it. But SO_LINGER is
>badly specified, and known to be broken in most tcp/ip stacks, so
>currently it's kind of a bad idea to use it, and we're stuck with the
>two server model.
>
>--
>Roger Espel Llima, [EMAIL PROTECTED]
>http://www.iagora.com/~espel/index.html
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Gunther Birznieks ([EMAIL PROTECTED])
eXtropia - The Web Technology Company
http://www.extropia.com/
