Re: SOMAXCONN (was Re: Solaris source code)

On Fri, Jul 13, 2001 at 07:53:02AM -0400, mlw wrote:
> Zeugswetter Andreas SB wrote:
> > I liked the idea of min(MaxBackends, PG_SOMAXCONN), since there is no use in
> > accepting more than your total allowed connections concurrently.
>
> I have been following this thread and I am confused why the queue
> argument to listen() has anything to do with Max backends. All the
> parameter to listen does is specify how long a list of sockets open
> and waiting for connection can be. It has nothing to do with the
> number of back end sockets which are open.

Correct.

> If you have a limit of 128 back end connections, and you have 127
> of them open, a listen with queue size of 128 will still allow 128
> sockets to wait for connection before turning others away.

Correct.

> It should be a parameter based on the time out of a socket connection
> vs the ability to answer connection requests within that period of
> time.

It's not really meaningful at all, at present.

> There are two was to think about this. Either you make this parameter
> tunable to give a proper estimate of the usability of the system, i.e.
> tailor the listen queue parameter to reject sockets when some number
> of sockets are waiting, or you say no one should ever be denied,
> accept everyone and let them time out if we are not fast enough.
>
> This debate could go on, why not make it a parameter in the config
> file that defaults to some system variable, i.e. SOMAXCONN.

With postmaster's current behavior there is no benefit in setting
the listen() argument to anything less than 1000. With a small
change in postmaster behavior, a tunable system variable becomes
useful.

But using SOMAXCONN blindly is always wrong; that is often 5, which
is demonstrably too small.

> BTW: on linux, the backlog queue parameter is silently truncated to
> 128 anyway.

The 128 limit is common, applied on BSD and Solaris as well.
It will probably increase in future releases.

Nathan Myers
ncm(at)zembu(dot)com