rcvbuf option

Maxim Dounin mdounin at mdounin.ru
Wed Feb 19 12:01:43 UTC 2014


On Tue, Feb 18, 2014 at 05:16:18PM -0500, atarob wrote:

> Maxim Dounin Wrote:
> -------------------------------------------------------
> > Hello!
> > 
> > On Tue, Feb 18, 2014 at 02:58:05PM -0500, atarob wrote:
> > 
> > > The config listen option rcvbuf which maps to the TCP SO_RCVBUF, is
> > applied
> > > to the listening socket, and not inherited by the accept()ed
> > connections. So
> > > if you have a high load application where the legitimate request is
> > bound to
> > > be no more than 4K, for instance, you could save a lot of RAM by
> > dropping
> > > the default here  without changing it at the system level.
> > > 
> > > I straced nginx and it does not seem to actually apply the settings
> > through
> > > a subsequent setsockopt to the accepted socket. Is this intentional
> > or an
> > > oversight?
> > 
> > What makes you think that SO_RCVBUF is not inherited by accepted 
> > connections?
> http://stackoverflow.com/a/12864681
> I didn't run it myself, because testing it on one platform isn't enough to
> assume either way. But why do you think that it is inherited in general?

This is how it works in BSD since BSD sockets introduction.
While I don't think it's guaranteed by any standard, there should be 
really good reasons to behave differently.

If you think there are OSes which behave differently and they are 
popular enough to care - feel free to name them.

> Also, these are inherently different settings. Even if it were inherited, to
> reduce the 10,000 connections on which I expect 2KB of upload, I have to
> drop the listening socket's buffer size? Isn't that used by the OS for
> clients that are trying to connect to the listening socket? If so, I
> wouldn't want to drop that. In fact, since there is only one (or a few)
> listening sockets, I might want to increase that while dropping the accepted
> sockets' buffer size.

Listening socket's buffer size isn't something used for 
anything.  Listening queue size aka backlog is what matters for 
listening sockets (see listen(2)), and there is a separate 
parameter of the "listen" directive to control it.

Maxim Dounin

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