Re: SOMAXCONN (was Re: Solaris source code) - Mailing list pgsql-hackers

Tom Lane wrote:
> 
> mlw <markw@mohawksoft.com> writes:
> > Nathan Myers wrote:
> >> But using SOMAXCONN blindly is always wrong; that is often 5, which
> >> is demonstrably too small.
> 
> > It is rumored that many BSD version are limited to 5.
> 
> BSD systems tend to claim SOMAXCONN = 5 in the header files, but *not*
> to have such a small limit in the kernel.  The real step forward that
> we have made in this discussion is to realize that we cannot trust
> <sys/socket.h> to tell us what the kernel limit actually is.
> 
> > Passing listen(5) would probably be sufficient for Postgres.
> 
> It demonstrably is not sufficient.  Set it that way in pqcomm.c
> and run the parallel regression tests.  Watch them fail.
>

That's interesting, I would not have guessed that. I have written a number of
server applications which can handle, litterally, over a thousand
connection/operations a second, which only has a listen(5). (I do have it as a
configuration parameter, but have never seen a time when I have had to change
it.)

I figured the closest one could come to an expert in all things socket related
would have to be the Apache web server source. They have a different take on
the listen() parameter:

>>>>> from httpd.h >>>>>>>>>>>   402 /* The maximum length of the queue of pending connections, as defined   403  * by
listen(2). Under some systems, it should be increased if you   404  * are experiencing a heavy TCP SYN flood attack.
405 *   406  * It defaults to 511 instead of 512 because some systems store it   407  * as an 8-bit datatype; 512
truncatedto 8-bits is 0, while 511 is   408  * 255 when truncated.   409  */   410   411 #ifndef DEFAULT_LISTENBACKLOG
412 #define DEFAULT_LISTENBACKLOG 511   413 #endif
 
<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

I have not found any other location in which DEFAULT_LISTENBACKLOG is defined,
but it is a configuration parameter, and here is what the Apache docs claim:

>>>>>>>>>>>> http://httpd.apache.org/docs/mod/core.html >>>>>>>>>>>>

ListenBacklog directive

Syntax: ListenBacklog backlog
Default: ListenBacklog 511
Context: server config
Status: Core
Compatibility: ListenBacklog is only available in Apache versions after 1.2.0. 

The maximum length of the queue of pending connections. Generally no tuning is
needed or desired, however on some systems it is desirable to increase this
when under a TCP SYN flood attack. See the backlog parameter to the listen(2)
system call. 

This will often be limited to a smaller number by the operating system. This
varies from OS to OS. Also note that many OSes do not use exactly what is
specified as the backlog, but use a number based on (but normally larger than)
what is set.
<<<<<<<<<<<<<<<<<<<<<<<

Anyway, why not just do what apache does, set it to some extreme default
setting, which even when truncated, is still pretty big, and allow the end user
to change this value in postgresql.conf.