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Socket limits

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Hi

I'm using all this free time to work on my slalom competition scoring 
system. It consists of a database server and a significant number of 
client applications, running anyt=where on a local network of RISC OS 
computers, mostly RPIs. The connections between them use TCP/IP.

I'm stress testing the system at the moment by running the maximum number 
of clients. In addition to exposing several latent bugs, that only get hit 
very infrequently, it has shown up limits in a couple of pieces of 
software that aren't mine. A couple of my bugs showed up after a 2 hour 
run with the maximum number of clients on the network. The last 3 days 
have been spent finding more quickly reproducable ways to trigger them, so 
that I could find out what was happening. (It turned out all to be 
problems reading data around the wrap in a ring buffer.)

I use SocketWatch to monitor incoming sockets and return poll reason 13 - 
pollword non-zero, in order to get the data read quickly. SocketWatch uses 
a bitmap to identify the sockets, and it's a 32-bit integer, meaning I can 
only monitor 32 sockets. This in turn limits the server to 28 clients, as 
4 bits are already in use. (The hardware timing system uses two to 
indicate that GPIO events have taken place, and there's a couple of others 
already tied up.)

Each client has 3 sockets - one to broadcast a request for the server, one 
to listen to incoming data, and one to send data to the server. The server 
uses one broadcast, and one listen, plus one send socket per client. 
Running all this on one machine, as I have been doing in testing, can tie 
up 112 sockets. I also have Hermes and Messenger Pro running, plus 
shareFS, and those together use the first 10 sockets.

It turns out Hermes and NewsHound both create a socket when they fetch, 
and delete it afterwards. This fails if socket number 95 is already in 
use, so I assume they are written with data allocated for handling 96 
sockets.

I think I can reduce my socket usage by creating the broadcast sockets 
when needed and deleting them afterwards. That will need some testing of 
course. It's not just the initial connection that uses them - if a 
connection drops or times out, the system attempts to reconnect 
automatically.

-- 
Alan Adams, from Northamptonshire
alan@adamshome.org.uk
http://www.nckc.org.uk/

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Socket limits Alan Adams <alan@adamshome.org.uk> - 2020-04-20 19:25 +0100
  Re: Socket limits Paolo Fabio Zaino <p.zaino@zfpsystems.com> - 2020-04-20 16:03 -0700
  Re: Socket limits druck <news@druck.org.uk> - 2020-04-21 00:59 +0100
    Re: Socket limits Alan Adams <alan@adamshome.org.uk> - 2020-04-21 15:39 +0100
      Re: Socket limits Alan Adams <alan@adamshome.org.uk> - 2020-04-22 12:59 +0100

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