Re: monitoring a very large number of channels

[Ralph Lange <Ralph.Lange@bessy.de>]

 Patrick,

interesting question - I would answer with a determined: it all depends.

There are a number of resource limitations that you could bump into when 
using Channel Access. Which one of those will be the first thing you 
hit, depends on many factors of your setup, not just the number of channels.

Here are a few limitations I can think of, ordered by guessed likelihood 
of causing trouble (descending):

Network bandwidth
Depends mainly on the type of data you are monitoring. If you are 
monitoring live color images at 30fps, a few channels will take all your 
available bandwidth even on a fast network, and you're screwed. If you 
are monitoring integers that update once every 10 seconds, without 
timestamps or alarm status, it will take a few 100,000 channels to put a 
noticable load on the same network. Unless you are on a 9600 baud modem 
line.

File descriptors
CA uses one TCP per client/IOC combination. Depending on your system 
configuration, one client trying to connect to more than 1024 IOCs could 
hit a limit, without even subscribing to a single channel.

Threads
On Linux, every thread counts against the maximum number of processes on 
the system. CA uses two threads per TCP circuit. Depending on your 
kernel configuration (cat /proc/sys/kernel/threads-max) and the other 
stuff running on your client machine, you might hit that limit by 
connecting to a large number of IOCs (again independent from number of 
channels).

Memory
CA allocates memory for each connection, and (much less) for each 
subscription. Its CPU usage depends on the amount of data it processes. 
If you enforce memory or CPU time limits on your client, you might hit 
those.

Numerical limits
CA uses unsigned 32 bit channel identifiers. So there there is a hard 
limit of ~4.3 billion channels per client. Which probably is still two 
or three orders of magnitude more than the largest existing EPICS systems.

For a usual client in a usual system, (e.g. the Channel Archiver or 
camonitor), the limiting factor will be bandwidth of their output 
(i.e.writing to disk or printing on your terminal) rather than any CA 
related limits.
As Bob pointed out, running archive engines that connect to a few 10,000 
channels is possible and common with larger installations.
How fast you can actually archive depends one the data size, the 
machine, the disks, the storage format, ..... should be in the order of 
between 5,000 and 15,000 updates/sec (double plus timestamp and status).

Good luck and do not worry!
Ralph


On 30.07.2010 05:51, Patrick Thomas wrote:
> Hi,
>
> I was wondering what the limiting factor is with regards to placing 
> monitors on a large number of channels. For example, how many channels 
> can camonitor or channel archiver watch?
>
> Thank you,
> Patrick