[cups] Error 1282 when server has more than 200 printers and domain socket is enabled

[Michael Sweet]

Alex,

> On Feb 10, 2015, at 6:58 AM, Alex Korobkin <korobkin+cups at gmail.com> wrote:
> 
> Yes, it queries it in parallel to speed up processing (with the number of
> printers being between 200 and 800 it is important).

There are a number of ways to achieve a speedup without causing a denial-of-service, including caching the PPDs (highly recommended) and dispatching N threads to process batches of printers, where N < number of printers, and probably a smallish number (16?) on regular hardware.

Regardless, you'll need to retry on failed connects and gets, with a back-off (sleep) of some sort.  Computers and servers do not have infinite resources.

> Is there any hardcoded limit that I can try to increase?

The kernel limits may or may not be configurable - depends on your OS and which limit is being tripped.  The listen backlog is normally hardcoded to a low number, and the total number of file descriptors somewhat higher (although the default per-process limit is normally 256 - cupsd increases its limit but maybe your application doesn't?)

> Also, cupsd.conf has MaxClientsPerHost 1000, does it apply to local client
> as well?

Yes, absolutely.  But unless you are seeing a "max clients exceeded" error (I don't see that in your log snippet) I'd guess you are hitting a kernel limit.


> 
> 2015-02-10 6:44 GMT-05:00 Michael Sweet <msweet at apple.com>:
> 
>> Alex,
>> 
>> Is the application querying the PPDs in parallel?  1282 == 0x502 ==
>> server-error-service-unavailable, which generally means the connect()
>> failed, either because you have exceeded the listen backlog or because
>> cupsd is throttling you.  It sounds like the former to me (listen backlog
>> exceeded...)
>> 
>> 
>>> On Feb 9, 2015, at 4:13 PM, Alex Korobkin <korobkin+cups at gmail.com>
>> wrote:
>>> 
>>> On a server with CUPS 1.7.5 (and patch for STR#4484) there is an local
>> app
>>> that queries a list of printers, and then reads their PPDs using CUPS
>> API.
>>> 
>>> If I add more than 200 printers to it, local app will frequently fail to
>>> retrieve a printer PPD or even get a list of printers, getting "1282
>> Transport
>>> endpoint is not connected" from CUPS. The more printers I add, the more
>>> often I will see error 1282. CUPS at the same time is working fine and is
>>> able to return the list of printers normally (as tested with lpstat -a
>> and
>>> Web UI).
>>> 
>>> If I disable domain socket (Listen /var/run/cups/cups.sock) and restart
>>> CUPS, the error will be gone. The app would query CUPS just fine all the
>>> time, regardless of the number of printers.
>>> 
>>> I don't see anything helpful in debug log, unfortunately.
>>> 
>>> The only suspicious activity is frequent opening/closing of connections
>> via
>>> local socket:
>>> 
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Accepted from localhost
>> (Domain)
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Waiting for request.
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] HTTP_STATE_WAITING Closing on
>> EOF
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Closing connection.
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Accepted from localhost
>> (Domain)
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Waiting for request.
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] HTTP_STATE_WAITING Closing on
>> EOF
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Closing connection.
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Accepted from localhost
>> (Domain)
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Waiting for request.
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] HTTP_STATE_WAITING Closing on
>> EOF
>>> D [03/Feb/2015:05:42:22 +0000] [Client 52] Closing connection.
>>> 
>>> --
>>> Alex
>>> _______________________________________________
>>> cups mailing list
>>> cups at cups.org
>>> https://www.cups.org/mailman/listinfo/cups
>> 
>> _________________________________________________________
>> Michael Sweet, Senior Printing System Engineer, PWG Chair
>> 
>> _______________________________________________
>> cups mailing list
>> cups at cups.org
>> https://www.cups.org/mailman/listinfo/cups
>> 
> 
> 
> 
> -- 
> -Alex
> _______________________________________________
> cups mailing list
> cups at cups.org
> https://www.cups.org/mailman/listinfo/cups

_________________________________________________________
Michael Sweet, Senior Printing System Engineer, PWG Chair