[Rd] [External] Re: Is it a good choice to increase the NCONNECTION value?

Wed Aug 25 09:02:49 CEST 2021

On 8/25/21 6:05 AM, Simon Urbanek wrote:
> Luke,
>
> sure, adjustment at run-time works just fine, the issue currently is that it is baked-in at compile time so there is no way to adjust it (re-building R is not an option in production environment where this usually happens).
>
> That said, I'm still not sure that connection limit is a good way to guard against the fd limit since there are so many other ways to use up descriptors (DLLs, sockets, pipes, etc. - packages and 3rd party libraries). Apparently we are actually already fiddling with the soft limit - we have R_EnsureFDLimit() and R_GetFDLimit() which is used at startup to raise it to 1024 by default regardless of the ulimit -n setting (comments say this is for DLLs). I guess based on that we know at least what to expect so we could trivially warn if the new setting is larger that the user limit.

Hi Simon,

I think the handling of the OS connections limit (querying, increasing, 
basing the real DLL limit on that and on a user request), which takes 
into account problems described by Martin and Luke, could be extended to 
cover the connections limit in question now. The DLL limit heuristics 
were chosen based on our R hard-limit on the number of connections.

Some background is in 
https://developer.r-project.org/Blog/public/2018/03/23/maximum-number-of-dlls/index.html

If it turns out too much work for the near future/next release (it will 
be a lot of work to get it right, including the heuristics and their 
interactions between the connections limit and the DLL limit), we could 
at least (perhaps temporarily) allow users who explicitly want to 
override and take the risk to do so, perhaps with some warnings when the 
overridden value seems too large given the OS-limit and the DLL-limit.

Cheers,
Tomas
>
> Cheers,
> Simon
>
>
>> On Aug 25, 2021, at 1:45 PM, luke-tierney using uiowa.edu wrote:
>>
>> We do need to be careful about using too many file descriptors.  The
>> standard soft limit on Linux is fairly low (1024; the hard limit is
>> usually quite a bit higher). Hitting that limit, e.g. with runaway
>> with code allocating lots of connections, can cause other things, like
>> loading packages, to fail with hard to diagnose error messages. A
>> static connection limit is a crude way to guard against that. Doing
>> anything substantially better is probably a lot of work. A simple
>> option that may be worth pursuing is to allow the limit to be adjusted
>> at runtime. Users who want to go higher would do so at their own risk
>> and may need to know how to adjust the soft limit on the process.
>>
>> Best,
>>
>> luke
>>
>> On Wed, 25 Aug 2021, Simon Urbanek wrote:
>>
>>> Martin,
>>>
>>> I don't think static connection limit is sensible. Recall that connections can be anything, not just necessarily sockets or file descriptions so they are not linked to the system fd limit. For example, if you use a codec then you will need twice the number of connections than the fds. To be honest the connection limit is one of the main reasons why in our big data applications we have always avoided R connections and used C-level sockets instead (others were lack of control over the socket flags, but that has been addressed in the last release). So I'd vote for at the very least increasing the limit significantly (at least 1k if not more) and, ideally, make it dynamic if memory footprint is an issue.
>>>
>>> Cheers,
>>> Simon
>>>
>>>
>>>> On Aug 25, 2021, at 8:53 AM, Martin Maechler <maechler using stat.math.ethz.ch> wrote:
>>>>
>>>>>>>>> GILLIBERT, Andre
>>>>>>>>>    on Tue, 24 Aug 2021 09:49:52 +0000 writes:
>>>>> RConnection is a pointer to a Rconn structure. The Rconn
>>>>> structure must be allocated independently (e.g. by
>>>>> malloc() in R_new_custom_connection).  Therefore,
>>>>> increasing NCONNECTION to 1024 should only use 8
>>>>> kilobytes on 64-bits platforms and 4 kilobytes on 32
>>>>> bits platforms.
>>>> You are right indeed, and I was wrong.
>>>>
>>>>> Ideally, it should be dynamically allocated : either as
>>>>> a linked list or as a dynamic array
>>>>> (malloc/realloc). However, a simple change of
>>>>> NCONNECTION to 1024 should be enough for most uses.
>>>> There is one important other problem I've been made aware
>>>> (similarly to the number of open DLL libraries, an issue 1-2
>>>> years ago) :
>>>>
>>>> The OS itself has limits on the number of open files
>>>> (yes, I know that there are other connections than files) and
>>>> these limits may quite differ from platform to platform.
>>>>
>>>> On my Linux laptop, in a shell, I see
>>>>
>>>> $ ulimit -n
>>>> 1024
>>>>
>>>> which is barely conformant with your proposed 1024 NCONNECTION.
>>>>
>>>> Now if NCONNCECTION is larger than the max allowed number of
>>>> open files and if R opens more files than the OS allowed, the
>>>> user may get quite unpleasant behavior, e.g. R being terminated brutally
>>>> (or behaving crazily) without good R-level warning / error messages.
>>>>
>>>> It's also not at all sufficient to check for the open files
>>>> limit at compile time, but rather at R process startup time
>>>>
>>>> So this may need considerably more work than you / we have
>>>> hoped, and it's probably hard to find a safe number that is
>>>> considerably larger than 128  and less than the smallest of all
>>>> non-crazy platforms' {number of open files limit}.
>>>>
>>>>> Sincerely
>>>>> Andr� GILLIBERT
>>>> [............]
>>>>
>>>> ______________________________________________
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>> -- 
>> Luke Tierney
>> Ralph E. Wareham Professor of Mathematical Sciences
>> University of Iowa                  Phone:             319-335-3386
>> Department of Statistics and        Fax:               319-335-3017
>>    Actuarial Science
>> 241 Schaeffer Hall                  email:   luke-tierney using uiowa.edu
>> Iowa City, IA 52242                 WWW:  http://www.stat.uiowa.edu
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