Re: Sv: Sv: Sv: Sv: Sv: Sv: Sv: Named pipes and multiple writers

[Ken Brown <kbrown@cornell.edu>]

On 4/1/2020 2:34 PM, Ken Brown via Cygwin wrote:
> On 4/1/2020 1:14 PM, sten.kristian.ivarsson@gmail.com wrote:
>>> On 4/1/2020 4:52 AM, sten.kristian.ivarsson@gmail.com wrote:
>>>>> On 3/31/2020 5:10 PM, sten.kristian.ivarsson@gmail.com wrote:
>>>>>>> On 3/28/2020 10:19 PM, Ken Brown via Cygwin wrote:
>>>>>>>> On 3/28/2020 11:43 AM, Ken Brown via Cygwin wrote:
>>>>>>>>> On 3/28/2020 8:10 AM, sten.kristian.ivarsson@gmail.com wrote:
>>>>>>>>>>> On 3/27/2020 10:53 AM, sten.kristian.ivarsson@gmail.com wrote:
>>>>>>>>>>>>> On 3/26/2020 7:19 PM, Ken Brown via Cygwin wrote:
>>>>>>>>>>>>>> On 3/26/2020 6:39 PM, Ken Brown via Cygwin wrote:
>>>>>>>>>>>>>>> On 3/26/2020 6:01 PM, sten.kristian.ivarsson@gmail.com wrote:
>>>>>>>>>>>>>>>> The ENIXIO occurs when parallel child-processes
>>>>>>>>>>>>>>>> simultaneously using O_NONBLOCK opening the descriptor.
>>>>>>>>>>>>>>>
>>>>>>>>>>>>>>> This is consistent with my guess that the error is
>>>>>>>>>>>>>>> generated by fhandler_fifo::wait.  I have a feeling that
>>>>>>>>>>>>>>> read_ready should have been created as a manual-reset
>>>>>>>>>>>>>>> event, and that more care is needed to make sure it's set
>> when it should be.
>>
>> [snip]
>>
>>>>>>>> Never mind.  I was able to reproduce the problem and find the cause.
>>>>>>>> What happens is that when the first subprocess exits,
>>>>>>>> fhandler_fifo::close resets read_ready.  That causes the second
>>>>>>>> and subsequent subprocesses to think that there's no reader open,
>>>>>>>> so their attempts to open a writer with O_NONBLOCK fail with ENXIO.
>>
>> [snip]
>>
>>>> I wrote in a previous mail in this topic that it seemed to work fine
>>>> for me as well, but when I bumped up the numbers of writers and/or the
>>>> number of messages (e.g. 25/25) it starts to fail again
>>
>> [snip]
>>
>>> Yes, it is a resource issue.  There is a limit on the number of writers
>> that can be open at one
>>> time, currently 64.  I chose that number arbitrarily, with no idea what
>> might actually be
>>> needed in practice, and it can easily be changed.
>>
>> Does it have to be a limit at all ? We would rather see that the application
>> decide how much resources it would like to use. In our particular case there
>> will be a process-manager with an incoming pipe that possible several
>> thousands of processes will write to
> 
> I agree.
> 
>> Just for fiddling around (to figure out if this is the limit that make other
>> things work a bit odd), where's this 64 limit defined now ?
> 
> It's MAX_CLIENTS, defined in fhandler.h.  But there seem to be other resource 
> issues also; simply increasing MAX_CLIENTS doesn't solve the problem.  I think 
> there are also problems with the number of threads, for example.  Each time your 
> program forks, the subprocess inherits the rfd file descriptor and its 
> "fifo_reader_thread" starts up.  This is unnecessary for your application, so I 
> tried disabling it (in fhandler_fifo::fixup_after_fork), just as an experiment.
> 
> But then I ran into some deadlocks, suggesting that one of the locks I'm using 
> isn't robust enough.  So I've got a lot of things to work on.
> 
>>> In addition, a writer isn't recognized as closed until a reader tries to
>> read and gets an error.
>>> In your example with 25/25, the list of writers quickly gets to 64 before
>> the parent ever tries
>>> to read.
>>
>> That explains the behaviour, but should there be some error returned from
>> open/write (maybe it is but I'm missing it) ?
> 
> The error is discovered in add_client_handler, called from thread_func.  I think 
> you'll only see it if you run the program under strace.  I'll see if I can find 
> a way to report it.  Currently, there's a retry loop in fhandler_fifo::open when 
> a writer tries to open, and I think I need to limit the number of retries and 
> then error out.

I pushed a few improvements and bug fixes, and your 25/25 example now runs 
without a problem.  I increased MAX_CLIENTS to 1024 just for the sake of this 
example, but I'll work on letting the number of writers increase dynamically as 
needed.

Ken