Sv: Sv: Sv: Sv: Sv: Sv: Sv: Sv: Named pipes and multiple writers
sten.kristian.ivarsson@gmail.com
sten.kristian.ivarsson@gmail.com
Thu Apr 2 08:05:49 GMT 2020
> 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.
I pulled it and tried it out and yes, the sample test program with 25/25
worked well and a whole bunch of our unit-tests passed with ok result now
We still do have some issues, but I cannot yet tell if they are related to
named pipes or not
It is great that you're looking into a totally dynamic solution
Kristian
> Ken
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