cygrunsrv + sshd + rsync = 20 times too slow -- throttled?

[Takashi Yano]

On Fri, 10 Sep 2021 22:17:21 -0400
Ken Brown wrote:
> On 9/10/2021 6:57 PM, Takashi Yano wrote:
> > On Fri, 10 Sep 2021 11:17:58 -0400
> > Ken Brown wrote:
> >> I've rerun your test with the latest version, and the test results are similar.
> >>    I've also run a suite of fifo tests that I've accumulated, and they all pass
> >> also, so I pushed your patch.
> >>
> >> I think we're in pretty good shape now.  The only detail remaining, AFAIK, is
> >> how to best avoid a deadlock if the pipe has been created by a non-Cygwin
> >> process.  I've proposed a timeout, but maybe there's a better idea.
> > 
> > I am not pretty sure what is the problem, but is not the following
> > patch enough?
> > 
> > diff --git a/winsup/cygwin/fhandler.h b/winsup/cygwin/fhandler.h
> > index d309be2f7..13fba9a14 100644
> > --- a/winsup/cygwin/fhandler.h
> > +++ b/winsup/cygwin/fhandler.h
> > @@ -1205,6 +1205,7 @@ public:
> >     select_record *select_except (select_stuff *);
> >     char *get_proc_fd_name (char *buf);
> >     int open (int flags, mode_t mode = 0);
> > +  void open_setup (int flags);
> >     void fixup_after_fork (HANDLE);
> >     int dup (fhandler_base *child, int);
> >     int close ();
> > diff --git a/winsup/cygwin/fhandler_pipe.cc b/winsup/cygwin/fhandler_pipe.cc
> > index 6994a5dce..d84e6ad84 100644
> > --- a/winsup/cygwin/fhandler_pipe.cc
> > +++ b/winsup/cygwin/fhandler_pipe.cc
> > @@ -191,6 +191,17 @@ out:
> >     return 0;
> >   }
> > 
> > +void
> > +fhandler_pipe::open_setup (int flags)
> > +{
> > +  fhandler_base::open_setup (flags);
> > +  if (get_dev () == FH_PIPER && !read_mtx)
> > +    {
> > +      SECURITY_ATTRIBUTES *sa = sec_none_cloexec (flags);
> > +      read_mtx = CreateMutexW (sa, FALSE, NULL);
> > +    }
> > +}
> > +
> >   off_t
> >   fhandler_pipe::lseek (off_t offset, int whence)
> >   {
> > 
> > 
> > AFAIK, another problem remaining is:
> > 
> > On Mon, 6 Sep 2021 14:49:55 +0200
> > Corinna Vinschen wrote:
> >> - What about calling select for writing on pipes read by non-Cygwin
> >>    processes?  In that case, we still can't rely on WriteQuotaAvailable,
> >>    just as before.
> 
> This is the problem I was talking about.  In this case the non-Cygwin process 
> might have a large pending read, so that the Cygwin process calling select on 
> the write side will see WriteQuotaAvailable == 0.  This could lead to a deadlock 
> with the Cygwin process waiting for write ready while the non-Cygwin process is 
> blocked trying to read.

Then, the above patch is for another issue.
The problem happes when:
1) Start command prompt.
2) Run 'echo AAAAAAAAAAAA | \cygwin64\bin\cat
This causes hang up in cat. In this case, pipe is created by cmd.exe.
Therefore, read_mtx is not created.
 
> My suggestion is that we impose a timeout in this situation, after which select 
> reports write ready.

Keeping read handle in write pipe (Corinna's query_hdl) causes problem
that write side cannot detect close on read side.
Is it possible to open read handle temporally when pipe_data_available()
is called?

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>