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Date: Sun, 29 Aug 2021 12:27:54 +0200
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Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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On Aug 29 00:43, Takashi Yano via Cygwin wrote:
> On Sat, 28 Aug 2021 13:58:08 +0200
> Corinna Vinschen wrote:
> > On Aug 28 18:41, Takashi Yano via Cygwin wrote:
> > > On Sat, 28 Aug 2021 10:43:27 +0200
> > > Corinna Vinschen wrote:
> > > > On Aug 28 02:21, Takashi Yano via Cygwin wrote:
> > > > > On Fri, 27 Aug 2021 12:00:50 -0400
> > > > > Ken Brown wrote:
> > > > > > Two years ago I thought I needed nt_create to avoid problems when calling 
> > > > > > set_pipe_non_blocking.  Are you saying that's not an issue?  Is 
> > > > > > set_pipe_non_blocking unnecessary?  Is that the point of your modification to 
> > > > > > raw_read?
> > > > > 
> > > > > Yes. Instead of making windows read function itself non-blocking,
> > > > > it is possible to check if the pipe can be read before read using
> > > > > PeekNamedPipe(). If the pipe cannot be read right now, EAGAIN is
> > > > > returned.
> > > > 
> > > > The problem is this:
> > > > 
> > > >   if (PeekNamedPipe())
> > > >     ReadFile(blocking);
> > > > 
> > > > is not atomic.  I. e., if PeekNamedPipe succeeds, nothing keeps another
> > > > thread from draining the pipe between the PeekNamedPipe and the ReadFile
> > > > call.  And as soon as ReadFile runs, it hangs indefinitely and we can't
> > > > stop it via a signal.
> > > 
> > > Hmm, you are right. Mutex guard seems to be necessary like pty code
> > > if we go this way.
> > > 
> > > > Is a blocking ReadFile actually faster than a non-blocking read?  Or
> > > > does it mainly depend on BYTE vs. MESSAGE mode?
> > > 
> > > Actually, I don't think so. Perhaps it is not essential problem of
> > > overlapped I/O but something is wrong with current pipe code.
> > > 
> > > > What if the pipe is created non-blocking and stays non-blocking all the
> > > > time and uses BYTE mode all the time?  Just as sockets, it would always
> > > > only emulate blocking mode.  Wouldn't that drop code size a lot and fix
> > > > most problems?
> > > 
> > > If 'non-blocking' means overlapped I/O, only the problem will be:
> > > https://cygwin.com/pipermail/cygwin/2021-March/247987.html
> > 
> > Sorry if that wasn't clear, but I was not talking about overlapped I/O,
> > which we should get rid off, but of real non-blocking mode, which
> > Windows pipes are fortunately capable of.
> 
> Do you mean, PIPE_NOWAIT flag? If this flags is specified in
> the read pipe, non-cygwin apps cannot read the pipe correctly.

Do you mean after execve?  If the child is a non-Cygwin process, we
could run a loop over the stdio descriptors in child_info_spawn::worker
setting pipes to blocking.

But that's a corner problem, easily solved.  It's more important that it
works nicely for Cygwin processes.


Corinna

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