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Date: Sat, 28 Aug 2021 13:58:08 +0200
To: cygwin AT cygwin DOT com
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 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.


Corinna

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