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Date: Sun, 29 Aug 2021 17:41:24 +0900
To: cygwin AT cygwin DOT com
Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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Hi Ken,

On Sat, 28 Aug 2021 16:55:52 -0400
Ken Brown wrote:
> On 8/28/2021 11:43 AM, 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.
> 
> While waiting for Corinna's response to this, I have one more question.  Do you 
> understand why nt_create() failed and you had to revert to create()?  Was it an 
> access problem because nt_create requested FILE_WRITE_ATTRIBUTES?  Or did I make 
> some careless mistake in writing nt_create?

I am sorry but no. I don't understand why piping C# program via
the pipe created by nt_create() has the issue. I tried to change
setup parameters in nt_create(), however, I did not succeed it to
work. I also couldn't find any mistake in nt_create() so far.

Win32 programs which use ReadFile() and WriteFile() work even
with the pipe created by nt_create() as well as overlapped I/O.

What does C# program differ from legacy win32 program at all?

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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