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Date: Sat, 28 Aug 2021 10:43:27 +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 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.

Is a blocking ReadFile actually faster than a non-blocking read?  Or
does it mainly depend on BYTE vs. MESSAGE mode?

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?


Corinna

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