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Date: Sat, 28 Aug 2021 18:41:02 +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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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


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

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