cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
Corinna Vinschen
corinna-cygwin@cygwin.com
Sat Aug 28 11:58:08 GMT 2021
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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