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Date: Mon, 30 Aug 2021 09:22:59 +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 Mon, 30 Aug 2021 09:13:14 +0900
Takashi Yano wrote:
> On Sun, 29 Aug 2021 17:04:56 -0400
> Ken Brown wrote:
> > On 8/29/2021 5:07 AM, Takashi Yano via Cygwin wrote:
> > > On Sat, 28 Aug 2021 18:41:02 +0900
> > > Takashi Yano 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.
> > > 
> > > I have found that set_pipe_non_blocking() succeeds for both read and
> > > write pipes if the write pipe is created by CreateNamedPipe() and the
> > > read pipe is created by CreateFile() contrary to the current create()
> > > code. Therefore, not only nt_create() but also PeekNamedPipe() become
> > > unnecessary.
> > > 
> > > Please see the revised patch attached.
> > 
> > I haven't had a chance to test this myself yet, but occurs to me that we might 
> > have a different problem after this patch: Does the write handle that we get 
> > from CreateNamedPipe() have FILE_READ_ATTRIBUTES access?
> 
> I have just checked this, and the answer is "No". Due to this problem,
> NtQueryInformationFile() call in select() fails on the write pipe.
> 
> It seems that we need more consideration...

We have two easy options:
1) Configure the pipe with PIPE_ACCESS_DUPLEX.
2) Use nt_create() again and forget C# program issue.



Even without this problem, select() for writing pipe has a bug
and does not wrok as expected. The following patch seems to be
needed.

diff --git a/winsup/cygwin/select.cc b/winsup/cygwin/select.cc
index 83e1c00e0..ac2fd227e 100644
--- a/winsup/cygwin/select.cc
+++ b/winsup/cygwin/select.cc
@@ -612,7 +612,6 @@ pipe_data_available (int fd, fhandler_base *fh, HANDLE h, bool writing)
           that.  This means that a pipe could still block since you could
           be trying to write more to the pipe than is available in the
           buffer but that is the hazard of select().  */
-      fpli.WriteQuotaAvailable = fpli.OutboundQuota - fpli.ReadDataAvailable;
       if (fpli.WriteQuotaAvailable > 0)
        {
          paranoid_printf ("fd %d, %s, write: size %u, avail %u", fd,

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

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