cygrunsrv + sshd + rsync = 20 times too slow -- throttled?

[Takashi Yano]

On Sat, 28 Aug 2021 11:00:24 +0900
Takashi Yano wrote:
> On Sat, 28 Aug 2021 02:21:11 +0900
> Takashi Yano wrote:
> 
> > On Fri, 27 Aug 2021 12:00:50 -0400
> > Ken Brown wrote:
> > 
> > > On 8/27/2021 7:24 AM, Takashi Yano wrote:
> > > > On Thu, 26 Aug 2021 18:18:29 -0400
> > > > Ken Brown wrote:
> > > >> On 8/26/2021 11:56 AM, Ken Brown via Cygwin wrote:
> > > [...]
> > > >> In case you want to try out my proposed change, I've just rebased the patches to
> > > >> the current master and pushed them to a new topic/pipe branch.
> > > > 
> > > > Hi Ken,
> > > > 
> > > > Thanks much! I tested topic/pipe branch.
> > > > 
> > > > [yano@cygwin-PC ~]$ scp test.dat yano@linux-server:.
> > > > yano@linux-server's password:
> > > > test.dat                                      100%  100MB  95.9MB/s   00:01
> > > > [yano@cygwin-PC ~]$ scp yano@linux-server:test.dat .
> > > > yano@linux-server's password:
> > > > test.dat                                      100%  100MB   8.0MB/s   00:12
> > > > 
> > > > yano@linux-server:~$ scp yano@cygwin-PC:test.dat .
> > > > yano@cygwin-PC's password:
> > > > test.dat                                      100%  100MB 109.7MB/s   00:00
> > > > yano@linux-server:~$ scp test.dat yano@cygwin-PC:.
> > > > yano@cygwin-PC's password:
> > > > test.dat                                      100%  100MB  31.4MB/s   00:03
> > > > 
> > > > As shown above, outgoing transfer-rate has been improved upto near
> > > > theoretical limit. However, incoming transfer-rate is not improved
> > > > much.
> > > > 
> > > > I digged further and found the first patch attached solves the issue
> > > > as follows.
> > > > 
> > > > [yano@cygwin-PC ~]$ scp yano@linux-server:test.dat .
> > > > yano@linux-server's password:
> > > > test.dat                                      100%  100MB 112.8MB/s   00:00
> > > > 
> > > > yano@linux-server2:~$ scp test.dat yano@cygwin-PC:.
> > > > yano@cygwin-PC's password:
> > > > test.dat                                      100%  100MB 102.5MB/s   00:00
> > > 
> > > Great!
> > > 
> > > > I also tested the case:
> > > >>>> https://cygwin.com/pipermail/cygwin/2021-March/247987.html
> > > >>>> which seems to be the same issue with
> > > >>>> https://stackoverflow.com/questions/10385424/good-alternatives-to-cygwin-cygwin-doesnt-support-natively-support-win32-app
> > > > 
> > > > Unfortunately, topic/pipe does not help.
> > > > 
> > > > I confirmed that applying the second patch attached, which reverts
> > > > to create() rather than nt_create(), and setting CYGWIN=pipe_byte
> > > > fixes the problem.
> > > > 
> > > > What do you think of this alternative implementation which does
> > > > not use nt_create()?
> > > 
> > > 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.
> 
> As for writint to pipe, set_pipe_non_blocking seems to take effect
> and be necessary.

So, the following patch seems to be appropriate.

diff --git a/winsup/cygwin/fhandler_pipe.cc b/winsup/cygwin/fhandler_pipe.cc
index b3af4a0a0..82f5de3f4 100644
--- a/winsup/cygwin/fhandler_pipe.cc
+++ b/winsup/cygwin/fhandler_pipe.cc
@@ -48,6 +48,8 @@ fhandler_pipe::set_pipe_non_blocking (bool nonblocking)
   IO_STATUS_BLOCK io;
   FILE_PIPE_INFORMATION fpi;

+  if ((openflags & O_ACCMODE) == O_RDONLY)
+    return; /* Do nothing for read pipe */
   fpi.ReadMode = FILE_PIPE_BYTE_STREAM_MODE;
   fpi.CompletionMode = nonblocking ? FILE_PIPE_COMPLETE_OPERATION
     : FILE_PIPE_QUEUE_OPERATION;

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>