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

[Takashi Yano]

On Tue, 24 Aug 2021 12:49:52 -0700
Chris Roehrig wrote:
> I have a network of Windows, Linux and Mac machines and I use rsync to synchronize various directories between them.
> 
> I'm trying to figure out why my rsync transfers are so slow (<4 MB/s) only when the remote endpoint is Cygwin rsync over sshd (with both a Linux or Cygwin rsync client).   In all other scenarios, I get the full 100MB/s as expected from gigabit ethernet.  This has been an ongoing problem for me for a couple of years over several Windows and Cygwin versions, and I'd like to try to fix it.    
> 
> If I run rsync --daemon --no-detach under mintty in the foreground on the remote Windows endpoint,  I get the full 100 MB/s transfers, so it seems like it has something to do with rsync.exe running in the background under the cygrunsrv+sshd service (which was installed normally using ssh-host-config).
> 
> If I do:
> 	pv /dev/zero | ssh $WINHOST "cat > /dev/null"
> or even
> 	pv /dev/urandom | ssh $WINHOST md5sum
> I also get the full 100 MB/s transfers, so it doesn't look like sshd itself is being throttled by bandwidth or CPU.
> 
> The machines have less than 15% CPU utilization while transferring, with each of the 4 cores less than 30%, so it doesn't look to be CPU issue.
> In Task Manager, sshd.exe and rsync.exe seem to be running normally using only few percent CPU, and show Power Throttling=Disabled, Priority=Normal.   Setting their Priority to High doesn't seem to change things.
> 
> Looking in Resource Monitor on the remote endpoint, the network usage is pretty much a flat horizontal line at about 18 Mbps (2.5 MB/s), so it sure looks to me as if rsync is somehow being bandwidth-throttled when run in the background under cygsshd.
> 
> It's almost as if rsync has an implicit --bwlimit override when it is run from cygrunsrv+sshd (I've tried --bwlimit=0 on the client which makes no difference).
> 
> 
> Any ideas?    Not sure where to go from here. 

In cygwin, just scp is very slow.

The transfer speed in my environment is as follows.
The tests were done with 100MB of test.dat file.

(1-1) From cygwin-PC,
[yano@cygwin-PC ~]$ scp test.dat yano@linux-server:.
yano@linux-server's password:
test.dat                                      100%  100MB   4.0MB/s   00:24
[yano@cygwin-PC ~]$ scp yano@linux-server:test.dat .
yano@linux-server's password:
test.dat                                      100%  100MB   8.0MB/s   00:12

(1-2) From linux-server,
yano@linux-server:~$ scp yano@cygwin-PC:test.dat .
yano@cygwin-PC's password:
test.dat                                      100%  100MB   4.0MB/s   00:24
yano@linux-server:~$ scp test.dat yano@cygwin-PC:.
yano@cygwin-PC's password:
test.dat                                      100%  100MB   4.1MB/s   00:24


I looked into this problem, and noticed that this is caused
by cygwin pipe implementation. Pipe in cygwin is configured
with FILE_FLAG_OVERLAPPED.

If the pipe is configured without FILE_FLAG_OVERLAPPED,
the transfer speed is much improved as follows.


(2-1) From cygwin-PC,
[yano@cygwin-PC ~]$ scp test.dat yano@linux-server:.
yano@linux-server's password:
test.dat                                      100%  100MB  85.5MB/s   00:01
[yano@cygwin-PC ~]$ scp yano@linux-server:test.dat .
yano@linux-server's password:
test.dat                                      100%  100MB  69.7MB/s   00:01

(2-2) From linux-server,
yano@linux-server:~$ scp yano@cygwin-PC:test.dat .
yano@cygwin-PC's password:
test.dat                                      100%  100MB  80.1MB/s   00:01
yano@linux-server:~$ scp test.dat yano@cygwin-PC:.
yano@cygwin-PC's password:
test.dat                                      100%  100MB  57.7MB/s   00:01

I am not sure why this happens and how to fix this.
Any idea?

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