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Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
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From: Ken Brown via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Ken Brown <kbrown AT cornell DOT edu>
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On 8/25/2021 7:18 AM, Takashi Yano via Cygwin wrote:
> 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 $WINHOST md5sum
>> I also get the full 100 MB/s transfers, so it doesn't look like sshd itself is being throttled by bandwidth or CPU.
>>
>> The machines have less than 15% CPU utilization while transferring, with each of the 4 cores less than 30%, so it doesn't look to be CPU issue.
>> In Task Manager, sshd.exe and rsync.exe seem to be running normally using only few percent CPU, and show Power Throttling=Disabled, Priority=Normal.   Setting their Priority to High doesn't seem to change things.
>>
>> Looking in Resource Monitor on the remote endpoint, the network usage is pretty much a flat horizontal line at about 18 Mbps (2.5 MB/s), so it sure looks to me as if rsync is somehow being bandwidth-throttled when run in the background under cygsshd.
>>
>> It's almost as if rsync has an implicit --bwlimit override when it is run from cygrunsrv+sshd (I've tried --bwlimit=0 on the client which makes no difference).
>>
>>
>> Any ideas?    Not sure where to go from here.
> 
> In cygwin, just scp is very slow.
> 
> The transfer speed in my environment is as follows.
> The tests were done with 100MB of test.dat file.
> 
> (1-1) From cygwin-PC,
> [yano AT cygwin-PC ~]$ scp test.dat yano AT linux-server:.
> yano AT linux-server's password:
> test.dat                                      100%  100MB   4.0MB/s   00:24
> [yano AT cygwin-PC ~]$ scp yano AT linux-server:test.dat .
> yano AT linux-server's password:
> test.dat                                      100%  100MB   8.0MB/s   00:12
> 
> (1-2) From linux-server,
> yano AT linux-server:~$ scp yano AT cygwin-PC:test.dat .
> yano AT cygwin-PC's password:
> test.dat                                      100%  100MB   4.0MB/s   00:24
> yano AT linux-server:~$ scp test.dat yano AT cygwin-PC:.
> yano AT 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 AT cygwin-PC ~]$ scp test.dat yano AT linux-server:.
> yano AT linux-server's password:
> test.dat                                      100%  100MB  85.5MB/s   00:01
> [yano AT cygwin-PC ~]$ scp yano AT linux-server:test.dat .
> yano AT linux-server's password:
> test.dat                                      100%  100MB  69.7MB/s   00:01
> 
> (2-2) From linux-server,
> yano AT linux-server:~$ scp yano AT cygwin-PC:test.dat .
> yano AT cygwin-PC's password:
> test.dat                                      100%  100MB  80.1MB/s   00:01
> yano AT linux-server:~$ scp test.dat yano AT cygwin-PC:.
> yano AT 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.

A couple years ago I had an idea for changing the pipe implementation to avoid 
overlapped I/O:

   https://cygwin.com/pipermail/cygwin-patches/2019q2/009393.html
   https://cygwin.com/pipermail/cygwin-patches/2019q2/009423.html

I never followed up on it.  But if you think it might help with this problem, I 
could dust it off and try to finish it.

Ken

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