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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 5:29 PM, Takashi Yano wrote:
> On Wed, 25 Aug 2021 13:52:19 -0400
> Ken Brown wrote:
>> 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.
> 
> Interesting.
> 
> It will be also helpfull for:
> 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
> 
> However, I wonder why scp dislikes overlapped I/O.

I agree that it would be good to understand this.  When I first proposed the 
change, I was thinking in terms of code simplification.  If it also improves 
performance (which we don't know yet), it becomes a higher priority, but in that 
case it would be nice to understand why it improves performance.

Ken

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