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Subject: Re: cygrunsrv + sshd + rsync = 20 times too slow -- throttled?
To: Chris Roehrig <croehrig AT house DOT org>
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On Tue, Aug 24, 2021, 09:50 Chris Roehrig <croehrig AT house DOT org> 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.
>
> Thanks,
> -- Chris
>
>
> Windows 10 v2004 (64-bit)
> CYGWIN_NT-10.0 .... 3.2.0(0.340/5/3) 2021-03-29 08:42 x86_64 Cygwin
> rsync version 3.2.4dev protocol version 31
> Linux Mint 20
>

Older versions of windows had a setting to optimize the OS for either
background services or foreground applications. One of the things this did
was throttle network usage. I don't know if windows 10 has the same setting
though.

>

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