Behaviour of system tray bandwidth indicator

[Patrick O'Callaghan]

On Thu, 2014-03-20 at 11:54 +0100, Kevin Kofler wrote:
> Patrick O'Callaghan wrote:
> > I'm using Dolphin to copy large files (several GB) to an NFS-mounted
> > NAS. This works fine except that the informational system tray pop-up is
> > showing absurd values for the copy bandwidth. e.g. a 2GB file is
> > declared to be finished when it's only just started. It looks like
> > what's being measured is the rate of handoff to network buffers. This
> > machine has a quad-core i7 CPU with 16GB of RAM and isn't doing much
> > else, so the entire file could be copied to system buffers very quickly.
> > However the actual LAN is 100Mbps so the real copy takes several
> > minutes, and it's only then that I get the pop-up saying it's finished.
> 
> The UI can only indicate what is visible to user space, it has no way to, 
> nor is it expected to, know what the kernel does behind the scenes. If the 
> kernel reports that it has processed the data, the UI shows it as processed. 
> That is exactly as designed.

Presumably 'cp' doesn't know what the kernel does behind the scenes
either, so I did a further experiment (using a 2.2GB test file):

1) NFS server mounted async:

$ time cp TestFile /storage/public/Media

real    4m4.959s
user    0m0.011s
sys     0m1.456s

2) NFS server mounted sync:

$ time cp TestFile /storage/public/Media

real    5m41.806s
user    0m0.113s
sys     0m8.609s

Clearly there is a difference in real time and system time which we can
put down to kernel buffering. However the difference is nothing like as
great as that shown by the notification widget under KDE, in which the
async case shows no transfer bandwidth bring consumed after a couple of
seconds, even though it still takes just as long to complete. In what
way is the Dolphin transfer different from cp?

poc