Re: Behaviour of system tray bandwidth indicator

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Am 20.03.2014 17:00, schrieb Patrick O'Callaghan:
> On Thu, 2014-03-20 at 13:14 +0100, Reindl Harald wrote:
>> Am 20.03.2014 12:29, schrieb 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.
>>>
>>> No doubt, however it's not how it's *read*. The interface could be
>>> clearer about what it's actually showing when the information is
>>> misleading.
>>
>> how should it?
>>
>> if you write 100 MB to disk and the kernel buffers it
>> from the application you are done, that's how any
>> modern operating system works for decades
>>
>> if you don't want that behavior disable any caches and
>> buffers but then your machine will be as slow as hardware
>> years ago
> 
> Then why does the pop-up show "copy completed" only after it has been
> transferred to the server? This is not consistent. In fact the more I
> think about it the more wrong it seems. I'm being told by the widget
> that X GB were sent in a fraction of a second (open to interpretation),
> that the copy is not currently consuming any bandwidth (false), but that
> it hasn't actually finished yet (true).

It is not that easy. from the popup point of view the data are
transfered but the copy is not confirmed as completed. On the way from
one point to another there are different caching strategies and
different "commit" strategies.

In NFS sync mode (NFS Server) and client async mode a data transfer is
buffered on client side and server side. A transfer is marked as
complete if the server has flushed the cache (data is on disk). On the
client side the data fransfer rate gets to 0 if all data is sent to the
cache (if the cache is big enough). If the cache is not big enough you
will see high transfere rates until the cache is filled and after that
something like zigzag: during writing data from cache to server the
transfer stale. If enough cache is back the transferrate will increase
to not realy available bandwith.

If you use NFS Server async mode the transfer "commit" from server comes
a little earlier: if the server received all data (in the server cache).
With 100MBit max network rate this will be almost identical (most modern
hard disks are fast enough for this rate). If you use 1GBit or 10GBit
hard disks may be slower. There may be several seconds between the
complete receive at server side and the commit to hard disks.

If you use async on the client side no cache will be used on client side
an therefore the transfere is almost identical to the network transfere
(if it is the only one of course).

The caching stuff is no problem if you usually transfere many small
files. If you transfer bigger files the caching is not of much use.

And be aware: this is only one part of file transfer: Metadata are
cached as well and these caches are more sensible than simple data.

So there is nothing the popup can do about this. It gets several
informations like transfer started, in progress, ended/finised and commited.

Regards
Martin

> 
> poc
> 
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