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Date: Thu, 25 Jun 2009 12:10:39 +0200
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
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Subject: Re: [1.7.0-50] scp progress counter flies through first 175 MB or 	so
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On Jun 24 13:17, Andrew Schulman wrote:
> Here's an odd one.
> 
> Using openssh 5.2p1-2 with Cygwin 1.7.0-50, when I scp any file, the
> progress counter appears to show ridiculously fast transfer rates, e.g.
> about 50 MB/s over a 750 KB/s connection, for the first 175 MB or so. After
> that the counter settles down to normal speed.  Then when the counter
> reaches the end, it "hangs" at 100% for the remaining time while the copy
> finishes.
> 
> At first I thought that the copy itself was being corrupted in the first
> 175 MB, but I'm no longer able to reproduce that.  I believe now that the
> copy is good and it's only the progress counter that's wrong.
> 
> When I revert to Cygwin 1.7.0-49, this problem doesn't occur.

I can reproduce that copying a file via scp from a Windows machine to
a Linux box.

It looks like the pipes between the local scp and the local ssh are now
exchanging the data quicker at the start than the ssh socket can send
them to the remote machine.  On my XP machine, scp advances quickly by
about 260 Megs (hard to tell, maybe it's exaclty 256 Megs for some
reason?), then keeps the advance roughly at that value until scp
finished.  At the end scp is just waiting for ssh which still has to
send the 256/260 Megs of data.

This is really weird, given that Cygwin does not create such a big
buffer for the pipe.  Consequentially Task Manager claims that the
memory is neither taken by scp, nor by ssh.  Both processes have normal
VM sizes < 10 Megs.  Per Task manager the memory is paged Kernel Memory.
A strange side effect is that the entire time taken by the data 
transmission is longer than with -49, by almost exactly the time it
takes to empty the big kernel cache.

Puzzeling.


Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat

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