Date: Thu, 25 Jun 2009 10:05:08 -0400
From: Christopher Faylor <cgf-use-the-mailinglist-please AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: [1.7.0-50] scp progress counter flies through first 175 MB or 	so
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On Thu, Jun 25, 2009 at 12:10:39PM +0200, Corinna Vinschen wrote:
>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.

Is ssh using non-blocking pipes opened for write?  Until a week or two
ago, Cygwin didn't support those and treated the non-blocking write as a
blocking write.

cgf

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