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Date: Thu, 13 Dec 2007 18:59:34 +0100
From: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>
To: cygwin AT cygwin DOT com
Subject: Re: VM and non-blocking writes
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On Dec 13 09:34, Wayne Christopher wrote:
> I have a server application that runs on XP under the latest cygwin, that 
> opens up a socket connection to a client on another system, makes that 
> socket non-blocking using fcntl(.... O_NDELAY), and then feeds the client a 
> large file (100's of MBs) by doing the following:
>
> 1. call write() with the entire size of the data not yet written
>
> 2. the return value of write is the number of bytes actually written 
> (should be limited by the socket buffer size - it is on linux)
>
> 3. select() for writable status on the socket (and do other things in the 
> mean time)
>
> 4. when the socket becomes writable, goto 1
>
> What I see is that no matter how large the size is that I give to write(), 
> the return value is always the full size.  Also, I see the virtual memory 
> used by my process go way up - in fact it goes up by much more than the 
> amount of data I've written.
>
> I tried putting in a limit of 10KB in the size given to the write() call.  
> I still see the VM size grow - more slowly this time, but it eventually 
> reaches 1.5GB and then I'm out of memory.
>
> Has anybody seen this behavior?  Should I not be using O_NDELAY?  Any other 
> workarounds?

I never saw this behaviour.  Nonblocking sockets are no problem, usually.
ssh is using them, too.  A quick scan through the call chain (write ->
writev -> sendmsg -> WSASendTo) doesn't show up any memory allocation
which wouldn't be free'd again.  Practically everything is done on the
stack.

The return code of write is an error code or the number of bytes
written as returned by the WSASendTo function.  If it really behaves as
you describe, there would be nothing Cygwin could do about that.
However, I'd expect that WSASendTo frequently returns SOCKET_ERROR with
the error code set to WSAEWOULDBLOCK, which translates to a return code
-1 from write with errno set to EAGAIN.

Are you absolutely sure you're not wasting the memory yourself, somehow?
Or, is it possible that there's a strange interaction with some piece of
firewall or virus scanner?

> I don't have a simple example program but I can make one if that will help.

Yes, please.  If it's actually a problem in Cygwin, it's occuring only
in some border cases.


Corinna

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

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