Severe performance degradation of writev

Corinna Vinschen
Mon Jul 7 13:41:00 GMT 2014

On Jul  7 12:10, Corinna Vinschen wrote:
> On Jul  7 07:28, jojelino wrote:
> > 2008-07-27  Corinna Vinschen  <>
> > 
> >        * (fhandler_socket::send_internal): Send never
> > more
> >        then 64K bytes at once.  For blocking sockets, loop until entire data
> >        has been sent or an error occurs.
> >        (fhandler_socket::sendto): Drop code which sends on 64K bytes.
> >        (fhandler_socket::sendmsg): Ditto.
> > 
> > This commit added workaround for KB823764. but it has brought another
> > performance issue when writev sends <64k of data.
> That's why the code contains that FIXME comment.  If you have a good
> idea for simple code to split a message into the least number of pieces
> to minimize the number of WsaSendTo calls...

I took a stab at the code and I think the new version improves writing
multiple small buffers a lot.  In my testing it still works in other
scenarios, too, but I would be very grateful if somebody could have a
critical look into my code changes as posted in

I uploaded a new developer snapshot to
Please give it a thorough try.


Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat
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