Re: Severe performance degradation of writev

[David Rothenberger <daveroth@acm.org>]

Corinna Vinschen wrote:
> On Jul  7 12:10, Corinna Vinschen wrote:
>> On Jul  7 07:28, jojelino wrote:
>>> 2008-07-27  Corinna Vinschen  <corinna@vinschen.de>
>>> 
>>> * fhandler_socket.cc (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 
> https://cygwin.com/ml/cygwin-cvs/2014-q3/msg00003.html
> 
> I uploaded a new developer snapshot to
> http://cygwin.com/snapshots/ Please give it a thorough try.

FWIW, this snapshot fixed a recent performance degradation for me when
doing ssh/rsync transfers within my local network at work. These
transfers had run at about 25 MB/s but recently degraded to about 500
kB/s. The snapshot restored the original performance.


-- 
David Rothenberger  ----  daveroth@acm.org

Issawi's Laws of Progress:
        The Course of Progress:
                Most things get steadily worse.
        The Path of Progress:
                A shortcut is the longest distance between two points.

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