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Re: Severe performance degradation of writev
- From: David Rothenberger <daveroth at acm dot org>
- To: cygwin at cygwin dot com
- Date: Wed, 09 Jul 2014 09:44:17 -0700
- Subject: Re: Severe performance degradation of writev
- Authentication-results: sourceware.org; auth=none
- References: <lpciht$pc5$1 at ger dot gmane dot org> <20140707101049 dot GI1803 at calimero dot vinschen dot de> <20140707134137 dot GK1803 at calimero dot vinschen dot de>
- Reply-to: cygwin at cygwin dot com
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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