Connections dropped under load

Bob Camp lists at
Wed Jan 5 17:59:32 CET 2011


Running simple load tests both on Apache directly, and on Varnish - both
seem to experience "long delays" on a small percentage of the requests. The
problem does not appear to happen with low loads. It does come up as CPU
usage becomes an issue. It also is hard to make happen with a single stream
of requests. It seems to come up much quicker with many requests done in

I've always *assumed* that the poor little TCP/IP hamster simply ran out of
breath and started dropping connections. 


-----Original Message-----
From: varnish-misc-bounces at
[mailto:varnish-misc-bounces at] On Behalf Of George
Sent: Wednesday, January 05, 2011 11:18 AM
To: varnish-misc at
Subject: Re: Connections dropped under load

Hello Cosimo,

Thank you for the quick reply. After your hint I had the tests run again 
but couldn't detect that pattern. What susprised me though after looking 
through the logs is that almost all requests by the load generator 
complete in a timely manner ( < 1 sec), but all requests generated by a 
real browser (IE, FF, Opera) will be served much later or even run into 
a timeout.

On 05.01.2011 16:30, Cosimo Streppone wrote:
> On Wed, 05 Jan 2011 16:20:31 +0100, George Georgovassilis 
> <g.georgovassilis at> wrote:
>> I'm having trouble with dropped connections under a loadtest.
>> The problem: As a measure for response, I am requesting an image from
>> the webapp running in Tomcat while the loadtest is underway. However
>> that either times out or is delivered after several seconds. Varnishlog
>> will often either not show the request (RxURL) at all, or show it
>> several seconds after the browser dispatched it.
> Hi George,
> if you measure the time you mention as "several seconds"
> and it's either 3 or 9 seconds, I think what you're seeing
> is a client-side TCP retransmit timeout.
> I experienced that, both under load testing,
> and in real production setups.

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