Connections dropped under load

George Georgovassilis g.georgovassilis at
Wed Jan 5 23:18:34 CET 2011

I removed the varnish instance so that the load generator is directly 
hitting Tomcat. Naturally, the request rate drops to 70 requests/sec 
with a CPU load of 100%... however connections don't drop anymore, no 
timeouts occur and the application remains pretty responsive. To recap, 
these are the possible scenarios:

1. The networking layer is overtaxed with the original 300 reqs/sec. I 
don't believe that, because the load generator doesn't record any 
dropped connections while a simple browser can't connect.

2. Tomcat is overtaxed. That also seems not plausible, since it is not 
servicing any requests under the load test - all is done by varnish. 
Even if, as I said when removing varnish from in between, it serves the 
requests just fine.

3. Varnish is overtaxed. Somehow that also doesn't make sense, since it 
is servicing the load generator just fine... but will refuse to serve 
browser requests.

4. Varnish, when under load, is picky about what connections to serve.

I'm stuck :-)

On 05.01.2011 17:59, Bob Camp wrote:
> Hi
> 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
> parallel.
> I've always *assumed* that the poor little TCP/IP hamster simply ran out of
> breath and started dropping connections.
> Bob
> -----Original Message-----
> From: varnish-misc-bounces at
> [mailto:varnish-misc-bounces at] On Behalf Of George
> Georgovassilis
> 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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