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: [email protected]
[mailto:[email protected]] On Behalf Of George
Georgovassilis
Sent: Wednesday, January 05, 2011 11:18 AM
To: [email protected]
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
<[email protected]> 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.
_______________________________________________
varnish-misc mailing list
[email protected]
http://www.varnish-cache.org/lists/mailman/listinfo/varnish-misc
_______________________________________________
varnish-misc mailing list
[email protected]
http://www.varnish-cache.org/lists/mailman/listinfo/varnish-misc
