This seems similar to what I've been seeing, described in an
earlier thread from before christmas. In my case it was not
during benchmarking, but when serving production load of around
300 req/s per server. Modern tcpip stacks on modern hardware
should handle this without blinking.
Did you have the chance to capture the problem with varnishlog so
you can replay/analyze it?
- Stig
On Wed, Jan 5, 2011 at 11:18 PM, George Georgovassilis
<[email protected] <mailto:[email protected]>>
wrote:
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]>
[mailto:[email protected]
<mailto:[email protected]>] On
Behalf Of George
Georgovassilis
Sent: Wednesday, January 05, 2011 11:18 AM
To: [email protected]
<mailto:[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]
<mailto:[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.
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--
Stig Bakken
CTO, Zedge.net - free your phone!
