unanswered requests from local host, high load

Carsten Otto otto at informatik.rwth-aachen.de
Thu Oct 28 17:49:28 MSD 2010

Dear list,

I'm running a public mirror server which currently handles about
5 GBit/sec with 15.000 concurrent connections and 1400 requests/second.
We only serve static files and basically all served files are in RAM
(e.g. the current Mozilla Firefox update).

In high load scenarios nginx sometimes fails to answer simple requests
in time, even when they originate from localhost. According to
SmokePing's "HTTP ping", which just GETs/HEADs the static index.html
file, the time needed for this answer changes dramatically and packet
loss is involved with up to 17% when the problems occur.

The Apache running in the background did not have this problem for
queries from localhost. When considering external queries (another
instance of SmokePing), I see gaps (100% packet loss) for both Apache
and nginx, where nginx also has packet loss up to 20% and higher
response times around that time.

If you want I can provide the four snippets of SmokePing graphs.

nginx 0.8.53, 64 workers, no keepalive, 5000 worker connections, epoll,
sendfile, tcp_nodelay
Linux 2.6.26 (Debian), max 131072 open files
two quadcore Xeons, 60 GByte DDR3, Intel 10 GBit NIC
10 TByte StorageTek disk backend

In case you need further information, I'm happy to provide it.

I just increased to 96 workers (as I already wanted, I accidently
reverted that config today), let's see what happens...

Best regards,
Carsten Otto           otto at informatik.rwth-aachen.de
LuFG Informatik 2      http://verify.rwth-aachen.de/otto/
RWTH Aachen            phone: +49 241 80-21211
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