Strange 3-second tcp connection latencies (Linux, HTTP)
- by user25417
Our webservers with static content are experiencing strange 3 second latencies occasionally. Typically, an ApacheBench run ( 10000 requests, concurrency 1 or 40, no difference, but keepalive off) looks like this:
Connection Times (ms)
min mean[+/-sd] median max
Connect: 2 10 152.8 3 3015
Processing: 2 8 34.7 3 663
Waiting: 2 8 34.7 3 663
Total: 4 19 157.2 6 3222
Percentage of the requests served within a certain time (ms)
50% 6
66% 7
75% 7
80% 7
90% 9
95% 11
98% 223
99% 225
100% 3222 (longest request)
I have tried many things:
- Apache2 2.2.9 with worker or prefork MPM, no difference (with KeepAliveTimeout 10-15)
- Nginx 0.6.32
- various tcp parameters (net.core.somaxconn=3000, net.ipv4.tcp_sack=0, net.ipv4.tcp_dsack=0)
- putting the files/DocumentRoot on tmpfs
- shorewall on or off (i.e. empty iptables or not)
- AllowOverride None is on for /, so no .htaccess checks (verified with strace)
- the problem persists whether the webservers are accessed directly or through a Foundry load balancer
Kernel is 2.6.32 (Debian Lenny backports), but it occurred with 2.6.26 also. IPv6 is enabled, but not used.
Does the issue look familiar to anyone? Help/suggestions are much appreciated. It sounds a bit like a SYN,ACK packet getting lost or ignored.
