Is there an nginx queue that isn't logged via $response_time

Server NGINX
#1

We have a setup that looks like this:

nginx->haproxy->app servers

We are terminating SSL with nginx and it sits in front of everything.
During
our peak load times, we are experiencing about a 2x performance hit.
Requests that would normally take 400 ms are taking 800ms. It’s taking
longer for the entire Internet.

The problem is, I have absolutely no sign of any slowdowns in my logs
and
graphs. New Relic shows all the app servers are responding correctly
with no
change in speed. Nginx and haproxy show nothing in their logs about
requests
slowing down, but we are slowing down at end users. Despite nginx
showing
that a particular request I tracked is taking 17ms ($response_time)
through
the entire stack, it took 1.5 seconds to curl it during peak load last
week.

So, that leaves me with two options:

  1. Network issues - I have more than enough pipe left according to
    graphs
    from the router. I’m only using 400 Mbps out of the 1 Gbps port and
    there
    are no errors in ifconfig or on the switch or routers. However,
    SoftLayer
    manages this gear, so I can’t verify this personally.

  2. nginx is holding up the request and either not logging it or I’m not
    logging the right thing. Is it possible that requests are being queued
    up
    because workers are busier and they’re not getting acted on as quickly?
    If
    this is in fact happening, what can I log in nginx other than
    $response_time, since that is showing no slowdown at all. And, if this
    is
    possible that requests are actually taking longer than $response_time is
    indicating, how do I go about tweaking the config to speed things up?

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,243170,243170#msg-243170

#2

Sorry, I meant $request_time.

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,243170,243171#msg-243171

#3

In case it helps, here at my sysctl and applicable nginx config values

Sysctl
net.ipv4.tcp_syncookies = 0
net.ipv4.tcp_synack_retries = 2
net.ipv4.ip_local_port_range = 1024 65535
net.ipv4.tcp_fin_timeout = 3
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
net.ipv4.tcp_rmem = 16777216 16777216 16777216
net.ipv4.tcp_wmem = 16777216 16777216 16777216
net.ipv4.tcp_max_tw_buckets = 16777216
net.ipv4.tcp_tw_reuse = 1
net.ipv4.tcp_max_syn_backlog = 262144
net.core.somaxconn = 262144
net.core.netdev_max_backlog = 15000
net.core.netdev_budget = 8196
net.ipv4.ip_local_port_range = 1024 65535
net.ipv4.ip_nonlocal_bind = 1

Applicable nginx configuration
user www-data;
worker_processes 20;
worker_rlimit_nofile 500000;
error_log /var/log/nginx/error.log;
pid /var/run/nginx.pid;

events {
use epoll;
multi_accept off;
accept_mutex off;
worker_connections 65536;
}

Posted at Nginx Forum:
http://forum.nginx.org/read.php?2,243170,243173#msg-243173