On 11/29/2010 11:25 PM, Travis Whitton wrote:
Hi,
We're experiencing some odd behavior regarding connections taking a
long time to establish to our website. We've been running Apache in
production for over three years now and have recently began
experiencing issues where the server-status page, static, and dynamic
content response times will slow anywhere from a few seconds to long
enough for the connection to timeout.
Initially thinking we might be hitting some hard limits with the OS,
we've thoroughly audited our sysctl variables, tried disabling
iptables and conntrack, and ensured that we're not running out of
ephemeral ports or anything along those lines. Looking at netstat, it
seems we have a pretty large number of connections in TIME_WAIT which
is understandable since this is a high traffic website, but I'm
wondering if this value could indicate we're backlogging on TCP
connections or something along those lines?
[r...@rhl073 ipv4]# netstat -an | awk '/^tcp/ {A[$(NF)]++} END {for (I
in A) {printf "%5d %s\n", A[I], I}}'
34723 TIME_WAIT
3 CLOSE_WAIT
275 FIN_WAIT1
74 FIN_WAIT2
8824 ESTABLISHED
815 SYN_RECV
102 CLOSING
30 LAST_ACK
10 LISTEN
In an effort to tune things, I've tried playing with the TCP timeout
settings a bit, and the response times have improved somewhat. Please
note that I've been testing response times using the loopback
interface to rule out any ethernet hardware issues.
echo 15> /proc/sys/net/ipv4/tcp_fin_timeout
echo 1> /proc/sys/net/ipv4/tcp_tw_recycle
echo 1> /proc/sys/net/ipv4/tcp_tw_reuse
We're running prefork, and have configured the client settings to what
seem to be reasonable limits for our hardware.
<IfModule prefork.c>
StartServers 100
MinSpareServers 100
MaxSpareServers 200
ServerLimit 1500
MaxClients 1500
MaxRequestsPerChild 1000000
</IfModule>
Forking new children is VERY expensive, compared to the alternatives.
If 1500 concurrent clients is common for your site, consider starting up
that many as well.
min/maxspare is only meant to handle bursts, not define your normal load.
Your settings mean "accept up to 1500 concurrent connections, but only
RUN 300 threads when you don't have that many clients"
Since apache will have to fork up to 1200 threads in rapid succession
when the load spikes, this will cause startup throttling after only a
few seconds, which is causing your timeouts.
You should change these to AT LEAST 1000 startup, 100 minspare and 200
maxspare - if 1500 is your actual max load, and not a limit you imposed
because you think it can't handle more.
It can handle many more, if you have the memory for them.
With 1500 concurrent connections, I would long ago have moved to worker
combined with proxying dynamic content to a separate prefork instance.
This will optimize memory and resource usage to such an extent that you
can easily run 5000 clients concurrently.
Worker threads are much more efficient and take far less memory than
prefork children, therefore they suffer far less from being short-lived
(due to low maxrequest settings)
Unless the majority of these requests are for dynamic content (they
rarely are), I predict you can increase performance several fold.
--
J.
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