https://bz.apache.org/bugzilla/show_bug.cgi?id=63296
Bug ID: 63296
Summary: mod_event: Connection timeouts with ab in case of
keepalive and high concurrency
Product: Apache httpd-2
Version: 2.4.6
Hardware: PC
OS: Linux
Status: NEW
Severity: critical
Priority: P2
Component: mpm_event
Assignee: [email protected]
Reporter: [email protected]
Target Milestone: ---
Problem description:
Running ab causes a connection timeout (/ serves an empty HTML file):
$ ab -n 10000 -c 128 -k -A Administrator:Faba8888 http://localhost:17080/
This is ApacheBench, Version 2.3 <$Revision: 1430300 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/
Benchmarking localhost (be patient)
Completed 1000 requests
Completed 2000 requests
Completed 3000 requests
Completed 4000 requests
Completed 5000 requests
Completed 6000 requests
Completed 7000 requests
Completed 8000 requests
Completed 9000 requests
apr_pollset_poll: The timeout specified has expired (70007)
Total of 9901 requests completed
We saw similar issues during load testing of our application with another load
testing tool.
Based on strace, it looks like connect(2) times out.
In case of mod_worker, it's clear that a concurrency level of 9+ must lead to
delays depending on KeepAliveTimeout because one thread handles one connection
throughout the lifetime of the connect. In case of mod_event, however, one
thread handles one request from any connection. A level of 9+ works with
mod_event, but 128 fails for reasons I cannot explain. Changing
KeepAliveTimeout affects mod_event in this scenario too.
In case of a proprietary MPM that handles connections and requests in a manner
similar to mod_event, I cannot reproduce the issue. In that implementation,
every thread handles new connections and incoming data based on epoll.
Workarounds:
1) Disabling keepalive connections (client or server) works around the issue.
2) Reducing the number of concurrent requests (-c) to 8 works around the issue.
Configuration:
MPM (only one process with a static number of threads):
<IfModule event.c>
StartServers 1
ServerLimit 1
MaxRequestWorkers 8
MinSpareThreads 8
MaxSpareThreads 8
ThreadsPerChild 8
</IfModule>
Keepalive:
KeepAlive On
MaxKeepAliveRequests 0
KeepAliveTimeout 120
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