Abort on overflow
After some analyzing kernel’s source, I figured out what this actually do. If it can’t create the child socket it just goes to listen_overflow. If ACK packet is valid, it tries to create skb and move socket to ESTABLISHED state. If it fails, it returns NULL if tcp_abort_on_overflow is disabled (default value), else it sets this packet as ACK and returns NULL.
I decided to verify how it really works. I’ve set up a custom web server with listen maximum backlog 5 and ran ab tool to generate traffic (to overflow backlog).
tcp_abort_on_overflow=0
% ab -n 300 -c 100 http://X.X.X.X/
This is ApacheBench, Version 2.3 <$Revision: 1604373 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/
Benchmarking X.X.X.X (be patient)
Completed 100 requests
Completed 200 requests
apr_pollset_poll: The timeout specified has expired (70007)
Total of 294 requests completed
tcp_abort_on_overflow=1
% ab -n 300 -c 100 http://X.X.X.X/
This is ApacheBench, Version 2.3 <$Revision: 1604373 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/
Benchmarking X.X.X.X (be patient)
Completed 100 requests
Completed 200 requests
Completed 300 requests
Finished 300 requests
Server Software:
Server Hostname: X.X.X.X
Server Port: 80
Document Path: /
Document Length: 13 bytes
Concurrency Level: 100
Time taken for tests: 4.630 seconds
Complete requests: 300
Failed requests: 61
(Connect: 0, Receive: 0, Length: 61, Exceptions: 0)
Total transferred: 23183 bytes
HTML transferred: 3107 bytes
Requests per second: 64.80 [#/sec] (mean)
Time per request: 1543.185 [ms] (mean)
Time per request: 15.432 [ms] (mean, across all concurrent requests)
Transfer rate: 4.89 [Kbytes/sec] received
Connection Times (ms)
min mean[+/-sd] median max
Connect: 0 823 1100.2 116 4437
Processing: 4 100 273.5 46 3337
Waiting: 0 42 57.4 15 246
Total: 66 923 1128.7 317 4555
As you noticed the first test just failed, due to overflowed backlog queue, the second passed without any burst.
More information about service backlog you can find in my another blog post.