http_load is yet another utility that does web server load testing. It can simulate a 33.6 Kbps modem connection (-throttle) and allows you to provide a file with a list of URLs that will be fetched randomly. You can specify how many parallel connections to run (-parallel N) and the number of requests to generate per second (-rate N). Finally, you can tell the utility when to stop by specifying either the test time length (-seconds N) or the total number of fetches (-fetches N).

Again, we will try to verify the results reported by ab (claiming that the script under test can handle about 855 requests per second on our machine). Therefore we run http_load with a rate of 860 requests per second, for 5 seconds in total. We invoke is on the file urls, containing a single URL:


Here is the generated output:

panic% http_load -rate 860 -seconds 5 urls
4278 fetches, 325 max parallel, 25668 bytes, in 5.00351 seconds
6 mean bytes/connection
855 fetches/sec, 5130 bytes/sec
msecs/connect: 20.0881 mean, 3006.54 max, 0.099 min
msecs/first-response: 51.3568 mean, 342.488 max, 1.423 min
HTTP response codes:
  code 200 -- 4278

This application also reports almost exactly the same response-rate capability: 855 requests per second. Of course, you may think that it's because we have specified a rate close to this number. But no, if we try the same test with a higher rate:

panic% http_load -rate 870 -seconds 5 urls
4045 fetches, 254 max parallel, 24270 bytes, in 5.00735 seconds
6 mean bytes/connection
807.813 fetches/sec, 4846.88 bytes/sec
msecs/connect: 78.4026 mean, 3005.08 max, 0.102 min

we can see that the performance goes down—it reports a response rate of only 808 requests per second.

The nice thing about this utility is that you can list a few URLs to test. The URLs that get fetched are chosen randomly from the specified file.

Note that when you provide a file with a list of URLs, you must make sure that you don't have empty lines in it. If you do, the utility will fail and complain:

./http_load: unknown protocol -