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http_load runs multiple http/https fetches in parallel, to test the
throughput of a web server. However unlike most such test clients,
it runs in a single process, so it doesn't bog down the client
machine.
You give it a file containing a list of URLs that may be fetched,
a flag specifying how to start connections (either by rate or by
number of simulated users), and a flag specifying when to quit
(either after a given number of fetches or a given elapsed time).
There are also optional flags for checksums, throttling, and progress
reports.
