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In tv_update_date(), we calculate the new global date based on the local one. It's very likely that other threads will end up with the exact same now_ms date (at 1 million wakeups/s it happens 99.9% of the time), and even the microsecond was measured to remain unchanged ~70% of the time with 16 threads, simply because sometimes another thread already updated a more recent version of it. In such cases, performing a CAS to the global variable requires a cache line flush which brings nothing. By checking if they're changed before writing, we can divide by about 6 the number of writes to the global variables, hence the overall contention. In addition, it's worth noting that all threads will want to update at the same time, so let's place a cpu relax call before trying again, this will spread attempts apart.
The HAProxy documentation has been split into a number of different files for ease of use. Please refer to the following files depending on what you're looking for : - INSTALL for instructions on how to build and install HAProxy - BRANCHES to understand the project's life cycle and what version to use - LICENSE for the project's license - CONTRIBUTING for the process to follow to submit contributions The more detailed documentation is located into the doc/ directory : - doc/intro.txt for a quick introduction on HAProxy - doc/configuration.txt for the configuration's reference manual - doc/lua.txt for the Lua's reference manual - doc/SPOE.txt for how to use the SPOE engine - doc/network-namespaces.txt for how to use network namespaces under Linux - doc/management.txt for the management guide - doc/regression-testing.txt for how to use the regression testing suite - doc/peers.txt for the peers protocol reference - doc/coding-style.txt for how to adopt HAProxy's coding style - doc/internals for developer-specific documentation (not all up to date)