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When profiling locks, it appears that the WQ's lock has become the most
contended one, despite the WQ being split by thread. The reason is that
each thread takes the WQ lock before checking if it it does have something
to do. In practice the WQ almost only contains health checks and rare tasks
that can be scheduled anywhere, so this is a real waste of resources.
This patch proceeds differently. Now that the WQ's lock was turned to RW
lock, we proceed in 3 phases :
1) locklessly check for the queue's emptiness
2) take an R lock to retrieve the first element and check if it is
expired. This way most visits are performed with an R lock to find
and return the next expiration date.
3) if one expiration is found, we perform the WR-locked lookup as
usual.
As a result, on a one-minute test involving 8 threads and 64 streams at
1.3 million ctxsw/s, before this patch the lock profiler reported this :
Stats about Lock TASK_WQ:
# write lock : 1125496
# write unlock: 1125496 (0)
# wait time for write : 263.143 msec
# wait time for write/lock: 233.802 nsec
# read lock : 0
# read unlock : 0 (0)
# wait time for read : 0.000 msec
# wait time for read/lock : 0.000 nsec
And after :
Stats about Lock TASK_WQ:
# write lock : 173
# write unlock: 173 (0)
# wait time for write : 0.018 msec
# wait time for write/lock: 103.988 nsec
# read lock : 1072706
# read unlock : 1072706 (0)
# wait time for read : 60.702 msec
# wait time for read/lock : 56.588 nsec
Thus the contention was divided by 4.3.
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 - 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)