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An interesting case was reported with threads and moderately sized stick-tables. Sometimes the watchdog would trigger during the purge. It turns out that the stick tables were sized in the 10s of K entries which is the order of magnitude of the possible number of connections, and that threads were used over distinct NUMA nodes. While at first glance nothing looks problematic there, actually there is a risk that a thread trying to purge the table faces 100% of entries still in use by a connection with (ts->ref_cnt > 0), and ends up scanning the whole table, while other threads on the other NUMA node are causing the cache lines to bounce back and forth and considerably slow down its progress to the point of possibly spending hundreds of milliseconds there, multiplied by the number of queued threads all failing on the same point. Interestingly, smaller tables would not trigger it because the scan would be faster, and larger ones would not trigger it because plenty of entries would be idle! The most efficient solution is to increase the table size to be large enough for this never to happen, but this is not reliable. We could have a parallel list of idle entries but that would significantly increase the storage and processing cost only to improve a few rare corner cases. This patch takes a more pragmatic approach, it considers that it will not visit more than twice the number of nodes to be deleted, which means that it accepts to fail up to 50% of the time. Given that very small batches are programmed each time (1/256 of the table size), this means the operation will finish quickly (128 times faster than now), and will reduce the inter-thread contention. If this needs to be reconsidered, it will probably mean that the batch size needs to be fixed differently. This needs to be backported to stable releases which extensively use threads, typically 2.0. Kudos to Nenad Merdanovic for figuring the root cause triggering this! |
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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)