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The problem with the way idle connections currently work is that it's easy for a thread to steal all of its siblings' connections, then release them, then it's done by another one, etc. This happens even more easily due to scheduling latencies, or merged events inside the same pool loop, which, when dealing with a fast server responding in sub-millisecond delays, can really result in one thread being fully at work at a time. In such a case, we perform a huge amount of takeover() which consumes CPU and requires quite some locking, sometimes resulting in lower performance than expected. In order to fight against this problem, this patch introduces a new server setting "pool-low-conn", whose purpose is to dictate when it is allowed to steal connections from a sibling. As long as the number of idle connections remains at least as high as this value, it is permitted to take over another connection. When the idle connection count becomes lower, a thread may only use its own connections or create a new one. By proceeding like this even with a low number (typically 2*nbthreads), we quickly end up in a situation where all active threads have a few connections. It then becomes possible to connect to a server without bothering other threads the vast majority of the time, while still being able to use these connections when the number of available FDs becomes low. We also use this threshold instead of global.nbthread in the connection release logic, allowing to keep more extra connections if needed. A test performed with 10000 concurrent HTTP/1 connections, 16 threads and 210 servers with 1 millisecond of server response time showed the following numbers: haproxy 2.1.7: 185000 requests per second haproxy 2.2: 314000 requests per second haproxy 2.2 lowconn 32: 352000 requests per second The takeover rate goes down from 300k/s to 13k/s. The difference is further amplified as the response time shrinks. |
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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)