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While the scheduler is priority-aware and class-aware, and consistently tries to maintain fairness between all classes, it doesn't make use of a fine execution budget to compensate for high-latency tasks such as TLS handshakes. This can result in many subsequent calls adding multiple milliseconds of latency between the various steps of other tasklets that don't even depend on this. An ideal solution would be to add a 4th queue, have all tasks announce their estimated cost upfront and let the scheduler maintain an auto- refilling budget to pick from the most suitable queue. But it turns out that a very simplified version of this already provides impressive gains with very tiny changes and could easily be backported. The principle is to reserve a new task flag "TASK_HEAVY" that indicates that a task is expected to take a lot of time without yielding (e.g. an SSL handshake typically takes 700 microseconds of crypto computation). When the scheduler sees this flag when queuing a tasklet, it will place it into the bulk queue. And during dequeuing, we accept only one of these in a full round. This means that the first one will be accepted, will not prevent other lower priority tasks from running, but if a new one arrives, then the queue stops here and goes back to the polling. This will allow to collect more important updates for other tasks that will be batched before the next call of a heavy task. Preliminary tests consisting in placing this flag on the SSL handshake tasklet show that response times under SSL stress fell from 14 ms before the patch to 3.0 ms with the patch, and even 1.8 ms if tune.sched.low-latency is set to "on".
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)