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The ring's offset currently contains a perpetually growing custor which is the number of bytes written from the start. It's used by readers to know where to (re)start reading from. It was made absolute because both the head and the tail can change during writes and we needed a fixed position to know where the reader was attached. But this is complicated, error-prone, and limits the ability to reduce the lock's coverage. In fact what is needed is to know where the reader is currently waiting, if at all. And this location is exactly where it stored its count, so the absolute position in the buffer (the seek offset from the first storage byte) does represent exactly this, as it doesn't move (we don't realign the buffer), and is stable regardless of how head/tail changes with writes. This patch modifies this so that the application code now uses this representation instead. The most noticeable change is the initialization, where we've kept ~0 as a marker to go to the end, and it's now set to the tail offset instead of trying to resolve the current write offset against the current ring's position. The offset was also used at the end of the consuming loop, to detect if a new write had happened between the lock being released and taken again, so as to wake the consumer(s) up again. For this we used to take a copy of the ring->ofs before unlocking and comparing with the new value read in the next lock. Since it's not possible to write past the current reader's location, there's no risk of complete rollover, so it's sufficient to check if the tail has changed. Note that the change also has an impact on the haring consumer which needs to adapt as well. But that's good in fact because it will rely on one less variable, and will use offsets relative to the buffer's head, and the change remains backward-compatible. |
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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)