revlog: add option to mmap revlog index Following on from Jun Wu's patch last October[1], we have found that using mmap for the revlog index in repos with large revlogs gives a noticable performance improvment (~110ms on each hg invocation), particularly for commands that don't touch the index very much. This changeset adds this as an option, activated by a new experimental config option so that it can be enabled on a per-repo basis. The configuration option specifies an index size threshold at which Mercurial will switch to using mmap to access the index. If the configuration option is not specified, the default remains to load the full file, which seems to be the best option for smaller repos. Some initial performance numbers for average of 5 invocations of `hg log -l 5` for different cache states: | Repo: | HG | FB | |---|---|---| | Index size: | 2.3MB | much bigger | | read (warm): | 237ms | 432ms | | mmap (warm): | 227ms | 321ms | | | (-3%) | (-26%) | | read (cold): | 397ms | 696ms | | mmap (cold): | 410ms | 888ms | | | (+3%) | (+28%) | [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-October/088737.html Test Plan: `hg log --config experimental.mmapindex=true` Differential Revision: https://phab.mercurial-scm.org/D477