mi-malloc: Environment Options

| mi-malloc 1.6 |

Environment Options

You can set further options either programmatically (using `mi_option_set`), or via environment variables.

- `MIMALLOC_SHOW_STATS=1`: show statistics when the program terminates.
- `MIMALLOC_VERBOSE=1`: show verbose messages.
- `MIMALLOC_SHOW_ERRORS=1`: show error and warning messages.
- `MIMALLOC_PAGE_RESET=0`: by default, mimalloc will reset (or purge) OS pages when not in use to signal to the OS that the underlying physical memory can be reused. This can reduce memory fragmentation in long running (server) programs. By setting it to `0` no such page resets will be done which can improve performance for programs that are not long running. As an alternative, the `MIMALLOC_RESET_DELAY=`<msecs> can be set higher (100ms by default) to make the page reset occur less frequently instead of turning it off completely.
- `MIMALLOC_LARGE_OS_PAGES=1`: use large OS pages (2MiB) when available; for some workloads this can significantly improve performance. Use `MIMALLOC_VERBOSE` to check if the large OS pages are enabled – usually one needs to explicitly allow large OS pages (as on Windows and Linux). However, sometimes the OS is very slow to reserve contiguous physical memory for large OS pages so use with care on systems that can have fragmented memory (for that reason, we generally recommend to use `MIMALLOC_RESERVE_HUGE_OS_PAGES` instead when possible).
- `MIMALLOC_RESERVE_HUGE_OS_PAGES=N`: where N is the number of 1GiB huge OS pages. This reserves the huge pages at startup and sometimes this can give a large (latency) performance improvement on big workloads. Usually it is better to not use `MIMALLOC_LARGE_OS_PAGES` in combination with this setting. Just like large OS pages, use with care as reserving contiguous physical memory can take a long time when memory is fragmented (but reserving the huge pages is done at startup only once). Note that we usually need to explicitly enable huge OS pages (as on Windows and Linux)). With huge OS pages, it may be beneficial to set the setting `MIMALLOC_EAGER_COMMIT_DELAY=N` (`N` is 1 by default) to delay the initial `N` segments (of 4MiB) of a thread to not allocate in the huge OS pages; this prevents threads that are short lived and allocate just a little to take up space in the huge OS page area (which cannot be reset).

Use caution when using `fork` in combination with either large or huge OS pages: on a fork, the OS uses copy-on-write for all pages in the original process including the huge OS pages. When any memory is now written in that area, the OS will copy the entire 1GiB huge page (or 2MiB large page) which can cause the memory usage to grow in big increments.