# mi-malloc: Environment Options

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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.

Advanced options:

- `MIMALLOC_ARENA_EAGER_COMMIT=2`: turns on eager commit for the large arenas (usually 1GiB) from which mimalloc allocates segments and pages. Set this to 2 (default) to only enable this on overcommit systems (e.g. Linux). Set this to 1 to enable explicitly on other systems as well (like Windows or macOS) which may improve performance (as the whole arena is committed at once). Note that eager commit only increases the commit but not the actual the peak resident set (rss) so it is generally ok to enable this.
- `MIMALLOC_PURGE_DELAY=N`: the delay in `N` milli-seconds (by default `10`) after which mimalloc will purge OS pages that are not in use. This signals to the OS that the underlying physical memory can be reused which can reduce memory fragmentation especially in long running (server) programs. Setting `N` to `0` purges immediately when a page becomes unused which can improve memory usage but also decreases performance. Setting `N` to a higher value like `100` can improve performance (sometimes by a lot) at the cost of potentially using more memory at times. Setting it to `-1` disables purging completely.
- `MIMALLOC_PURGE_DECOMMITS=1`: By default "purging" memory means unused memory is decommitted (`MEM_DECOMMIT` on Windows, `MADV_DONTNEED` (which decresease rss immediately) on `mmap` systems). Set this to 0 to instead "reset" unused memory on a purge (`MEM_RESET` on Windows, generally `MADV_FREE` (which does not decrease rss immediately) on `mmap` systems). Mimalloc generally does not "free" OS memory but only "purges" OS memory, in other words, it tries to keep virtual address ranges and decommits within those ranges (to make the underlying physical memory available to other processes).

Further options for large workloads and services:

- `MIMALLOC_USE_NUMA_NODES=N`: pretend there are at most `N` NUMA nodes. If not set, the actual NUMA nodes are detected at runtime. Setting `N` to 1 may avoid problems in some virtual environments. Also, setting it to a lower number than the actual NUMA nodes is fine and will only cause threads to potentially allocate more memory across actual NUMA nodes (but this can happen in any case as NUMA local allocation is always a best effort but not guaranteed).
- `MIMALLOC_ALLOW_LARGE_OS_PAGES=1`: use large OS pages (2 or 4MiB) when available; for some workloads this can significantly improve performance. When this option is disabled, it also disables transparent huge pages (THP) for the process (on Linux and Android). Use `MIMALLOC_VERBOSE` to check if the large OS pages are enabled – usually one needs to explicitly give permissions for 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 whenever 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_ALLOW_LARGE_OS_PAGES=1` 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 give permission for 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 purged as huge OS pages are pinned to physical memory). The huge pages are usually allocated evenly among NUMA nodes. We can use `MIMALLOC_RESERVE_HUGE_OS_PAGES_AT=N` where `N` is the numa node (starting at 0) to allocate all the huge pages at a specific numa node instead.

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 large increments.