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Some pages (e.g., hugetlb pages) cannot be purged, and should be prioritized for reuse. A custom extent_alloc hook signals this by OR'ing EXTENT_ALLOC_FLAG_PINNED into the low bits of the returned pointer; jemalloc strips the flag bits and caches pinned extents in a dedicated ecache_pinned, separate from the dirty/muzzy decay pipeline. Pinned extents do not coalesce eagerly, except for ones larger than SC_LARGE_MINCLASS. A prefer-small policy reuses the smallest fitting pinned extent, to avoid unnecessary split/fragmentation.
jemalloc is a general purpose malloc(3) implementation that emphasizes fragmentation avoidance and scalable concurrency support. jemalloc first came into use as the FreeBSD libc allocator in 2005, and since then it has found its way into numerous applications that rely on its predictable behavior. In 2010 jemalloc development efforts broadened to include developer support features such as heap profiling and extensive monitoring/tuning hooks. Modern jemalloc releases continue to be integrated back into FreeBSD, and therefore versatility remains critical. Ongoing development efforts trend toward making jemalloc among the best allocators for a broad range of demanding applications, and eliminating/mitigating weaknesses that have practical repercussions for real world applications. The COPYING file contains copyright and licensing information. The INSTALL file contains information on how to configure, build, and install jemalloc. The ChangeLog file contains a brief summary of changes for each release. URL: https://jemalloc.net/