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When allocating from dirty extents (which we always prefer if available), large active extents can get split even if the new allocation is much smaller, in which case the introduced fragmentation causes high long term damage. This new option controls the threshold to reuse and split an existing active extent. We avoid using a large extent for much smaller sizes, in order to reduce fragmentation. In some workload, adding the threshold improves virtual memory usage by >10x.
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: http://jemalloc.net/