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copied!<p>Generally, you will have less fragmentation with jemalloc or tcmalloc, than with libc malloc. This is due to 4 factors:</p> <ul> <li><p>more granular allocation classes for jemalloc and tcmalloc. It reduces internal fragmentation, especially when Redis has to allocate a lot of very small objects.</p></li> <li><p>better algorithms and data structures to prevent external fragmentation (especially for jemalloc). Obviously, the gain depends on your long term memory allocation patterns.</p></li> <li><p>support of "malloc size". Some allocators offer an API to return the size of allocated memory. With glibc (Linux), malloc does not have this capability, so it is emulated by explicitly adding an extra prefix to each allocated memory block. It increases internal fragmentation. With jemalloc and tcmalloc (or with the BSD libc malloc), there is no such overhead.</p></li> <li><p>jemalloc (and tcmalloc with some setting changes) can be more aggressive than glibc to release memory to the OS - but again, it depends on the allocation patterns.</p></li> </ul> <p>Now, how is it possible to get inconsistent values for mem_fragmentation_ratio?</p> <p>As stated in <a href="http://redis.io/commands/info" rel="nofollow">the INFO documentation</a>, the mem_fragmentation_ratio value is calculated as the ratio between memory resident set size of the process (RSS, measured by the OS), and the total number of bytes allocated by Redis using the allocator.</p> <p>Now, if more memory is allocated with libc (compared to jemalloc,tcmalloc), or if more memory is used by some other processes on your system during your benchmarks, Redis memory may be swapped out by the OS. It will reduce the RSS (since a part of Redis memory will not be in main memory anymore). The resulting fragmentation ratio will be less than 1.</p> <p>In other words, this ratio is only relevant if you are sure Redis memory has not been swapped out by the OS (if it is not the case, you will have performance issues anyway).</p>

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