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+This directory contains all Redis dependencies, except for the libc that
+should be provided by the operating system.
+
+* **Jemalloc** is our memory allocator, used as replacement for libc malloc on Linux by default. It has good performances and excellent fragmentation behavior. This component is upgraded from time to time.
+* **hiredis** is the official C client library for Redis. It is used by redis-cli, redis-benchmark and Redis Sentinel. It is part of the Redis official ecosystem but is developed externally from the Redis repository, so we just upgrade it as needed.
+* **linenoise** is a readline replacement. It is developed by the same authors of Redis but is managed as a separated project and updated as needed.
+* **lua** is Lua 5.1 with minor changes for security and additional libraries.
+* **hdr_histogram** Used for per-command latency tracking histograms.
+
+How to upgrade the above dependencies
+===
+
+Jemalloc
+---
+
+Jemalloc is modified with changes that allow us to implement the Redis
+active defragmentation logic. However this feature of Redis is not mandatory
+and Redis is able to understand if the Jemalloc version it is compiled
+against supports such Redis-specific modifications. So in theory, if you
+are not interested in the active defragmentation, you can replace Jemalloc
+just following these steps:
+
+1. Remove the jemalloc directory.
+2. Substitute it with the new jemalloc source tree.
+3. Edit the Makefile located in the same directory as the README you are
+   reading, and change the --with-version in the Jemalloc configure script
+   options with the version you are using. This is required because otherwise
+   Jemalloc configuration script is broken and will not work nested in another
+   git repository.
+
+However note that we change Jemalloc settings via the `configure` script of Jemalloc using the `--with-lg-quantum` option, setting it to the value of 3 instead of 4. This provides us with more size classes that better suit the Redis data structures, in order to gain memory efficiency.
+
+If you want to upgrade Jemalloc while also providing support for
+active defragmentation, in addition to the above steps you need to perform
+the following additional steps:
+
+5. In Jemalloc tree, file `include/jemalloc/jemalloc_macros.h.in`, make sure
+   to add `#define JEMALLOC_FRAG_HINT`.
+6. Implement the function `je_get_defrag_hint()` inside `src/jemalloc.c`. You
+   can see how it is implemented in the current Jemalloc source tree shipped
+   with Redis, and rewrite it according to the new Jemalloc internals, if they
+   changed, otherwise you could just copy the old implementation if you are
+   upgrading just to a similar version of Jemalloc.
+
+#### Updating/upgrading jemalloc
+
+The jemalloc directory is pulled as a subtree from the upstream jemalloc github repo. To update it you should run from the project root:
+
+1. `git subtree pull --prefix deps/jemalloc https://github.com/jemalloc/jemalloc.git <version-tag> --squash`<br>
+This should hopefully merge the local changes into the new version.
+2. In case any conflicts arise (due to our changes) you'll need to resolve them and commit.
+3. Reconfigure jemalloc:<br>
+```sh
+rm deps/jemalloc/VERSION deps/jemalloc/configure
+cd deps/jemalloc
+./autogen.sh --with-version=<version-tag>-0-g0
+```
+4. Update jemalloc's version in `deps/Makefile`: search for "`--with-version=<old-version-tag>-0-g0`" and update it accordingly.
+5. Commit the changes (VERSION,configure,Makefile).
+
+Hiredis
+---
+
+Hiredis is used by Sentinel, `redis-cli` and `redis-benchmark`. Like Redis, uses the SDS string library, but not necessarily the same version. In order to avoid conflicts, this version has all SDS identifiers prefixed by `hi`.
+
+1. `git subtree pull --prefix deps/hiredis https://github.com/redis/hiredis.git <version-tag> --squash`<br>
+This should hopefully merge the local changes into the new version.
+2. Conflicts will arise (due to our changes) you'll need to resolve them and commit.
+
+Linenoise
+---
+
+Linenoise is rarely upgraded as needed. The upgrade process is trivial since
+Redis uses a non modified version of linenoise, so to upgrade just do the
+following:
+
+1. Remove the linenoise directory.
+2. Substitute it with the new linenoise source tree.
+
+Lua
+---
+
+We use Lua 5.1 and no upgrade is planned currently, since we don't want to break
+Lua scripts for new Lua features: in the context of Redis Lua scripts the
+capabilities of 5.1 are usually more than enough, the release is rock solid,
+and we definitely don't want to break old scripts.
+
+So upgrading of Lua is up to the Redis project maintainers and should be a
+manual procedure performed by taking a diff between the different versions.
+
+Currently we have at least the following differences between official Lua 5.1
+and our version:
+
+1. Makefile is modified to allow a different compiler than GCC.
+2. We have the implementation source code, and directly link to the following external libraries: `lua_cjson.o`, `lua_struct.o`, `lua_cmsgpack.o` and `lua_bit.o`.
+3. There is a security fix in `ldo.c`, line 498: The check for `LUA_SIGNATURE[0]` is removed in order to avoid direct bytecode execution.
+
+Hdr_Histogram
+---
+
+Updated source can be found here: https://github.com/HdrHistogram/HdrHistogram_c
+We use a customized version based on master branch commit e4448cf6d1cd08fff519812d3b1e58bd5a94ac42.
+1. Compare all changes under /hdr_histogram directory to upstream master commit e4448cf6d1cd08fff519812d3b1e58bd5a94ac42
+2. Copy updated files from newer version onto files in /hdr_histogram.
+3. Apply the changes from 1 above to the updated files.
+