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.

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.

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
   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:
   

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 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
   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 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.

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.

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.