From 634b0966106b48cbf2c51fadafa7482a5af77e64 Mon Sep 17 00:00:00 2001 From: antirez <antirez@gmail.com> Date: Wed, 11 Jan 2017 17:24:49 +0100 Subject: [PATCH] Defrag: document the feature in redis.conf. --- redis.conf | 55 ++++++++++++++++++++++++++++++++++++++++++++++++------ 1 file changed, 49 insertions(+), 6 deletions(-) diff --git a/redis.conf b/redis.conf index 18ba9fb3f..c54dba392 100644 --- a/redis.conf +++ b/redis.conf @@ -1230,21 +1230,64 @@ aof-rewrite-incremental-fsync yes # lfu-decay-time 1 ########################### ACTIVE DEFRAGMENTATION ####################### +# +# WARNING THIS FEATURE IS EXPERIMENTAL. However it was stress tested +# even in production and manually tested by multiple engineers for some +# time. +# +# What is active defragmentation? +# ------------------------------- +# +# Active (online) defragmentation allows a Redis server to compact the +# spaces left between small allocations and deallocations of data in memory, +# thus allowing to reclaim back memory. +# +# Fragmentation is a natural process that happens with every allocator (but +# less so with Jemalloc, fortunately) and certain workloads. Normally a server +# restart is needed in order to lower the fragmentation, or at least to flush +# away all the data and create it again. However thanks to this feature +# implemented by Oran Agra for Redis 4.0 this process can happen at runtime +# in an "hot" way, while the server is running. +# +# Basically when the fragmentation is over a certain level (see the +# configuration options below) Redis will start to create new copies of the +# values in contiguous memory regions by exploiting certain specific Jemalloc +# features (in order to understand if an allocation is causing fragmentation +# and to allocate it in a better place), and at the same time, will release the +# old copies of the data. This process, repeated incrementally for all the keys +# will cause the fragmentation to drop back to normal values. +# +# Important things to understand: +# +# 1. This feature is disabled by default, and only works if you compiled Redis +# to use the copy of Jemalloc we ship with the source code of Redis. +# This is the default with Linux builds. +# +# 2. You never need to enable this feature if you don't have fragmentation +# issues. +# +# 3. Once you experience fragmentation, you can enable this feature when +# needed with the command "CONFIG SET activedefrag yes". +# +# The configuration parameters are able to fine tune the behavior of the +# defragmentation process. If you are not sure about what they mean it is +# a good idea to leave the defaults untouched. -# enabled active defragmentation +# Enabled active defragmentation # activedefrag yes -# minimum amount of fragmentation waste to start active defrag +# Minimum amount of fragmentation waste to start active defrag # active-defrag-ignore-bytes 100mb -# minimum percentage of fragmentation to start active defrag +# Minimum percentage of fragmentation to start active defrag # active-defrag-threshold-lower 10 -# maximum percentage of fragmentation at which we use maximum effort +# Maximum percentage of fragmentation at which we use maximum effort # active-defrag-threshold-upper 100 -# minimal effort for defrag in CPU percentage +# Minimal effort for defrag in CPU percentage # active-defrag-cycle-min 25 -# maximal effort for defrag in CPU percentage +# Maximal effort for defrag in CPU percentage # active-defrag-cycle-max 75 +