Terminating due to java.lang.OutOfMemoryError: Java heap space

[air3ijai]

Describe the bug
Migration failed due to the out of memory

To Reproduce

1. Start to sync RDB (~ 21GB) to remote Redis

   rmt -s /data/dump.rdb -m redis://redis-7:6379
   

2. Get the error

   \[ 18.4GB| 91.9MB/s]Terminating due to java.lang.OutOfMemoryError: Java heap space
   

3. Information about free memory before and after OOM

   free -g
                 total        used        free      shared  buff/cache   available
   Mem:             62           9          26           0          26          51
   Swap:             0           0           0
   
   free -g
                 total        used        free      shared  buff/cache   available
   Mem:             62           3          32           0          26          58
   Swap:             0           0           0
   

Expected behavior
If application require more memory, we should be able to specify it

Version(run rct --version or rct -V and paste the information):

redis rdb cli: v0.9.1 (1f96c81dd4935e2c7fc39f6bb227beede0c8ede4: 2022-05-01T08:10:04+0000)
home: /data/redis-rdb-cli/bin/..
java version: 11.0.17, vendor: Ubuntu
java home: /usr/lib/jvm/java-11-openjdk-amd64
default locale: en, platform encoding: UTF-8
os name: Linux, version: 5.11.20-300.fc34.x86_64, arch: amd64

Additional context
We have tried to set export JAVA_OPTS="-Xmx30G", but nothing changed.

[leonchen83]

I think there is a big key in this rdb. so you should use following method to migrate rdb to target redis
step1 rct -f resp -s /data/dump.rdb -o /data/dump.aof -r change rdb to aof file
step2 cat /data/dump.aof | /path/to/redis/src/redis-cli -p 6379 --pipe

[leonchen83]

This tool designed for small keys(less than 512mb) migration.
so if you have big key in rdb. you have two ways to handle this.
first way
step1: use rdt generate an rdb without big keys(use --key option filter big keys). after that use rmt migrate filtered rdb to target.
step2: change that big key to aof file. rct -f resp -s /path/to/dump.rdb --key ${big_key} -o /path/to/bigkey.aof
step3: migrate that bigkey aof file alone. cat /data/bigkey.aof | /path/to/redis/src/redis-cli -p 6379 --pipe

second way
step1 rct -f resp -s /data/dump.rdb -o /data/dump.aof -r change rdb to aof file
step2 cat /data/dump.aof | /path/to/redis/src/redis-cli -p 6379 --pipe

[leonchen83]

Tips: Avoiding big keys is one of the best practices for redis

[air3ijai]

Using your comment

export JAVA_TOOL_OPTIONS="-Xms4g -Xmx4g"

/opt/redis-rdb-cli/bin/rmt \
  -s /data/dump.rdb \
  -m redis://localhost:6379?authPassword=password

Picked up JAVA_TOOL_OPTIONS: -Xms4g -Xmx4g
/[ 20.5GB| 95.0MB/s]

# Source
db0:keys=1994658,expires=0,avg_ttl=0

# Destination
db0:keys=1994658,expires=0,avg_ttl=0

So, 4gb was enough in our case to work correctly (still OOM with 2gb), and an option proto-max-bulk-len 1024mb on Redis side to increase import limit.

Thank you for your support!

[leonchen83]

So, 4gb was enough in our case to work correctly (still OOM with 2gb), and an option proto-max-bulk-len 1024mb on Redis side to increase import limit.

Thread count affect OOM
by default redis-rdb-cli use 4 threads to migrate data to target.
so you need set Xms, Xmx at least 4*{max key size} memory

[air3ijai]

Let's try to tune this. Based on our max key size it would be: 792.1MB * 4 = 3168.4MB

I did some tests with 3000m - 1500m, with a 100m step - just a single run. And noted the following

1600m - 3 x Ok in a row

\[ 20.4GB| 96.5MB/s]
/[ 20.5GB| 94.8MB/s]
/[ 20.4GB| 94.1MB/s]

1500m - 3 x OOM in a row

\[  7.2GB| 99.4MB/s]Terminating due to java.lang.OutOfMemoryError: Java heap space
/[  7.4GB| 97.0MB/s]Terminating due to java.lang.OutOfMemoryError: Java heap space
\[  7.2GB|133.1MB/s]Terminating due to java.lang.OutOfMemoryError: Java heap space

But with all that, my precedent experiment

Picked up JAVA_TOOL_OPTIONS: -Xms2g -Xmx2g
\[ 11.2GB| 92.3MB/s]Terminating due to java.lang.OutOfMemoryError: Java heap space

And without any memory specification in an initial comment

\[ 18.4GB| 91.9MB/s]Terminating due to java.lang.OutOfMemoryError: Java heap space

On my node MaxHeapSize = 8417968128 ~ 8GB and it is higher than what I set and the limit I've found during the latest tests.

java -XX:+PrintFlagsFinal -version | grep HeapSize
    uintx ErgoHeapSizeLimit                         = 0                                   {product}
    uintx HeapSizePerGCThread                       = 87241520                            {product}
    uintx InitialHeapSize                          := 526385152                           {product}
    uintx LargePageHeapSizeThreshold                = 134217728                           {product}
    uintx MaxHeapSize                              := 8417968128                          {product}
openjdk version "1.8.0_352"
OpenJDK Runtime Environment (build 1.8.0_352-8u352-ga-1~22.04-b08)
OpenJDK 64-Bit Server VM (build 25.352-b08, mixed mode)

[leonchen83]

Hi
About MaxHeapSize = 8417968128 please refer to this doc

792.1MB * 4 = 3168.4MB means 4 threads at the same time to migrate 4 big keys. you can take it as MaxHeapSize. But it will be less than this value in actual use.