Redis

Check the running version from the command line:

redis-cli -p 6370 info | grep -i redis_version

Check that the Redis command-line interface (CLI) connects to the Redis server.

redis-cli -p 6370 ping
PONG

Check max memory

By default redis is not set with a memory limit, so it's best to set one
grep ^maxmemory /etc/redis/redis.conf 
maxmemory 536870912

Set with 0 by default which means unlimited
redis-cli -p 6370
127.0.0.1:6370> config get maxmemory
1) "maxmemory"
2) "0"

Emergency increase Redis maxmemory
redis-cli -p 6370
127.0.0.1:6370> CONFIG SET maxmemory 8GB
OK

Troubleshooting Memory

'maxmemory_human' must be larger than 'used_memory_peak_human' in case of using the 'volatile-lru' policy.
redis-cli -p 6370 info | grep memory | grep 'human\|policy'

When memory is fully used, the number of 'evicted_keys' will be large and growing fast.  The maxmemory should be increased in this case even when it's using 'allkeys-lru' to prevent high amount of cache miss.
redis-cli -p 6370 info | grep evicted_keys

More info here
https://redis.io/topics/lru-cache

Logging errors that show that the maxmemory limit is being hit.
cron.log: OOM command not allowed when used memory > 'maxmemory'
error.log: PHP Fatal error:  Uncaught Exception: Warning: session_write_close(): Failed to write session data using user defined save handler.

Troubleshooting Throughput

# if input or output is greater than 100 mbps then there can be an application issue
redis-cli -p 6370 info stats | grep instantaneous

Troubleshooting Latency

# Latency monitoring is enabled by setting a threshold measured in milliseconds
redis-cli -p 6370
CONFIG SET latency-monitor-threshold 100

# this command will generate a human-readable report of the logged latency events and recommendations on how to resolve them
latency doctor

# More info here
https://redis.io/topics/latency-monitor

Check keys expiration

$ redis-cli -p 6370
KEYS *session*
TTL name_of_session_key