I have a cluster with 8 webserver nodes and a redis cache instance running on a separate server.
During high load, the throughput on the redis server goes through the roof, approximately 35-40 MB/s (megabytes). I inspected the redis commands with MONITOR on the redis instance and I discovered that there were a disproportionate amount of EVALSHA calls against the redis cache with the config tags.
This means, the web servers were sending data to the redis cache at a pace of 14 MB/s.
If I disable the config cache in the backend, the received bytes counter on the redis server came all the way down to 1 MB/s, which is reasonable.
I suspect that the config cache is getting invalidated somehow.
Have any of you seen anything like this before?
These are my config settings:
<cache> <backend>Cm_Cache_Backend_Redis</backend> <backend_options> <server>10.254.253.1</server> <!-- or absolute path to unix socket --> <port>6380</port> <persistent></persistent> <!-- Specify unique string to enable persistent connections. E.g.: sess-db0; bugs with phpredis and php-fpm are known: https://github.com/nicolasff/phpredis/issues/70 --> <database>0</database> <!-- Redis database number; protection against accidental data loss is improved by not sharing databases --> <password></password> <!-- Specify if your Redis server requires authentication --> <force_standalone>0</force_standalone> <!-- 0 for phpredis, 1 for standalone PHP --> <connect_retries>2</connect_retries> <!-- Reduces errors due to random connection failures; a value of 1 will not retry after the first failure --> <read_timeout>10</read_timeout> <!-- Set read timeout duration; phpredis does not currently support setting read timeouts --> <automatic_cleaning_factor>0</automatic_cleaning_factor> <!-- Disabled by default --> <compress_data>1</compress_data> <!-- 0-9 for compression level, recommended: 0 or 1 --> <compress_tags>1</compress_tags> <!-- 0-9 for compression level, recommended: 0 or 1 --> <compress_threshold>2048</compress_threshold> <!-- Strings below this size will not be compressed --> <compression_lib>snappy</compression_lib> <!-- Supports gzip, lzf, lz4 (as l4z) and snappy --> <use_lua>1</use_lua> <!-- Set to 1 if Lua scripts should be used for some operations --> </backend_options> </cache>