About Memory Usage

In certain scenarios would be ideal to estimate how much memory Fluent Bit could be using, this is very useful for containerized environments where memory limits are a must.

In order to estimate we will assume that the input plugins have set the Mem_Buf_Limit option (you can learn more about it in the Backpressure section).

Estimating

Input plugins append data independently, so in order to do an estimation a limit should be imposed through the Mem_Buf_Limit option. If the limit was set to 10MB we need to estimate that in the worse case, the output plugin likely could use 20MB.

Fluent Bit has an internal binary representation for the data being processed, but when this data reach an output plugin, this one will likely create their own representation in a new memory buffer for processing. The best example are the InfluxDB and Elasticsearch output plugins, both needs to convert the binary repesentation to their respective-custom JSON formats before to talk to their backend servers.

So, if we impose a limit of 10MB for the input plugins and considering the worse case scenario of the output plugin consuming 20MB extra, as a minimum we need (30MB x 1.2) = 36MB.

Glibc and Memory Fragmentation

Is well known that in intensive environments where memory allocations happens in the order of magnitude, the default memory allocator provided by Glibc could lead to a high fragmentation, reporting a high memory usage by the service.

It's strongly suggested that in any production environment, Fluent Bit be build with Jemalloc enabled (-DFLB_JEMALLOC=On), which is an alternative memory allocator that have better strategies to reduce fragmentation among others for better performance.

You can check if Fluent Bit have been built with Jemalloc using the following command:

$ bin/fluent-bit -h|grep JEMALLOC

The output should looks like:

Build Flags =  JSMN_PARENT_LINKS JSMN_STRICT FLB_HAVE_TLS FLB_HAVE_SQLDB
FLB_HAVE_TRACE FLB_HAVE_FLUSH_LIBCO FLB_HAVE_VALGRIND FLB_HAVE_FORK
FLB_HAVE_PROXY_GO FLB_HAVE_JEMALLOC JEMALLOC_MANGLE FLB_HAVE_REGEX
FLB_HAVE_C_TLS FLB_HAVE_SETJMP FLB_HAVE_ACCEPT4 FLB_HAVE_INOTIFY

If the FLB_JEMALLOC option is listed in Build Flags, everything will be fine.

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