which performance metric gets improved by bootstrap.mlockall:true( disables swapping) in config file?
The RAM waste metric
I recommend against setting this myself.
It locks the initial JVM heap in memory, which means that it will always sit resident in physical RAM. This can help reduce garbage collection pause times (a terrible thing that can happen is the heap is swapped out, then a full garbage collection is initiated and the JVM must touch every page of the heap leading to massive disk I/O). There are other ways to reduce the propensity to swap, and this is but one of them.