Most efficient initial capacity size for StringBuilder?

Question

I\'m writing lots of stuff to log in bursts, and optimizing the data path. I build the log text with StringBuilder. What would be the most efficient initial cap

Answer 1

Well, I ended up testing this briefly myself, and then testing some more after comments, to get this edited answer.

Using JDK 1.7.0_07 and test app reporting VM name "Java HotSpot(TM) 64-Bit Server VM", granularity of StringBuilder memory usage is 4 chars, increasing at even 4 chars.

Answer: any multiple of 4 is equally good capacity for StringBuilder from memory allocation point of view, at least on this 64-bit JVM.

Tested by creating 1000000 StringBuilder objects with different initial capacities, in different test program executions (to have same initial heap state), and printing out ManagementFactory.getMemoryMXBean().getHeapMemoryUsage().getUsed() before and after.

Printing out heap sizes also confirmed, that amount actually allocated from heap for each StringBuilder's buffer is an even multiple of 8 bytes, as expected since Java char is 2 bytes long. In other words, allocating 1000000 instances with initial capacity 1..4 takes about 8 megabytes less memory (8 bytes per instace), than allocating same number of isntances with initial capacity 5...8.