问题
If I use the Runtime class (freeMemory(), totalMemory(), and gc()), then it gives me memory above MB (i.e. 1,000,000 bytes).
But if I run the same code on any online compiler, then they show memory used in KB (i.e. 1000 bytes). This is a huge difference.
This means Runtime does not show the actual memory used by the program.
I need to calculate actual memory used by the program. What is the way these online compilers use to calculate memory used by the program?
回答1:
First calculate the memory used before your code execution i.e. first line of your code.
long beforeUsedMem=Runtime.getRuntime().totalMemory()-Runtime.getRuntime().freeMemory();
Calculate the memory used after your code execution:-
long afterUsedMem=Runtime.getRuntime().totalMemory()-Runtime.getRuntime().freeMemory();
Then you can do:
long actualMemUsed=afterUsedMem-beforeUsedMem;
It will give you actual memory utilized by your program.
For further memory analysis your best bet is any profiler tool like jvisualvm.
- Do remember it, that Runtime.getRuntime().totalMemory(), this memory is total memory available to your JVM.
- java -Xms512m -Xmx1024m , it means that total memory of your program will start with 512MB, which may lazily loaded to max of 1024MB.
- So if you are running same program, on different JVMs, Runtime.getRuntime().totalMemory() might give you different results.
回答2:
You can use top command in ubuntu to check % CPU use or % memory use while running the java programme.
top command will give you these many information.
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
just type
top and hit enter
in your terminal & check java in command section.
来源：https://stackoverflow.com/questions/37916136/how-to-calculate-memory-usage-of-a-java-program