2018 Concurrency Training Note

-
2018 Concurrency Training Node
  1. Hotspot Compiler Brief
  2. Java Synchronization: Keyword Synchronize
  3. Java Synchronization: The Lock
  4. Java Synchronization: The Reader
  5. Volatile 

Comments

Post a Comment

Popular posts from this blog

Java Object Size and Overhead

-
Let's look at the details few details of object header and calculate the memory size an object occupies inside JVM Heap. Each Object contains following information. • The Object Header. • The memory for primitive types. 
 • The memory for reference types. 
 • Offset / alignment -  in fact, these are a few unused bytes that are placed after the data object itself. This is done in order that an address in memory was always a multiple of machine word, to speed up the memory read + reduce the number of bits for a pointer to an object. It is also worth noting that in java a size of any object is multiple of 8 bytes! •   Object Header :             In case of 32-bit system, the header size is 8 bytes, in the case of 64-bit system, respectively is 16 bytes. It contains following information. 1   Hash Code - 2   Garbage Collection Information  -  each java object contains the i...
Read more

Java Synchronization: Keyword “synchronized”

-
Image
What does it mean by “synchronized”? 1) Mutual Exclusive Only one thread can enter synchronized block. 2) Guarantee no reordering Instruction inside synchronized block can reordered between themselves. E.g. 4, 6, 5 Instruction outside synchronized block can reorder between themselves. E.g. 2, 3 ,1. Instruction in synchronized block cannot be reordered to mix with outside block (1, 5, 2, 3) Instruction outside synchronized block cannot be reordered to mix with in block (7, 5, 8, 9) 3) Guarantee to get value from memory all the time, not from cache 4) Volatile variable observe next page (need validation) After all instruction in a synchronized block get executed. All change values are pushed into memory from local cache of the thread. Also as we know data move does not work on single variable, therefore the cache line which contains the changed value got pushed to memory too. Cost of the lock in "synchronized" 1) Memory Synchronization 2) Mutual Exclusi...
Read more

Java Hotspot Compiler Brief

-
Image
1)       Other name: JIT Compiler, Dynamic Compiler 2)       How a java program runs in JVM? -           javac: translate java code to byte code -           JVM Interpreter runs instruction by instruction -           In JVM, there is a lightweight profiler detects common block of instructions running (for example loop) and try to optimize the execution ON-FLY, immediate speed up. For example: we run 5*5*5 for 1000 time, it gets faster after some runs. Why do I care?       You may run bench-marking test. The result will come out funny and you will wonder why.
Read more