Volatile Vs Atomic

Question

I read somewhere below line      Java volatile keyword doesn t means atomic  its common misconception   that after declaring volatile     operation will be atomic  to make   the operation atomic you still need to ensure exclusive access using   synchronized method or block in Java    So what will happen if two threads attack a volatile primitive variable at same time    Does this mean that whosoever takes lock on it  that will be setting its value first  And if in meantime  some other thread comes up and read old value while first thread was changing its value  then doesn t new thread will read its old value   What is the difference between Atomic and volatile keyword

User · Answer

There are two important concepts in multithreading environment:

The volatile keyword eradicates visibility problems, but it does not deal with atomicity. volatile will prevent the compiler from reordering instructions which involve a write and a subsequent read of a volatile variable; e.g. k++. Here, k++ is not a single machine instruction, but three:

copy the value to a register;
increment the value;
place it back.

So, even if you declare a variable as volatile, this will not make this operation atomic; this means another thread can see a intermediate result which is a stale or unwanted value for the other thread.

On the other hand, AtomicInteger, AtomicReference are based on the Compare and swap instruction. CAS has three operands: a memory location V on which to operate, the expected old value A, and the new value B. CAS atomically updates V to the new value B, but only if the value in V matches the expected old value A; otherwise, it does nothing. In either case, it returns the value currently in V. The compareAndSet() methods of AtomicInteger and AtomicReference take advantage of this functionality, if it is supported by the underlying processor; if it is not, then the JVM implements it via spin lock.

User · Answer

As Trying as indicated  volatile deals only with visibility   Consider this snippet in a concurrent environment   boolean isStopped   false                   while   isStopped               do some kind of work         The idea here is that some thread could change the value of isStopped from false to true in order to indicate to the subsequent loop that it is time to stop looping   Intuitively  there is no problem   Logically if another thread makes isStopped equal to true  then the loop must terminate   The reality is that the loop will likely never terminate even if another thread makes isStopped equal to true   The reason for this is not intuitive  but consider that modern processors have multiple cores and that each core has multiple registers and multiple levels of cache memory that are not accessible to other processors   In other words  values that are cached in one processor s local memory are not visisble to threads executing on a different processor   Herein lies one of the central problems with concurrency  visibility   The Java Memory Model makes no guarantees whatsoever about when changes that are made to a variable in one thread may become visible to other threads   In order to guarantee that updates are visisble as soon as they are made  you must synchronize   The volatile keyword is a weak form of synchronization   While it does nothing for mutual exclusion or atomicity  it does provide a guarantee that changes made to a variable in one thread will become visible to other threads as soon as it is made   Because individual reads and writes to variables that are not 8-bytes are atomic in Java  declaring variables volatile provides an easy mechanism for providing visibility in situations where there are no other atomicity or mutual exclusion requirements

User · Answer

So what will happen if two threads attack a volatile primitive variable at same time    Usually each one can increment the value   However sometime  both will update the value at the same time and instead of incrementing by 2 total  both thread increment by 1 and only 1 is added      Does this mean that whosoever takes lock on it  that will be setting its value first      There is no lock   That is what synchronized is for      And in if meantime  some other thread comes up and read old value while first thread was changing its value  then doesn t new thread will read its old value    Yes      What is the difference between Atomic and volatile keyword    AtomicXxxx wraps a volatile so they are basically same  the difference is that it provides higher level operations such as CompareAndSwap which is used to implement increment   AtomicXxxx also supports lazySet   This is like a volatile set  but doesn t stall the pipeline waiting for the write to complete   It can mean that if you read a value you just write you might see the old value  but you shouldn t be doing that anyway   The difference is that setting a volatile takes about 5 ns  bit lazySet takes about 0 5 ns

User · Answer

Volatile and Atomic are two different concepts  Volatile ensures  that a certain  expected  memory  state is true across different threads  while Atomics ensure that operation on variables are performed atomically   Take the following example of two threads in Java   Thread A   value   1  done   true    Thread B   if  done    System out println value     Starting with value   0 and done   false the rule of threading tells us  that it is undefined whether or not Thread B will print value  Furthermore value is undefined at that point as well  To explain this you need to know a bit about Java memory management  which can be complex   in short  Threads may create local copies of variables  and the JVM can reorder code to optimize it  therefore there is no guarantee that the above code is run in exactly that order  Setting done to true and then setting value to 1 could be a possible outcome of the JIT optimizations   volatile only ensures  that at the moment of access of such a variable  the new value will be immediately visible to all other threads and the order of execution ensures  that the code is at the state you would expect it to be  So in case of the code above  defining done as volatile will ensure that whenever Thread B checks the variable  it is either false  or true  and if it is true  then value has been set to 1 as well   As a side-effect of volatile  the value of such a variable is set thread-wide atomically  at a very minor cost of execution speed   This is however only important on 32-bit systems that i E  use long  64-bit  variables  or similar   in most other cases setting reading a variable is atomic anyways  But there is an important difference between an atomic access and an atomic operation  Volatile only ensures that the access is atomically  while Atomics ensure that the operation is atomically   Take the following example   i   i   1    No matter how you define i  a different Thread reading the value just when the above line is executed might get i  or i   1  because the operation is not atomically  If the other thread sets i to a different value  in worst case i could be set back to whatever it was before by thread A  because it was just in the middle of calculating i   1 based on the old value  and then set i again to that old value   1  Explanation   Assume i   0 Thread A reads i  calculates i 1  which is 1 Thread B sets i to 1000 and returns Thread A now sets i to the result of the operation  which is i   1   Atomics like AtomicInteger ensure  that such operations happen atomically  So the above issue cannot happen  i would either be 1000 or 1001 once both threads are finished

User · Answer

The volatile keyword is used    to make non atomic 64-bit operations atomic  long and double   all other  primitive accesses are already guaranteed to be atomic   to make variable updates guaranteed to be seen by other threads   visibility effects  after writing to a volatile variable  all the variables that where visible before writing that variable become visible to another thread after reading the same volatile variable  happen-before ordering     The java util concurrent atomic   classes are  according to the java docs      A small toolkit of classes that support lock-free thread-safe   programming on single variables  In essence  the classes in this   package extend the notion of volatile values  fields  and array   elements to those that also provide an atomic conditional update   operation of the form           boolean compareAndSet expectedValue  updateValue     The atomic classes are built around the atomic compareAndSet      function that maps to an atomic CPU instruction  The atomic classes introduce the happen-before ordering as the volatile variables do   with one exception  weakCompareAndSet         From the java docs      When a thread sees an update to an atomic variable caused by a   weakCompareAndSet  it does not necessarily see updates to any other   variables that occurred before the weakCompareAndSet    To your question      Does this mean that whosoever takes lock on it  that will be setting   its value first  And in if meantime  some other thread comes up and   read old value while first thread was changing its value  then doesn t   new thread will read its old value    You don t lock anything  what you are describing is a typical race condition that will happen eventually if threads access shared data without proper synchronization  As already mentioned declaring a variable volatile in this case will only ensure that other threads will see the change of the variable  the value will not be cached in a register of some cache that is only seen by one thread       What is the difference between AtomicInteger and volatile int    AtomicInteger provides atomic operations on an int with proper synchronization  eg  incrementAndGet    getAndAdd             volatile int will just ensure the visibility of the int to other threads

User · Answer

The effect of the volatile keyword is approximately that each individual read or write operation on that variable is atomic   Notably  however  an operation that requires more than one read write -- such as i    which is equivalent to i   i   1  which does one read and one write -- is not atomic  since another thread may write to i between the read and the write   The Atomic classes  like AtomicInteger and AtomicReference  provide a wider variety of operations atomically  specifically including increment for AtomicInteger

[java] Volatile Vs Atomic

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