JMM

The Java Memory Model is critical for ensuring correct behavior in multi-threaded applications by establishing visibility and ordering guarantees. However, enforcing these guarantees (via synchronization, volatile, or atomic operations) introduces performance trade-offs due to locking overhead, memory barriers, and potential contention

The JMM ensures that when one thread modifies a shared variable, that change becomes visible to other threads under certain conditions. For example, using synchronization primitives like synchronized blocks or volatile variables creates a “happens-before” relationship, ensuring that a write by one thread is visible to subsequent reads by another.

Atomic Operations and CAS:
The JMM enables lock-free programming through atomic classes (like AtomicInteger) that use Compare-And-Swap (CAS) operations. CAS operations are efficient but can suffer from the “ABA problem” or excessive retries under high contention, which might hurt performance.

Avoid False Sharing:
Be mindful of how data is laid out in memory. Use padding or specialized libraries to avoid false sharing when multiple threads update adjacent data.

The JMM allows compilers and processors to reorder instructions for optimization, as long as the reordering does not violate the “happens-before” guarantees. However, improper synchronization can lead to threads seeing stale or out-of-order values, causing inconsistent behavior.

CAS

Compare-and-Swap (CAS) is a lock-free, atomic instruction used in concurrent programming to achieve synchronization without explicit locking. It works by comparing the current value of a variable to an expected value and, only if they match, swapping it with a new value. This operation is performed atomically by the hardware, ensuring that no other thread can interfere between the comparison and the update.

• Threads assume that conflicts will be rare and proceed without locking. If a conflict occurs (CAS fails), the thread retries the operation.

• Building Blocks for Atomic Classes:
  CAS is used in the implementation of many atomic classes in java.util.concurrent.atomic (e.g., AtomicInteger, AtomicReference). Example: **AtomicInteger:** The counteris an instance ofAtomicInteger. When we call incrementAndGet()`. these following happens

• The current value is read.

• The expected value is set to this current value.

• The new value is computed (current value + 1).

• CAS is used to atomically compare the counter’s current value to the expected value. If they match, it updates the counter to the new value.

• If the value was updated by another thread in the meantime (i.e., the current value no longer equals the expected value), the CAS operation fails and the process repeats until it succeeds.

With multiple threads concurrently calling increment(), CAS ensures that each increment operation is atomic, and no updates are lost. This allows us to safely update the counter without using explicit locks.

False Sharing

Analogy Imagine two people (cores) working on separate tasks (modifying counter1 and counter2). They each have their own desk (cache). But, because the tools for both tasks are stored on the same shared clipboard (cache line), every time one person uses a tool, the other person has to put the clipboard back in the main toolbox, wait for the first person to put it back, and then get the clipboard again from the main toolbox, even if they only needed their own tool. This constant back-and-forth for a shared resource (the clipboard/cache line) is the performance overhead.

In modern multicore processors, data is transferred between the main memory and the CPU cores in fixed-size blocks called cache lines (typically 64 bytes). When two or more independent variables that are frequently accessed by different CPU cores happen to reside within the same cache line, it can lead to a performance degradation known as false sharing

False sharing in multicore multithreading system can be avoided by using padding

	public volatile long counter1 = 0;
    public volatile long counter2 = 0;
This can cause counter1 and counter2 to be stored in same cache line and cause false sharing.

	  public volatile long p1, p2, p3, p4, p5, p6, p7;
	  public volatile long counter = 0;
      public volatile long q1, q2, q3, q4, q5, q6, q7;
padding can avoid false sharing