Week 3 - Synchronisation

Question 1

what is a critical section

Answer 1

A sequence of instructions that only one thread should be able to access at a time otherwise it leads to unpredicatble outcomes

Question 2

what is a lock

Answer 2

lock is a construct that declares a critical section indicating one thread access only at a time. just before a thread about to access crtifical section it checks if lock is free :

if successful it aqquires lock and releases the lock when it exits the critical section

if failure its blocked indefinitely until the lock is released

Question 3

intrinsic lock

Answer 3

locks on entire instance of an object

Locks entire instance not just method
- If one thread gets the lock on that particular instance no other synchronised method can be used on that object

Question 4

why are synchronised blocks good

Answer 4

So yes — synchronized blocks are more precise. They let you lock exactly what you need, and release the lock as early as possible, improving concurrency.

Question 5

READ VERY IMPORTANT

Answer 5

speed wise
cpu -> cache -> RAM (fastest ->slowest)

cache holds frequently accessed data
ram is slow and holds important stuff like heap and data ( objects are instance variables stored here)

lets say on a multi core system ( share main memory , but have their own caches and cpu)

They each cacche frequently accessed data (objects and instance variables)
when one thread modifies an instance variable not immediately propagated to main memoryu (also it may read values from cache which could be out of data) this means lets say another thread reads that instance variable it hasnt received the update and so when modifying it would mofify wrong value (lost update)

leads to race conditions (from an architectural sense)

Question 6

volatile introduction

youre an idiot if you dont read this crucial

Answer 6

To deal with issue we discussed previously (variables cached locally by each thread causing inconsistency , us not writing to main memory straight away , reading from cache with old values) we use the volatile keyword

Question 7

volatile

Answer 7

when changed its straight away flushed to main memory

when accessed its only accessed from main memory NOT CACHE AS THIS MAY HAVE OLD VALUES

Question 8

volatile visibility guarantee

Answer 8

if a variable marked as volatile to ram flushed then all other variables flushed

Question 9

why is instruction reordering disasterous

hint think about volatile

Answer 9

so essentially jvm can choose to reorder lines in program for efficiency but this may be disasterous in volatile cases as a thread may read a value before another thread has actually modified it

so what volatile ensures is
anything before a volatile write must remain there ( so no unintended consequences)
anything after volatile write must remain there

excellent example:
✅ What volatile guarantees
When you do:

buffer.add(…); // Step 1
newDataAvailable = true; // Step 2 (volatile write)

The JVM guarantees:

Nothing before the volatile write (like the buffer update) can be moved after it

Nothing after can be moved before it either