Understanding Hazard Pointers for Safe Memory Reclamation

Hazard Pointers: Safe Memory Reclamation for Lock-Free Objects Maged M. Michael Presented by Abdulai Sei

Recap • So far we have seen many lock-free dynamic objects algorithms • How many reclaim memory of reused nodes? • How many of these return memory to the operating system? • What happens if a thread frees a node and another thread has a pointer to that node?

Related paper -Type Stable Memory (TSM) • Similar concept to HP, non-blocking • Three basic extension to conventional type implementation • type descriptor remains a valid instance of the type even when inactive (in the free list) • allows multiple memory allocation for the same type • type change over time (descriptor has to be inactive before reallocation) • Guarantee a pointer to point to a descriptor of the type • readers can continue reading but does not guarantee correct data

Hazard Pointers (HP) • A memory management methodology that allows memory reclamation for arbitrary reuse • Memory is finite and needs to be reused • Thread failures and dead lock leave chunks of memory unclaimed for reuse by the operating system

HP Claims • Suitable for user-level application as well as system level program without dependency on kernel or scheduler • Very efficient algorithm for dynamic lock-free object • Requires only single-word reads and writes • Solves the ABA problem • Wait-free

ABA Problem • Common problem with CAS, CAS2, DCAS • Affects almost all lock-free algorithms • Thread A reads a value (XX) from a shared memory, then another thread B reads the same value XX, change it to XY and then back to XX. When thread A reads the value for the second time, it sees XX and assume it was not changed • Must be prevented regardless of memory reclamation • Applying HP to ABA-safe algorithm makes it safe

Methodology • Based on the observation that • A thread holds a small number of references • References do not need further validation for accessing the content of dynamic nodes • References are not ABA-prone • Core ideas • Single-writer multi-reader shared pointers called HP • HP varies among threads – typically one to two • Communicates with the algorithm through HP • Consists of two parts • One for processing retired nodes • Memory reclamation and ABA prevention

Algorithm – (reader only) HP List READER (add2) – dereference add1 add1* add2

Algorithm – (reader + writer) HP List WRITER READER (add2) – dereference add1 allocate add3 Deference add1 copy add2 to add3 modify add3 add1* add3 add2

Algorithm (reader + writer) HP List WRITER READER (add2) – reader has already deference add1* before writer start allocate add3 copy add2 to add3 modify add3 CAS retired add2 add1* add3 add2

Questions for class • Is the reader reading the right data after the writer perform CAS? See slide 13, the conditions • Can the garbage collector free add2? • Any new problems? • yes, readers have to write to the HP list • others?

Code

The Condition • A thread announces to other threads when it assigns a reference to one of it HP • Other threads refrain from reclaiming or reusing node • At time t, each node n is in one of the states • Allocated, reachable, removed, retired, free, unavailable, undefined • A thread holds a reference to a node when it is safe • Question: • Can a thread create a new hazardous reference to a node when it is retired?

Applying HP • Apply to existing lock-free algorithms based on conditions • Examine the target algorithm • Identify HP and hazard reference they use • Determine where hazard reference is created and last hazard pointer that uses it • Determine the max # that can be hazardous • Insert the following in target algorithm • Write the address of nodes that are target for reference to an unavailable hazard reference • Validate the node is safe • Example of existing algorithms to apply the use of HP • FIFO Queues • LIFO Stacks • List-Based Sets and Hash Tables • Single-Writer Multireader Dynamic Structures

Applications – FIFO queues

Applications – Lock-free stack using HP

Experiment • Comparing performance of new methodology to lock-free memory management system • FIFI queues, LIFO stacks, chaining hash table – all implemented using HP • Comparison with commonly use locks • Using test-and-test and set with exponential backoff • Hash table with 100 separate locks • IBM RS /6000 multiprocessor with 4 processors • Align cache data structure • Locks, single-word, double-word CAS implemented using LL/SC • All implementation compiled at highest optimization • Ran each experiment 5 times and reported the average

Performance Comparison Hash tables load factor 1 FIFO Queue LIFO stacks Hash tables load factor 5

Conclusion • HP provided unrestricted memory reclamation for dynamic lock-free objects • Takes constant amortized time per retired node • Uses only single-word instructions • Comparable performance to that of lock-based implementation • Both under no contention and no multiprogramming • Outperform them significantly under moderate multiprogramming and/or contention • Guarantee progress even if a thread fails or delays (prevent deadlock)

Understanding Hazard Pointers for Safe Memory Reclamation