1. Shared Memory Systems
Miodrag Bolic

2. Overview
Next 4 to 5 lectures

3. Outline Characteristics of shared memory systems
Programming shared-memory multiprocessors
Hardware implementation
Architectures
Memory access
Caches
Synchronization

4. Characteristics of shared memory systems [3]
Any processor can directly reference any memory location.
Communication occurs implicitly as result of loads and stores.
Location of data in memory is transparent to the programmer.
Inherently provided on wide range of platforms (standard processors today have specific extra hardware for share memory systems)
Memory may be physically distributed among processors.

5. Requirements [3] Support for memory coherency
The machine must make sure that all of the processing nodes have an accurate picture of the most up-to-date memory.
Support for atomic operations on data
The machine must allow for only one processor to change data at a time.
Nonatomic operation: One processor requests data and before the request is answered, another processor changes that data.

6. Shared Memory Program [3]
Sum all the elements of an array of size n.
INITIALIZE; //assign proc_nums and num_procs
read_array(array_to_sum, size); //read the array and array size from file
if (proc_num == 0) //initialize the sum {
LOCK(global_sum);
global_sum = 0;
UNLOCK(global_sum); }
local_sum = 0; size_to_sum = size/num_procs;
lower_ind = size_to_sum * proc_num;
upper_ind = size_to_sum * (proc_num + 1);
for (i = lower_ind; i < upper_ind; i++)
local_sum += array_to_sum[i];
//if size =100, num_proc=4, processor 0 sums 0 to 24, proc 1 sums 25 to 49, etc
LOCK(global_sum); //locks the sum variable so only this process can change it
global_sum += local_sum;
UNLOCK(global_sum); //gives the sum back so other procs can add to it
BARRIER(num_procs); //waits for num_procs to get to this point in the program
if (proc_num == 0)
printf("sum is %d", global_sum);
END;

7. Multiprocessor Software Functions – Example [3]
INITIALIZE – assigns a number (proc_num) to each processor in the system; assigns the total number of processors (num_procs).
LOCK(data)
Allows a processor to “check out” a certain piece of shared data.
While one processor has the data locked, no other processors can obtain the lock.
The lock is blocking, so once a LOCK is encountered, execution of the program cannot proceed until the LOCK is obtained.
UNLOCK(data) – releases a lock so that other processors can obtain it.
BARRIER(n_procs) – When a BARRIER is encountered, a processor waits at that BARRIER until n_procs processors reach the BARRIER, then execution can proceed.

8. Architecture [3]
An example of shared-bus architecture with 4 processors
Both static and dynamic networks can be used to connect processors and shared memory

9. Natural Extensions of Memory System [4]

10. Memory Organization [1]

11. Caches and Cache Coherence [4]
Caches play key role in all cases
Reduce average data access time
Reduce bandwidth demands placed on shared interconnect
But private processor caches create a problem
Copies of a variable can be present in multiple caches
A write by one processor may not become visible to others
They’ll keep accessing stale value in their caches
Cache coherence problem
Need to take actions to ensure visibility

12. Inconsistency in Data Sharing
Suppose two processors each use (read) a data item X from a shared memory. Then each processor’s cache will have a copy of X that is consistent with the shared memory copy.
Now suppose one processor modifies X (to X’). Now that processor’s cache is inconsistent with the other processor’s cache and the shared memory.
With a write-through cache, the shared memory copy will be made consistent, but the other processor still has an inconsistent value (X).
With a write-back cache, the shared memory copy will be updated eventually, when the block containing X (actually X’) is replaced or invalidated.

13. Cache coherence

14. Mutual Exclusion [4] Provided by LOCK-UNLOCK around critical section
Set of operations we want to execute atomically
Implementation of LOCK/UNLOCK must guarantee mutual excl.
Can lead to significant serialization if contended
Especially since expect non-local accesses in critical section
Mutex stands for “mutual exclusion”

15. Simple Software Lock [4]
lock: ld register, location /* copy location to register*/
cmp register, #0 /* compare with 0 */
bnz lock /* if not 0, try again */
st location, #1 / * store 1 to mark it locked */
ret /* return control to caller */
and
unlock: st location, #0 /* write 0 to location */
ret /* return control to caller*/
Problem: lock needs atomicity in its own implementation
Read (test) and write (set) of lock variable by a process not atomic
Solution: atomic read-modify-write or exchange instructions
atomically test value of location and set it to another value, return success or failure somehow

16. Atomic Exchange Instruction [4]
Specifies a location and register. In atomic operation:
Value in location read into a register
Another value (function of value read or not) stored into location
Many variants
Simple example: test&set
Value in location read into a specified register
Constant 1 stored into location
Successful if value loaded into register is 0
Other constants could be used instead of 1 and 0
Can be used to build locks

17. Simple Test&Set Lock [4]
lock: t&s register, location
bnz lock /* if not 0, try again */
ret /* return control to caller */
unlock: st location, #0 /* write 0 to location */
Other read-modify-write primitives can be used too
Swap

18. Mutual Exclusion: Altera [1]
A mutex allows cooperating processors to agree that one of them should be allowed mutually exclusive access to a hardware resource in the system.
Component that is added in SOPC builder
Shared memory can be accessed without using mutexes

19. Altera Mutex [2]

20. Example: Opening and locking a mutex [2]
#include <altera_avalon_mutex.h>
/* get the mutex device handle */
alt_mutex_dev* mutex = altera_avalon_mutex_open( “/dev/mutex” );
/* acquire the mutex, setting the value to one */
altera_avalon_mutex_lock( mutex, 1 ); /*
* Access a shared resource here. */
/* release the lock */
altera_avalon_mutex_unlock( mutex );

21. Performance Criteria (T&S Lock) [4]
Uncontended Latency
Very low if repeatedly accessed by same processor; indept. of p
Traffic
Lots if many processors compete; poor scaling with p
Each t&s generates invalidations, and all rush out again to t&s
Storage
Very small (single variable); independent of p
Fairness
Poor, can cause starvation
Test&set with backoff similar, but less traffic
Luckily, better hardware primitives as well as algorithms exist

22. Improved Hardware Primitives: LL-SC [4]
Goals:
Test with reads
Failed read-modify-write attempts don’t generate invalidations
Nice if single primitive can implement range of r-m-w operations
Load-Locked (or -linked), Store-Conditional
LL reads variable into register
Follow with arbitrary instructions to manipulate its value
SC tries to store back to location if and only if no one else has written to the variable since this processor’s LL
If SC succeeds, means all three steps happened atomically
If fails, doesn’t write or generate invalidations (need to retry LL)
Success indicated by condition codes;

23. Simple Lock with LL-SC [4]
lock: ll reg1, location /* LL location to reg1 */
bnz reg1, lock /* if location is locked, try again*/
sc location, reg2 /* SC reg2 into location*/
beqz lock /* if failed, start again */
ret /* return controll to the caller of lock */
* */
unlock: st location, #0 /* write 0 to location */
ret
SC can fail (without putting transaction on bus) if:
Tries to get bus but another processor’s SC gets bus first
LL, SC are not lock, unlock respectively
Only guarantee no conflicting write to lock variable between them
But can use directly to implement simple operations on shared variables

24. A Simple Centralized Barrier [2]
Shared counter maintains number of processes that have arrived
increment when arrive (lock), check until reaches numprocs
struct bar_type {
int counter;
struct lock_type lock;
int flag = 0;} bar_name;
BARRIER (bar_name, p) {
LOCK(bar_name.lock);
if (bar_name.counter == 0)
bar_name.flag = 0; /* reset flag if first to reach*/
mycount = bar_name.counter++; /* mycount is private */
UNLOCK(bar_name.lock);
if (mycount == p) { /* last to arrive */
bar_name.counter = 0; /* reset for next barrier */
bar_name.flag = 1; /* release waiters */ }
else while (bar_name.flag == 0) {}; /* busy wait for release */

25. References
Altera Corp., Creating Multiprocessor NIOS II System Tutorial, 2005.
Altera Corp. ,Altera Embedded Peripherals Handbook, 2005.
J. Kowalczyk, “Multiprocessor Systems,” Xilinx, 2003.
D. Culler, J. P. Singh, Parallel Computer Architectures, A Hardware/Software Approach, Morgan Kaufman, 1999.