Ch 4 Synchronization Models of Memory Consistency
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Transcript of Ch 4 Synchronization Models of Memory Consistency
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Unit IIISynchronization & Memory consistency Models
CS2354 Advanced Computer Architecture
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Synchronization
Why we need Synchronization?
important because while using
multiprocessors architectures Need to know when
it is safe for different processes to use shared data
When Synchronization is important?
different processor uses shared
data, we must have some mechanism to establish
the synchronization in process
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Synchronization
The synchronization mechanism are built with the
user level software routines relay on hardware
supplies synchronization instructions
For small multiprocessors, uninterruptable
instruction are used to fetch and update thememory which is referred to as the atomic
operation
The operation used to fetch and update memory atsame time (read and write) referred to as atomic
operation
In large scale multiprocessors synchronization can
be a bottleneck
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Synchronization
several techniques have been proposed to reduce
contention and latency in synchronization
We have to first examine the hardware primitives
to implement synchronization and then construct
synchronization routines
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Synchronization
Hardware primitives
Basic requirements to implement synchronization in
multiprocessor is the set of hardware primitives with
the ability to automatically read and modify the
memory location(In one step) Interchanging value in a register for a value in memory
automatically referred to as atomic and exchange
The key property of these atomic primitive is that they
read and update memory value atomically (ie) without
subdividing into number of steps
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In a simple lock,
Value 0 represents the lock is free
Value 1 represents the lock is unavailable
For example, 2 processors tries to do the
exchange simultaneously:
This race is broken down since exactly one of the
processors will perform the exchange.
First returning 0, the second processor will return1 when it does the exchange
Synchronization
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Synchronization In addition to atomic exchange the other option used in old
multiprocessors are Atomic exchange: interchange a value in a register for a value in
memory
0 synchronization variable is free
1 synchronization variable is locked and unavailable
Set register to 1 & swap
New value in register determines success in getting lock0 if you succeeded in setting the lock (you were first)1 if other processor had already claimed access
Key is that exchange operation is indivisibleTest-and-set: tests a value and sets it if the value passes the test
Fetch-and-increment: it returns the value of a memory location andatomically increments it
0 synchronization variable is free
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Synchronization
To implement synchronization, a processor tries to set the
lock by exchange of 1, which is in the register ,with thememory address corresponding to the lock
The value returned from the exchange instruction is 1, if some
other processor had already claimed access , otherwise value
returned is 0 The synchronization is locked and unavailable if some other
processor has already claimed access otherwise value=0
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Uninterruptable Instruction to
Fetch and Update Memory
Hard to have read & write in 1 instruction: use 2
instead
Load linked (or load locked) + store conditional
Load linked returns the initial value
Store conditional returns 1 if it succeeds (no other
store to same memory location since preceding load)
and 0 otherwise
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Uninterruptable Instruction to
Fetch and Update Memory
Example doing atomic swap with LL & SC:try: mov R3,R4 ; mov exchange value
ll R2,0(R1) ; load linkedsc R3,0(R1) ; store conditional
beqz R3,try ; branch store fails (R3 = 0)mov R4,R2 ; put load value in R4
At the end of this sequence the contents of R4 & thememory location specified by R1 have been atomically
exchanged Anytime a processor intervenes & modifies the value in
memory b/w LL & SC instructions
SC returns 0 in R3, causing the code sequence to try again
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Uninterruptable Instruction to
Fetch and Update Memory
Another Example doing fetch & increment with LL
& SC:
try: ll R2,0(R1) ; load linked
addi R2,R2,#1 ; increment (OK if regreg)sc R2,0(R1) ; store conditional
beqz R2,try ; branch store fails (R2 = 0)
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Implementing locks using
coherence(reverse of atomic exchange) Spin locks: processor continuously tries to acquire, spinning
around a loop trying to get the lock
li R2,#1
lockit: exch R2,0(R1) ;atomic exchange
bnez R2,lockit ;already locked?
Problem: exchange includes a write, which invalidates all other
copies; this generates considerable bus traffic
Solution: start by simply repeatedly reading the variable; when
it changes, then try exchange (test and test&set):
try: li R2,#1
lockit: lw R3,0(R1) ;load var
bnez R3,lockit ; 0 not free spin
exch R2,0(R1) ;atomic exchange
bnez R2,try ;already locked?
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Lock&Unlock: Test&Set
/* Test&Set */
==============
loadi R2, #1
lockit: exch R2, location /* atomic operation*/
bnez R2, lockit /* test*/
unlock: store location, #0 /* free the lock (write
0) */
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Lock&Unlock: Test and Test&Set
/* Test and Test&Set */
=======================
lockit: load R2, location /* read lock varijable */bnz R2, lockit /* check value */
loadi R2, #1
exch R2, location /* atomic operation */
bnz reg, lockit /* if lock is not acquired,
repeat */
unlock: store location, #0 /* free the lock (write
0) */
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Lock&Unlock: Test and Test&Set
/* Load-linked and Store-Conditional */
=======================================
lockit: ll R2, location /* load-linked read */
bnz R2, lockit /* if busy, try again */load R2, #1
sc location, R2 /* conditional store */
beqz R2, lockit /* if sc unsuccessful, try
again */
unlock: store location, #0 /* store 0 */
A th MP I
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Another MP Issue:
Memory Consistency Models What is consistency? When must a processor see consistent view of
memory . e.g., seems thatP1: A = 0; P2: B = 0;
..... .....A = 1; B = 1;
L1: if (B == 0) ... L2: if (A == 0) ...
Impossible for both if statements L1 & L2 to be true? What if write invalidate is delayed & processor continues?
The processes are assumed to be running on different processors
and location A,B are originally cached by both processors with initial
value 0.
If write always take immediate effect and immediately seen byother processors it is impossible to for both statement (L1,L2) to be
true
A th MP I
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Another MP Issue:
Memory Consistency Models Reaching the if statement means that either A or B must be assigned
the value to 1 Suppose the invalidate delays, there is no possible that p1 & p2
seen invalidation respectively This behavior came be overcome by using model for memory
consistency
h
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Another MP Issue:
Memory Consistency Models
Rules for the cases :
Three consistency Models
strict consistency model
sequential consistency model
relaxed consistency model
h
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Another MP Issue:
Memory Consistency Models
Strict Consistency Model
P1: write ( Vrble) at the time of t1
P2: Read ( Vrble ) at the time of t2
A h MP I
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Another MP Issue:
Memory Consistency Models Sequential consistency model:
Result of any execution is the same as if the accesses of
each processor were kept in order and the accesses
among different processors were interleaved
assignments before ifs above
SC: delay all memory accesses until all invalidates
done
Relaxed consistency model:
works by allowing read and write to complete out of
order but uses synchronization to enforce ordering
among the writes
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Relaxed Consistency Models: The Basics
Key idea: allow reads and writes to complete out of
order, but to use synchronization operations to
enforce ordering, so that a synchronized program
behaves as if the processor were sequentially
consistent
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Relaxed Consistency Models: The Basics
By relaxing orderings, may obtain performance
advantages
Also specifies range of legal compiler
optimizations on shared data
Unless synchronization points are clearly defined
and programs are synchronized, compiler could
not interchange read and write of 2 shared data
items because might affect the semantics of the
program
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Relaxed Consistency Models: The Basics
3 major sets of relaxed orderings:
1. WR ordering (all writes completed before next read)
Because retains ordering among writes, many programs
that operate under sequential consistency operate under
this model, without additional synchronization. Called
processor consistency
1. W W ordering (all writes completed before next write)
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Relaxed Consistency Models: The Basics
3 major sets of relaxed orderings: ( Cont )
3. R W and R R orderings, a variety of models depending
on ordering restrictions and how synchronization operations
enforce ordering
Many complexities in relaxed consistency models; defining
precisely what it means for a write to complete; deciding
when processors can see values that it has written
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Write Consistency
For now assume
1. A write does not complete (and allow the next write
to occur) until all processors have seen the effect of
that write
2. The processor does not change the order of anywrite with respect to any other memory access
if a processor writes location A followed by location
B, any processor that sees the new value of B must
also see the new value of A
These restrictions allow the processor to reorder
reads, but forces the processor to finish writes in
program order
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Memory Consistency Model Schemes faster execution to sequential consistency
Not an issue for most programs; they are synchronized A program is synchronized if all access to shared data are
ordered by synchronization operations
write (x)...release (s) {unlock}...acquire (s) {lock}...
read(x) Several Relaxed Models for Memory Consistency since
most programs are synchronized; characterized by theirattitude towards: RAR, WAR, RAW, WAWto different addresses