Multiprocessors and Thread-Level Parallelism
1. Explain Symmetric Shared-Memory Architecture and How to reduce Cache
coherence problem in Symmetric Shared Memory architectures:
Symmetric Shared Memory Architectures:
The Symmetric Shared Memory Architecture consists of several processors with a single
physical memory shared by all processors through a shared bus which is shown below.
` Small-scale shared-memory machines usually support the caching of both shared
and private data. Private data is used by a single processor, while shared data is used by
multiple processors, essentially providing communication among the processors through
reads and writes of the shared data. When a private item is cached, its location is
migrated to the cache, reducing the average access time as well as the memory bandwidth
required. Since no other processor uses the data, the program behavior is identical to that
in a uniprocessor.
Cache Coherence in Multiprocessors:
Introduction of caches caused a coherence problem for I/O operations, The same problem
exists in the case of multiprocessors, because the view of memory held by two different
processors is through their individual caches. Figure 6.7 illustrates the problem and
shows how two different processors can have two different values for the same location.
This difficulty s generally referred to as the cache-coherence problem.
Cache contents for
Cache contents for
Memory contents for location X
0 1 1 CPU A reads X 1 1 2 CPU B reads X 1 1 1 3 CPU A stores 0 into X 0 1 0
FIGURE 6.7 The cache-coherence problem for a single memory location (X), read
and written by two processors (A and B). We initially assume that neither cache
contains the variable and that X has the value 1. We also assume a write-through cache; a
write-back cache adds some additional but similar complications. After the value of X
has been written by A, A’s cache and the memory both contain the new value, but B’s
cache does not, and if B reads the value of X, it will receive 1!
Informally, we could say that a memory system is coherent if any read of a data item
returns the most recently written value of that data item. This simple definition contains
two different aspects of memory system behavior, both of which are critical to writing
correct shared-memory programs. The first aspect, called coherence, defines what values
can be returned by a read. The second aspect, called consistency, determines when a
written value will be returned by a read. Let’s look at coherence first.
A memory system is coherent if
1 A read by a processor, P, to a location X that follows a write by P to X, with no
writes of X by another processor occurring between the write and the read by P, always
returns the value written by P.
2 A read by a processor to location X that follows a write by another processor to X
returns the written value if the read and write are sufficiently separated in time and no
other writes to X occur between the two accesses.
3 Writes to the same location are serialized: that is, two writes to the same location
by any two processors are seen in the same order by all processors. For example, if the
values 1 and then 2 are written to a location, processors can never read the value of the
location as 2 and then later read it as 1.
Coherence and consistency are complementary: Coherence defines the behavior of reads
and writes to the same memory location, while consistency defines the behavior of reads
and writes with respect to accesses to other memory locations.
Basic Schemes for Enforcing Coherence
Coherent caches provide migration, since a data item can be moved to a local cache
and used there in a transparent fashion. This migration reduces both the latency to access
a shared data item that is allocated remotely and the bandwidth demand on the shared
Coherent caches also provide replication for shared data that is being simultaneously
read, since the caches make a copy of the data item in the local cache. Replication
reduces both latency of access and contention for a read shared data item.
The protocols to maintain coherence for multiple processors are called cache-
coherence protocols. There are two classes of protocols, which use different techniques
to track the sharing status, in use:
Directory based—The sharing status of a block of physical memory is kept in just one
location, called the directory; we focus on this approach in section 6.5, when we discuss
scalable shared-memory architecture.
Snooping—Every cache that has a copy of the data from a block of physical memory also
has a copy of the sharing status of the block, and no centralized state is kept. The caches
are usually on a shared-memory bus, and all cache controllers monitor or snoop on the
bus to determine whether or not they have a copy of a block that is requested on the bus.
The method which ensure that a processor has exclusive access to a data item before it
writes that item. This style of protocol is called a write invalidate protocol because it
invalidates other copies on a write. It is by far the most common protocol, both for
snooping and for directory schemes. Exclusive access ensures that no other readable or
writable copies of an item exist when the write occurs: all other cached copies of the item
Figure 6.8 shows an example of an invalidation protocol for a snooping bus with write-
back caches in action To see how this protocol ensures coherence, consider a write
followed by a read by another processor: Since the write requires exclusive access, any
copy held by the reading processor must be invalidated (hence the protocol name). Thus,
when the read occurs, it misses in the cache and is forced to fetch a new copy of the data.
For a write, we require that the writing processor have exclusive access, preventing any
other processor from being able to write simultaneously. If two processors do attempt to
write the same data simultaneously, one of them wins the race, causing the other
processor’s copy to be invalidated. For the other processor to complete its write, it must
obtain a new copy of the data, which must now contain the updated value. Therefore, this
protocol enforces write serialization.
Contents of Contents of Contents of memory Processor activity
Bus activity CPU A’s cache
CPU B’s cache
0 CPU A reads X Cache miss for X 0 0 CPU B reads X Cache miss for X 0 0 0 CPU A writes a 1 to X Invalidation for X 1
CPU B reads X Cache miss for X 1 1 1
FIGURE 6.8 An example of an invalidation protocol working on a snooping bus for
a single cache block (X) with write-back caches.
The alternative to an invalidate protocol is to update all the cached copies of a
data item when that item is written. This type of protocol is called a write update or write
broadcast protocol. Figure 6.8 shows an example of a write update protocol in operation.
In the decade since these protocols were developed, invalidate has emerged as the winner
for the vast majority of designs.
Contents of Contents of Contents of memory Processor activity
Bus activity CPU A’s cache
CPU B’s cache
0 CPU A reads X Cache miss for X 0 0 CPU B reads X Cache miss for X 0 0 0 CPU A writes a 1 to X
Write broadcast of X 1 1 1
CPU B reads X 1 1 1
FIGURE 6.9 An example of a write update or broadcast protocol working on a
snooping bus for a single cache block (X) with write-back caches.
The performance differences between write update and write invalidate protocols
arise from three characteristics:
Multiple writes to the same word with no intervening reads require multiple write
broadcasts in an update protocol, but only one initial invalidation in a write
With multiword cache blocks, each word written in a cache block requires a write
broadcast in an update protocol, although only the first write to any word in the
block needs to generate an invalidate in an invalidation protocol. An invalidation
protocol works on cache blocks, while an update protocol must work on
individual words (or bytes, when bytes are written). It is possible to try to merge
writes in a write broadcast scheme.
The delay between writing a word in one processor and reading the written value
in another processor is usually less in a write update scheme, since the written
data are immediately updated in the reader’s cache
Basic Implementation Techniques
The serialization of access enforced by the bus also forces serialization of writes, since
when two processors compete to write to the same location, one must obtain bus access
before the other. The first processor to obtain bus access will cause the other processor’s
copy to be invalidated, causing writes to be strictly serialized. One implication of this
scheme is that a write to a shared data item cannot complete until it obtains bus access.
For a write-back cache, however, the problem of finding the most recent data value is
harder, since the most recent value of a data item can be in a cache rather than in
memory. Happily, write-back caches can use the same snooping scheme both for caches
misses and for writes: Each processor snoops every address placed on the bus. If a
processor finds that it has a dirty copy of the requested cache block, it provides that cache
block in response to the read request and causes the memory access to be aborted. Since
write-back caches generate lower requirements for memory bandwidth, they are greatly
preferable in a multiprocessor, despite the slight increase in complexity. Therefore, we
focus on implementation with write-back caches.
The normal cache tags can be used to implement the process of snooping, and the valid
bit for each block makes invalidation easy to implement. Read misses, whether generated
by an invalidation or by some other event, are also straightforward since they simply rely
on the snooping capability. For writes we’d like to know whether any other copies of the
block are cached, because, if there are no other cached copies, then the write need not be
placed on the bus in a write-back cache. Not sending the write reduces both the time
taken by the write and the required bandwidth.
2. Explain Distributed Shared-Memory Architectures.
There are several disadvantages in Symmetric Shared Memory architectures. First,
compiler mechanisms for transparent software cache coherence are very limited. Second,
without cache coherence, the multiprocessor loses the advantage of being able to fetch
and use multiple words in a single cache block for close to the cost of fetching one word.
Third, mechanisms for tolerating latency such as prefetch are more useful when they can
fetch multiple words, such as a cache block, and where the fetched data remain coherent;
we will examine this advantage in more detail later.
These disadvantages are magnified by the large latency of access to remote memory
versus a local cache. For these reasons, cache coherence is an accepted requirement in
small-scale multiprocessors. For larger-scale architectures, there are new challenges to
extending the cache-coherent shared-memory model. Although the bus can certainly be
replaced with a more scalable interconnection network and we could certainly distribute
the memory so that the memory bandwidth could also be scaled, the lack of scalability of
the snooping coherence scheme needs to be addressed is known as Distributed Shared
The first coherence protocol is known as a directory protocol. A directory keeps the
state of every block that may be cached. Information in the directory includes which
caches have copies of the block, whether it is dirty, and so on.
To prevent the directory from becoming the bottleneck, directory entries can be
distributed along with the memory, so that different directory accesses can go to different
locations, just as different memory requests go to different memories. A distributed
directory retains the characteristic that the sharing status of a block is always in a single
known location. This property is what allows the coherence protocol to avoid broadcast.
Figure 6.27 shows how our distributed-memory multiprocessor looks with the directories
added to each node.
FIGURE 6.27 A directory is added to each node to implement cache coherence in a
Directory-Based Cache-Coherence Protocols: The Basics
There are two primary operations that a directory protocol must implement: handling a
read miss and handling a write to a shared, clean cache block. (Handling a write miss to a
shared block is a simple combination of these two.) To implement these operations, a
directory must track the state of each cache block. In a simple protocol, these states could
be the following:
Shared—One or more processors have the block cached, and the value in memory is up
to date (as well as in all the caches)
Uncached—No processor has a copy of the cache block
Exclusive—Exactly one processor has a copy of the cache block and it has written the
block, so the memory copy is out of date. The processor is called the owner of the block.
In addition to tracking the state of each cache block, we must track the processors that
have copies of the block when it is shared, since they will need to be invalidated on a
write. The simplest way to do this is to keep a bit vector for each memory block. When
the block is shared, each bit of the vector indicates whether the corresponding processor
has a copy of that block. We can also use the bit vector to keep track of the owner of the
block when the block is in the exclusive state. For efficiency reasons, we also track the
state of each cache block at the individual caches.
A catalog of the message types that may be sent between the processors and the
directories. Figure 6.28 shows the type of messages sent among nodes. The local node is
the node where a request originates. The home node is the node where the memory
location and the directory entry of an address reside. The physical address space is
statically distributed, so the node that contains the memory and directory for a given
physical address is known. For example, the high-order bits may provide the node
number, while the low-order bits provide the offset within the memory on that node. The
local node may also be the home node. The directory must be accessed when the home
node is the local node, since copies may exist in yet a third node, called a remote node.
A remote node is the node that has a copy of a cache block, whether exclusive (in
which case it is the only copy) or shared. A remote node may be the same as either the
local node or the home node. In such cases, the basic protocol does not change, but
interprocessor messages may be replaced with intraprocessor messages.
Message type Source Destination Message contents Function of this message
Read miss Local cache Home directory
P, A Processor P has a read miss at address A; request data and make P a read sharer.
Write miss Local cache Home directory
P, A Processor P has a write miss at address A; — request data and make P the exclusive owner.
Invalidate Home directory
Remote cache A Invalidate a shared copy of data at address A.
Fetch Home directory
Remote cache A Fetch the block at address A and send it to its home directory; change the state of A in the remote cache to shared.
Remote cache A Fetch the block at address A and send it to its home directory; invalidate the block in the cache.
Data value reply
Local cache D Return a data value from the home memory.
Data write back Remote cache
A, D Write back a data value for address A.
FIGURE 6.28 The possible messages sent among nodes to maintain coherence are
shown with the source and destination node, the contents (where P=requesting
processor number), A=requested address, and D=data contents), and the function of
3. What is Synchronization? Explain various Hardware Primitives
Synchronization mechanisms are typically built with user-level software routines
that rely on hardware-supplied synchronization instructions. The efficient spin locks can
be built using a simple hardware synchronization instruction and the coherence
Basic Hardware Primitives
The key ability we require to implement synchronization in a multiprocessor is a set of
hardware primitives with the ability to atomically read and modify a memory location.
Without such a capability, the cost of building basic synchronization primitives will be
too high and will increase as the processor count increases. There are a number of
alternative formulations of the basic hardware primitives, all of which provide the ability
to atomically read and modify a location, together with some way to tell if the read and
write were performed atomically. These hardware primitives are the basic building blocks
that are used to build a wide variety of user-level synchronization operations, including
things such as locks and barriers.
One typical operation for building synchronization operations is the atomic exchange,
which interchanges a value in a register for a value in memory. Use this to build a basic
synchronization operation, assume that we want to build a simple lock where the value 0
is used to indicate that the lock is free and a 1 is used to indicate that the lock is
unavailable. A processor tries to set the lock by doing an exchange of 1, which is in a
register, with the memory address corresponding to the lock. The value returned from the
exchange instruction is 1 if some other processor had already claimed access and 0
otherwise. In the latter case, the value is also changed to be 1, preventing any competing
exchange from also retrieving a 0.
There are a number of other atomic primitives that can be used to implement
synchronization. They all have the key property that they read and update a memory
value in such a manner that we can tell whether or not the two operations executed
atomically. One operation, present in many older multiprocessors, is test-and-set, which
tests a value and sets it if the value passes the test. For example, we could define an
operation that tested for 0 and set the value to 1, which can be used in a fashion similar to
how we used atomic exchange.
Another atomic synchronization primitive is fetch-and-increment: it returns the value
of a memory location and atomically increments it. By using the value 0 to indicate that
the synchronization variable is unclaimed, we can use fetch-and-increment, just as we
used exchange. There are other uses of operations like fetch-and-increment.
Implementing Locks Using Coherence
We can use the coherence mechanisms of a multiprocessor to implement spin locks:
locks that a processor continuously tries to acquire, spinning around a loop until it
succeeds. Spin locks are used when a programmer expects the lock to be held for a very
short amount of time and when she wants the process of locking to be low latency when
the lock is available. Because spin locks tie up the processor, waiting in a loop for the
lock to become free, they are inappropriate in some circumstances.
The simplest implementation, which we would use if there were no cache coherence,
would keep the lock variables in memory. A processor could continually try to acquire
the lock using an atomic operation, say exchange, and test whether the exchange returned
the lock as free. To release the lock, the processor simply stores the value 0 to the lock.
Here is the code sequence to lock a spin lock whose address is in R1 using an atomic
lockit: EXCH R2,0(R1) ; atomic exchange
BNEZ R2,lockit ; already locked?
If our multiprocessor supports cache coherence, we can cache the locks using the
coherence mechanism to maintain the lock value coherently. Caching locks has two
advantages. First, it allows an implementation where the process of “spinning” (trying to
test and acquire the lock in a tight loop) could be done on a local cached copy rather than
requiring a global memory access on each attempt to acquire the lock. The second
advantage comes from the observation that there is often locality in lock accesses: that is,
the processor that used the lock last will use it again in the near future. In such cases, the
lock value may reside in the cache of that processor, greatly reducing the time to acquire
Synchronization Performance Challenges
One additional common synchronization operation in programs with parallel
loops is a barrier. A barrier forces all processes to wait until all the processes reach the
barrier and then releases all of the processes. A typical implementation of a barrier can be
done with two spin locks: one used to protect a counter that tallies the processes arriving
at the barrier and one used to hold the processes until the last process arrives at the
Synchronization Mechanisms for Larger-Scale Multiprocessors
The major difficulty with our spin-lock implementation is the delay due to
contention when many processes are spinning on the lock. One solution is to artificially
delay processes when they fail to acquire the lock. The best performance is obtained by
increasing the delay exponentially whenever the attempt to acquire the lock fails. Figure
6.41 shows how a spin lock with exponential back-off is implemented. Exponential back-
off is a common technique for reducing contention in shared resources, including access
to shared networks and buses. This implementation still attempts to preserve low latency
when contention is small by not delaying the initial spin loop. The result is that if many
processes are waiting, the back-off does not affect the processes on their first attempt to
acquire the lock. We could also delay that process, but the result would be poorer
performance when the lock was in use by only two processes and the first one happened
to find it locked. ADDUI R3,R0,#1 ;R3 = initial delay lockit: LL R2,0(R1) ;load linked BNEZ R2,lockit ;not available-spin DADDUI R2,R2,#1 ;get locked value SC R2,0(R1) ;store conditional BNEZ R2,gotit ;branch if store succeeds DSLL R3,R3,#1 ;increase delay by factor of 2 PAUSE R3 ;delays by value in R3 J lockit gotit: use data protected by lock IGURE 6.41 A spin lock with exponential back-off.
Another technique for implementing locks is to use queuing locks. Queuing locks work
by constructing a queue of waiting processors; whenever a processor frees up the lock, it
causes the next processor in the queue to attempt access. This eliminates contention for a
lock when it is freed. We show how queuing locks operate in the next section using a
hardware implementation, but software implementations using arrays can achieve most of
the same benefits Before we look at hardware primitives,
In this section we look at two hardware synchronization primitives. The first primitive
deals with locks, while the second is useful for barriers and a number of other user-level
operations that require counting or supplying distinct indices. In both cases we can create
a hardware primitive where latency is essentially identical to our earlier version, but with
much less serialization, leading to better scaling when there is contention.
The major problem with our original lock implementation is that it introduces a large
amount of unneeded contention. For example, when the lock is released all processors
generate both a read and a write miss, although at most one processor can successfully
get the lock in the unlocked state. This sequence happens on each of the 20 lock/unlock
We can improve this situation by explicitly handing the lock from one waiting
processor to the next. Rather than simply allowing all processors to compete every time
the lock is released, we keep a list of the waiting processors and hand the lock to one
explicitly, when its turn comes. This sort of mechanism has been called a queuing lock.
Queuing locks can be implemented either in hardware, or in software using an array to
keep track of the waiting processes.
4. What is Multithreading? How to exploiting TLP.
Multithreading: Exploiting Thread-Level Parallelism within a Processor
Multithreading allows multiple threads to share the functional units of a single processor
in an overlapping fashion. To permit this sharing, the processor must duplicate the
independent state of each thread. For example, a separate copy of the register file, a
separate PC, and a separate page table are required for each thread.
There are two main approaches to multithreading.
Fine-grained multithreading switches between threads on each instruction, causing the
execution of multiples threads to be interleaved. This interleaving is often done in a
round-robin fashion, skipping any threads that are stalled at that time.
Coarse-grained multithreading was invented as an alternative to fine-grained
multithreading. Coarse-grained multithreading switches threads only on costly stalls,
such as level two cache misses. This change relieves the need to have thread-switching be
essentially free and is much less likely to slow the processor down, since instructions
from other threads will only be issued, when a thread encounters a costly stall.
Simultaneous Multithreading: Converting Thread-Level Parallelism into Instruction-
Simultaneous multithreading (SMT) is a variation on multithreading that uses the
resources of a multiple issue, dynamically-scheduled processor to exploit TLP at the
same time it exploits ILP. The key insight that motivates SMT is that modern multiple-
issue processors often have more functional unit parallelism available than a single thread
can effectively use. Furthermore, with register renaming and dynamic scheduling,
multiple instructions from independent threads can be issued without regard to the
dependences among them; the resolution of the dependences can be handled by the
dynamic scheduling capability.
Figure 6.44 conceptually illustrates the differences in a processor’s ability to exploit
the resources of a superscalar for the following processor configurations:
n a superscalar with no multithreading support,
n a superscalar with coarse-grained multithreading,
n a superscalar with fine-grained multithreading, and
n a superscalar with simultaneous multithreading.
Superscalar Coarse MT Fine MT SMT
In the superscalar without multithreading support, the use of issue slots is limited by a
lack of ILP.
In the coarse-grained multithreaded superscalar, the long stalls are partially hidden by
switching to another thread that uses the resources of the processor.
In the fine-grained case, the interleaving of threads eliminates fully empty slots. Because
only one thread issues instructions in a given clock cycle.
In the SMT case, thread-level parallelism (TLP) and instruction-level parallelism (ILP)
are exploited simultaneously; with multiple threads using the issue slots in a single clock
Figure 6.44 greatly simplifies the real operation of these processors it does illustrate
the potential performance advantages of multithreading in general and SMT in particular.
Design Challenges in SMT processors
There are a variety of design challenges for an SMT processor, including:
Dealing with a larger register file needed to hold multiple contexts,
Maintaining low overhead on the clock cycle, particularly in critical steps such
as instruction issue, where more candidate instructions need to be considered,
and in instruction completion, where choosing what instructions to commit may
be challenging, and
Ensuring that the cache conflicts generated by the simultaneous execution of
multiple threads do not cause significant performance degradation.
In viewing these problems, two observations are important. In many cases, the potential
performance overhead due to multithreading is small, and simple choices work well
enough. Second, the efficiency of current super-scalars is low enough that there is room
for significant improvement, even at the cost of some overhead.