Lecture-Cap8-Part23-Cache Coherence Protocol

Study Flashcards Play Quiz

Questions and Answers

What action is taken when a write miss occurs for a shared block in a coherence protocol?

Invalidate this block as there was an attempt to write a shared block

What is the consequence of attempting to read a shared block that is exclusive elsewhere?

Place cache block on bus, write back the cache block, and abort memory access.

What is the primary issue with the snooping-based coherence protocol?

The need for broadcasting messages, which can negatively impact the benefits of DSM memory organization.

What information does the directory in a directory protocol contain?

Which caches have copies of the block, whether it is dirty, and the block status (shared, uncached, modified) Signup and view all the answers

What is the main advantage of the directory protocol over snooping-based coherence protocols?

It eliminates the need for broadcasting messages. Signup and view all the answers

How is the directory distributed in a directory protocol?

Along with the memory, as shown in Fig. 8.5. Signup and view all the answers

What does a coherent view of memory guarantee in a scenario where a processor P1 reads from location X, following a write by processor P2 to X, if the write and read are sufficiently separated in time and no other writes to X occur between the two accesses?

The written value is returned. Signup and view all the answers

What is the significance of the serialization notion in maintaining cache coherence?

Writes to the same location are seen in the same order by all processors. Signup and view all the answers

What is the purpose of a status bit associated with a cache block in cache coherence protocols?

To keep track of the status of any sharing of a data block. Signup and view all the answers

What is the main difference between snooping and directory-based cache coherence protocols?

Snooping tracks sharing status in every cache, while directory-based protocols track it in a central location. Signup and view all the answers

Why is cache coherence essential in multiprocessor systems?

To ensure that processors see the same order of writes to the same location. Signup and view all the answers

What is the primary advantage of using a directory-based cache coherence protocol over snooping-based protocols?

It is more scalable and efficient for large-scale systems. Signup and view all the answers

What is the state change of a cache block when it is first read, even if there is only one copy?

The state changes from invalid to shared. Signup and view all the answers

What triggers the transition from the shared state to the exclusive state in a cache coherence protocol?

A CPU write hits the shared block. Signup and view all the answers

What is the purpose of the coherence action in a cache coherence protocol?

To place an invalidate on the bus. Signup and view all the answers

What is the main limitation of a single directory approach in a multicore environment?

It is not scalable. Signup and view all the answers

What happens when a CPU read miss occurs for a cache block in the invalid state?

A regular miss is placed on the bus. Signup and view all the answers

What is the significance of the two perspectives shown in Fig. 8.4?

They illustrate the same finite-state machine from the CPU and bus perspectives. Signup and view all the answers

How do directories in a directory-based protocol track caches?

They track caches that share the memory addresses of the memory portion in the node. Signup and view all the answers

What is the key difference between the implementation of the directory-based protocol and snooping-based coherence protocols?

The directory-based protocol is based on message passing between nodes, whereas snooping-based coherence protocols are based on snooping on the bus. Signup and view all the answers

Where is the finite-state machine controller typically implemented?

In each core. Signup and view all the answers

Why is there no cache coherence problem with respect to multicomputers based on message passing?

Because each processor has its own independent memory and address space, and there is no other computer writing on its memory. Signup and view all the answers

What is the main advantage of multithreading?

It allows multiple threads to share the functional units of a single processor in an overlapping way. Signup and view all the answers

What is the key difference between multithreading and ILP?

Multithreading is a form of explicit parallelism, whereas ILP is a form of implicit parallelism. Signup and view all the answers

What is the main difference between multithreading and multiprocessing?

Multithreading shares most of the processor core among a set of threads, and duplicates just private state, whereas multiprocessing has multiple independent threads operating at once and in parallel. Signup and view all the answers

What is the primary advantage of simultaneous multithreading (SMT) over fine-grained and coarse-grained multithreading?

SMT can execute multiple instructions from independent threads without noticing dependencies among them, due to register renaming and dynamic scheduling. Signup and view all the answers

What is the main reason why fine-grained multithreading switches between threads on each clock cycle?

To interleave the execution of instructions from multiple threads and skip stalled threads. Signup and view all the answers

How do modern processors, such as Intel Core i7 and IBM Power7, implement multithreading?

They use simultaneous multithreading (SMT) to execute multiple instructions from independent threads concurrently. Signup and view all the answers

What is the primary drawback of coarse-grained multithreading?

It only switches threads on costly stalls, such as L2 or L3 cache misses, which can lead to inefficient processor utilization. Signup and view all the answers

What is the primary benefit of multithreading over a single-threaded processor?

Multithreading can execute multiple threads concurrently, improving thread-level parallelism and overall processor utilization. Signup and view all the answers

Study Notes

Cache Coherence

Cache coherence is a protocol that ensures all processors in a multi-processor system have a coherent view of memory.
In a coherent view of memory, a read by a processor to a location returns the written value if the write and read are sufficiently separated in time and no other writes to that location occur between the two accesses.

Cache Coherence Protocols

There are two main types of cache coherence protocols: snooping-based and directory-based.
Snooping-based protocols track the sharing status of a block in each cache containing a copy of the data from a physical memory block.
Directory-based protocols keep the sharing status of a particular block of physical memory in one location, the directory.

Snooping Coherence Protocols

Each cache has a copy of both memory block data and "share status" of a block, e.g., shared/non-shared.
The state changes from invalid to shared on the first reading of the block, even if there is only one copy.
On the first write, the state becomes exclusive.

Directory-Based Protocols

The directory is responsible for tracking caches that share the memory addresses of the memory portion in the node.
Each directory is responsible for tracking caches that share the memory addresses of the memory portion in the node.
The protocol needs to know which node has the block to make the invalidation.

Multicomputers

Multicomputers are processors with independent memories and address spaces.
There is no cache coherence problem in multicomputers based on message passing because each computer only writes to its own memory.

Thread-Level Parallelism

Multithreading is a way to exploit thread-level parallelism (TLP) in a processor.
TLP allows multiple threads to share the functional units of a single processor in an overlapping way.
Hardware approaches for multithreading include fine-grained, coarse-grained, and simultaneous multithreading (SMT).
Fine-grained multithreading switches between threads on each clock cycle, coarse-grained multithreading switches threads only on costly stalls, and SMT is the most common implementation.

Simultaneous Multithreading (SMT)

SMT is a variation of fine-grained multithreading implemented on top of a multiple-issue, dynamically scheduled processor.
SMT allows multiple instructions from independent threads to be executed without noticing dependencies among them.
Examples of processors that use SMT include Intel Core i7 and IBM Power7.

Studying That Suits You

Use AI to generate personalized quizzes and flashcards to suit your learning preferences.