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What role does object technology play in structuring distributed services?
Object technology facilitates inheritance and reuse, which helps in organizing distributed services effectively.
Why is academia considered a rich source of technological innovation?
Academia encourages out-of-the-box thinking unhindered by market pressures, allowing for groundbreaking innovations.
How can byproducts of thought experiments influence technological advancements?
Byproducts of thought experiments can lead to unexpected innovations, such as sticky notes originating from space research.
What is the significance of memory in distributed systems?
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What is the main question addressed by a Global Memory System (GMS)?
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What is the goal of Distributed Shared Memory (DSM) in a cluster?
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How does the Distributed File System (DFS) utilize cluster memory?
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What common theme is explored across the three distributed subsystems discussed?
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What role does GMS play in utilizing global memory as a substitute for disk paging?
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How does Host P adjust its memory after copying a shared page from Host Q?
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What happens to Host R's memory if the globally oldest page it evicts comes from its local memory?
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Why does the total memory pressure in the cluster increase when page X is shared between Host P and Host Q?
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What does GMS not handle regarding shared pages?
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How do idle nodes function within the GMS framework?
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What does the dynamic local-global memory split signify in GMS?
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Explain the significance of age management in GMS.
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What must Host P do when it experiences a page fault on page X while it is shared with Host Q?
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Describe the impact of a globally oldest page being evicted from global memory.
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What happens to Host Q's memory when page X is copied for Host P?
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How is cluster-wide memory pressure managed within GMS?
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What is the purpose of the epoch-based management concept in GMS?
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What triggers the end of an epoch in the GMS system?
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How does the initiator node determine which pages should be replaced?
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What information do nodes share at the start of each epoch?
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What does a node's weight represent in the GMS system?
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How are dynamic initiator assignments beneficial in GMS?
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What happens to a page with an age greater than the minimum age during a page fault?
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How does the GMS system approximate global LRU page replacement?
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What role does historical page usage play in the GMS algorithm?
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What determines the next initiator node in the GMS system?
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What is one significant task performed by the initiator node?
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What is the expected outcome of a page fault when the page's age is less than the minimum age?
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Why is it important for nodes to receive weight and age information during an epoch?
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What is the significance of the min age in the context of page replacement?
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What is the primary role of GMS in memory management across a cluster?
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How does GMS collect age information for approximating global LRU?
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What is the purpose of the Page Frame Directory (PFD) in GMS?
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Define the components of a Universal ID (UID) used by GMS.
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What is the function of the Global Cache Directory (GCD) in the GMS architecture?
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Explain the role of the Page Ownership Directory (POD) in GMS.
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How does the pageout daemon work with GMS?
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What happens when a page fault occurs in a distributed memory system managed by GMS?
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Why is the Distributed Hash Table important for the Global Cache Directory (GCD)?
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What distinguishes a private page from a shared page in the context of GMS?
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How does GMS enable efficient distributed memory management?
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What is the significance of intercepting read/write calls in the unified buffer cache (UBC)?
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Describe the data structure utilized to ensure that every node has up-to-date ownership information in GMS.
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What approach does GMS use to manage the lifecycle of free page frames in a cluster?
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What is the primary goal of the Global Memory System (GMS)?
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How does the role of the Virtual Memory Manager differ between traditional systems and GMS?
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What is memory pressure in the context of distributed systems?
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Explain the concept of working set in relation to virtual memory.
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What happens during a page fault in a GMS environment?
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What is the difference between private pages and shared pages in GMS?
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Why can network memory access be faster than disk access in GMS?
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What dynamic adjustments are made to physical memory in GMS based on memory pressure?
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How does GMS ensure fault tolerance during node crashes?
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What is meant by the term 'dirty pages' in the context of GMS?
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What advantages does GMS provide in handling memory pressure compared to traditional paging systems?
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In the memory hierarchy with GMS, where is cluster memory positioned?
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Why is the coherence of shared pages a concern in GMS, and who is responsible for it?
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How does GMS handle the eviction of pages from physical memory?
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What happens in a system when a node misses retrieving a page due to stale POD information?
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In the common case, why is page fault handling considered fast?
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What triggers a node's paging daemon to start evicting pages?
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What role does weight information play in the page eviction process?
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How does the paging daemon in GMS differ from traditional paging daemons in virtual memory systems?
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What process must occur after a page is sent to a new node during eviction?
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What is the purpose of retrying after a miss in page retrieval?
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Why are misses considered relatively infrequent in page retrieval processes?
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What is the significance of handling page evictions in batches?
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How does GMS ensure efficient management of page ages across the cluster?
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What is the primary goal of the Virtual Memory (VM) system in OSF/1?
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How does the Unified Buffer Cache (UBC) enhance file system performance?
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What modification was necessary for integrating GMS into the OSF/1 memory system?
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What role does the Pageout Daemon play in memory management?
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In the GMS system, what does the Virtual Memory Manager do when a page fault occurs?
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What is the primary function of the Free List in OSF/1's memory management?
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Explain the significance of collecting age information for pages in GMS.
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What challenge does the GMS face in tracking age information for anonymous pages?
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What happens to dirty pages during the page eviction process in GMS?
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How does GMS handle page faults compared to the traditional system?
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Why is the modification of handling global page replacement significant in GMS?
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What does the TLB (Translation Lookaside Buffer) store, and why is it utilized in GMS?
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What change occurs in the UBC when handling page faults in the GMS?
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How does the GMS integration influence the overall architecture of OSF/1?
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What is the purpose of the Page Ownership Directory (POD) in a distributed system?
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How does a node determine which Page Frame Directory (PFD) contains the mapping for a given UID?
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Explain how a node handles a page fault in a distributed system.
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What happens if the page is no longer in memory during a page retrieval?
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Why is the communication with the POD always local?
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Describe the efficiency considerations related to network communication during page fault handling.
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What is a common case optimization during page fault handling in a distributed system?
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What are the potential consequences of stale POD information?
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How does the node retrieve a page located at another node?
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What is the significance of the Global Cache Directory (GCD)?
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What role does the Page Frame Directory (PFD) play in memory management?
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How can page replacement affect the request fulfillment process?
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What occurs when a node sends a request to another node during a page fault?
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Summarize the overall process of handling a page fault.
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What is the primary function of GMS in memory management?
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How does GMS decide which page to replace when memory pressure increases?
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Describe the memory structure of each host in GMS.
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What occurs in Host P's memory allocation when it encounters a page fault for page X and finds it in Host Q's global cache?
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In the case of extreme memory pressure on Host P, what is the strategy for handling a page fault when no global memory is available?
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What happens when a page fault occurs and the required page is not available in any cluster memory?
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Explain the memory adjustment process for Host R when it receives an evicted page from Host P.
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What is the impact on total physical memory of a host when page transfer occurs between Host P and Host Q?
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How does GMS handle the coherence of shared pages in memory?
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What key technical challenge does GMS face when managing page replacement?
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In GMS, what adjustment needs to occur in Host P's memory allocation during a page fault when it increases local memory?
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When Host R receives a page from Host P, what condition determines whether Host R can simply discard its oldest page?
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Identify the role of global memory in the context of GMS.
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What effect does a page fault have on Host P's memory distribution during standard page fault handling?
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What is the significance of dynamically managing memory allocation between local and global memory in GMS?
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Study Notes
Distributed Systems: Definitions and Principles
- Object technology, specifically inheritance and reuse, is key to structuring distributed services.
- Technological innovation frequently originates in academia where researchers are free from market pressures and existing product lines.
- Java arose from a failed attempt to create a video-on-demand system in the 1990s, highlighting the potential for valuable byproducts to emerge from research.
- Distributed systems often prioritize efficient memory utilization.
- Three primary subsystems, each focused on memory optimization:
- Global Memory System (GMS) - efficient paging across a LAN.
- Distributed Shared Memory (DSM) - simulating shared memory abstractions for programmers.
- Distributed File System (DFS) - using cluster memory for file caching.
Memory Management in Distributed Systems
- Memory is a critical resource in distributed systems where idle memory on peers within a network can be leveraged for local memory shortages.
- The Global Memory System (GMS) solves the challenge of scarce local memory by utilizing idle memory from other nodes on the network, reducing the need for disk access.
- Network speed (Gigabit Ethernet, 10Gbps) is essential for making network memory access viable and potentially faster than disk access.
- GMS keeps a copy of all pages on the disk for fault tolerance, mitigating the loss of clean pages in the event of a node crash.
Global Memory System (GMS) - Core Functionality
- The memory hierarchy with GMS includes: Processor, Caches, Main Memory, Cluster Memory, and Disk (used only for data not in cluster memory).
- For page faults, GMS first examines cluster memory before accessing the disk, reducing dependence on slower disk.
- Evicted pages can be sent to peer nodes instead of the disk, improving access time when needed later.
Community Memory Service (CMS)
- GMS splits physical memory into local and global sections.
- Local: Stores the working set of running processes (private or shared).
- Global: Stores swapped-out pages from peers, functioning as a memory cache for peers.
- This split is dynamic, adjusting based on local memory pressure.
- GMS does not handle coherence for shared pages. Responsibility for ensuring coherence lies with the application.
- Page replacement employs a globally oldest page policy, where the oldest page in the cluster is replaced, which requires managing age information across nodes.
GMS Page Fault Handling
- Case 1: Page Found in Global Cache: Host P encounters a page fault, locates the page in global memory, retrieves the page from the host with the global cache.
- Case 2: Extreme Memory Pressure: Hosts global memory is depleted and must evict a page from its working set to accommodate a new page.
- Case 3: Page Only on Disk: The page must be fetched from the disk. One page in the global cache is evicted and sent to a peer host with the globally oldest page.
- Case 4: Handling Shared Page: GMS copies the shared page to the requesting host, increasing the cluster's memory pressure. GMS does not handle coherence for shared pages.
GMS Behavior and Node Activity
- GMS swaps pages between nodes, allowing idle nodes to act as memory caches for other nodes.
- Idle nodes gradually transition into memory servers, dedicating their resources to storing pages for peers.
- The local and global memory split is dynamic, constantly changing based on node activity and memory pressure.
Age Management in GMS
- Aims to identify the globally oldest page for replacement across nodes in a distributed system.
- Prioritizes distributed management to avoid overloading any single node.
Epoch-Based Management
- Management is divided into epochs, each with a maximum duration (T) and a maximum number of page replacements (M).
- Epochs end when either T or M is reached.
- A new manager node, called the initiator, is selected for the next epoch.
Process During Each Epoch
- Nodes send age information (indicating page usage) to the initiator at the start of each epoch.
- Smaller age indicates more relevant pages (recently used), larger age indicates less relevant pages.
- The initiator calculates the minimum age of the pages to be replaced in the upcoming epoch.
- Pages older than the minimum age are replacement candidates.
- Pages younger than the minimum age survive to the next epoch.
- The initiator assigns each node a replacement weight, representing the fraction of pages anticipated for replacement from that node.
- Nodes receive the minimum age, their own weight, and the weights of other nodes.
Dynamic Initiator Assignment
- The initiator role is not static to prevent overburdening a single node.
- The least active node (with the highest weight) becomes the initiator for the next epoch.
- All nodes receive the weight distribution from the current initiator and can locally determine the next initiator.
Age Management and Replacement Decision
- The minimum age represents the M oldest pages across all nodes.
- A page fault triggers replacement of a page (page y), depending on its age.
- If page y's age is greater than the minimum age, it's a replacement candidate and can be discarded.
- If page y's age is less than the minimum age, it's active and should be sent to a peer for storage in their global cache.
Choosing a Peer
- The node uses the weight distribution to choose a peer to send the page.
- Higher weight nodes are more likely candidates for eviction, but the choice is not rigid.
- Weight influences the decision but does not completely control it.
Approximation of Global LRU
- The GMS algorithm approximates global least recently used (LRU) page replacement.
- This is achieved by gathering information from all nodes about page ages and computing minimum age and weight at the start of each epoch.
- Each node then makes localized decisions based on the age and weight information during the epoch.
GMS Implementation in Unix
- The GMS concept requires modifications to the operating system to redirect page fault handling to cluster memory instead of the disk.
- Modifications are made to the virtual memory (VM) and unified buffer cache (UBC) to interact with the GMS.
- GMS collects age information from both VM and UBC, which can be challenging for anonymous pages in VM, requiring periodic TLB dumps.
- The pageout daemon collaborates with GMS to place clean pages in cluster memory instead of discarding them.
- The free list interacts with GMS for distributed page frame management across the cluster.
Data Structures in GMS
- GMS uses distributed data structures for cluster-wide virtual memory management.
- UID (Universal ID): A global identifier derived from the virtual address.
- Page Frame Directory (PFD): Maps UIDs to physical page frames or disk locations, similar to a cluster-wide page table.
- Global Cache Directory (GCD): Manages the mapping between UIDs and nodes that hold their PFDs.
- Page Ownership Directory (POD): Tracks ownership information for pages and ensures memory consistency across the cluster.
Virtual Address to UID Conversion
- Virtual address (VA) is local to a single processor.
- UID is a global identifier, combining components like IP address, disk partition, i-node, and offset.
Data Structures Continued
- Global Cache Directory (GCD): A partitioned hash table spread across the cluster, avoiding the need for broadcasting.
- Page Ownership Directory (POD): Replicated on all nodes, ensuring all nodes have consistent ownership information.
- Page Frame Directory (PFD): Provides the final mapping between UID and physical page frame or disk location.
Page Fault Handling Workflow
- The virtual address is converted to a UID.
- The POD is consulted to determine which node has the relevant portion of the GCD.
- The GCD is queried to find the PFD corresponding to the UID.
- The PFD is then queried for the UID to determine the page's location.
Dynamic Nature of the POD
- The POD is updated only when nodes are added or removed from the cluster.
- When updates occur, the POD is redistributed and replicated across all nodes.
Key Takeaways
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The GCD enables efficient mapping of UIDs to the corresponding PFD, avoiding broadcasts.
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The POD provides information about which part of the GCD to consult.
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The PFD provides the final mapping, facilitating efficient distributed page fault handling.### Handling Page Faults
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When a page fault occurs, the node responsible for the fault converts the virtual address to a UID (Universal ID), a global identifier for the page within the cluster.
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The node then consults the local copy of the Page Ownership Directory (POD) which is replicated across all nodes. The POD indicates which node manages the Global Cache Directory (GCD) for the given UID.
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The node sends the UID to the owner node, which then queries its Global Cache Directory (GCD) to determine which node holds the Page Frame Directory (PFD) for the UID. The owner node forwards the UID to the node with the PFD.
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The node with the PFD then retrieves the page from its memory or disk and sends it back to the node that encountered the fault.
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Retrieving the page from another node is considered comparable to disk access in traditional systems, making the network communication overhead acceptable.
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In the common scenario where the page is not shared, the POD and GCD reside on the same node, eliminating extra network communication for lookup. This significantly speeds up the process.
Possible Miss Scenarios
- When a page is evicted from a node's memory, the GCD might not be updated immediately.
- If the node that encountered the page fault requests the page before the GCD update, the retrieval will fail, resulting in a "miss."
- Stale information in the POD, caused by changes in the UID space distribution, can also lead to misses.
- This occurs when a node contacts the wrong node based on outdated POD information, resulting in an incorrect routing of the request.
- Misses are relatively infrequent and are handled by retrying the retrieval process after consulting the updated POD or GCD.
Page Eviction Process
- The paging daemon in GMS manages page evictions in batches, triggering them when free memory falls below a certain threshold.
- When a node decides to evict a page, it sends the page to another node in the cluster, using weight information to select a suitable candidate.
- Unlike traditional systems where pages are sent to disk on eviction, GMS sends evicted pages to another node's memory.
- The paging daemon also updates the GCD to reflect the new location of the evicted page.
- This process involves sending an update message to the UID owner, ensuring the GCD accurately maps the UID to the new node's PFD.
GMS Overview
- GMS leverages network memory as a paging device, bypassing traditional disk access for faster performance.
- Efficient global age management across the cluster ensures balanced load distribution, preventing any single node from being overwhelmed.
- Cluster-wide memory management in GMS utilizes distributed data structures (POD, GCD, PFD) to manage pages across the cluster. The paging daemon serves as a central coordinator for local page management and GMS interactions, ensuring smooth and efficient page evictions and updates.
- GMS is designed to minimize overhead while maintaining good performance during page faults and evictions.
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Description
Explore the foundational concepts and principles of distributed systems, focusing on memory management techniques. This quiz covers object technology, subsystems like Global Memory System, Distributed Shared Memory, and Distributed File System, as well as the significance of efficient memory utilization. Understanding these topics is essential for anyone studying or working in distributed computing.