MESIF protocol

The MESIF protocol is a cache coherency mechanism developed by Intel for maintaining consistency across multiple processor caches in multiprocessor systems, particularly those employing point-to-point interconnects such as the QuickPath Interconnect (QPI).^[1] It extends the standard MESI (Modified, Exclusive, Shared, Invalid) protocol by adding a fifth state, Forward (F), which designates a single clean copy of shared data as the primary source for supplying additional copies to other caches, thereby enabling efficient cache-to-cache transfers and reducing latency in multi-socket environments.^[2] Invented by Herbert H. J. Hum and James R. Goodman and patented in 2005, MESIF was first proposed in 2001 and served as the foundation for QPI implementations in products like the Intel Core i7 processors and subsequent Xeon Scalable series.^[2]^[3] In MESIF, cache lines can reside in one of five states: M (Modified) for a dirty, writable copy held exclusively by one cache; E (Exclusive) for a clean, writable copy also held exclusively; S (Shared) for clean, read-only copies that multiple caches may hold without forwarding privileges; I (Invalid) indicating no valid data; and F (Forward), a specialized shared state where exactly one cache acts as the "first among equals" to forward data directly to requesters, minimizing broadcast traffic and ensuring at most one response per request.^[1]^[2] The protocol operates via source-snooping, where requests are broadcast from the requesting node to all others over point-to-point links, with the home node (associated with the memory address) coordinating acknowledgments and conflict resolution to maintain serializability.^[3] This design achieves two-hop latency for common operations like cache hits in E, M, or F states—compared to three hops in traditional directory-based protocols—while scaling hierarchically without requiring a central directory, offering 6-11% performance improvements in bandwidth-bound workloads on four-node systems at 4 GHz.^[3] MESIF supports both source-snoop modes for low-latency small systems and home-snoop modes with directory assistance for larger configurations, as implemented in Intel's Xeon processors to handle non-uniform memory access (NUMA) architectures efficiently.^[1]^[4]

Overview

Definition and Purpose

The MESIF protocol is a five-state cache coherency mechanism—comprising Modified (M), Exclusive (E), Shared (S), Invalid (I), and Forward (F) states—that extends the traditional MESI protocol to support cache coherent non-uniform memory access (ccNUMA) architectures in multi-core and multi-socket processor systems.^[1]^[3] It was developed to maintain data consistency across distributed caches connected via point-to-point interconnects, such as Intel's QuickPath Interconnect (QPI), without relying on a shared bus.^[1] The addition of the Forward state designates a single cache as the authoritative source for supplying additional shared copies of data, preventing redundant transmissions and ensuring efficient coherence enforcement.^[2] The primary purpose of MESIF is to guarantee that all processors in a multi-processor system observe a consistent view of memory, resolving the cache coherence problem where multiple caches might hold copies of the same data line.^[3] By implementing source snooping—where requests are routed through a home agent that forwards snoops to potential holders of the data—MESIF optimizes bandwidth usage in scalable systems, particularly by avoiding the need for a central directory that could become a bottleneck.^[1] This approach is especially beneficial for shared read operations, as the Forward state allows one designated cache to supply data directly to requesters, minimizing directory traffic and enabling 2-hop latency for common cache-to-cache transfers.^[3] MESIF addresses the limitations of earlier bus-based protocols, which struggled with scalability beyond a few processors due to broadcast overhead, by leveraging the high bandwidth and low latency of point-to-point links.^[3] In contrast to directory-based protocols that often require three or more hops for coherence actions, MESIF achieves comparable or lower latency for frequent operations like reads while scaling to larger node counts through hierarchical snooping mechanisms, thus reducing overall system bandwidth pressure without a centralized coherence directory.^[1]^[3]

History and Development

The MESIF protocol originated in 2001 as a source-snooping cache coherence mechanism designed for point-to-point interconnects in multiprocessor systems. It was proposed by Herbert H. J. Hum, an Intel engineer, and James R. Goodman, a researcher at the University of Wisconsin-Madison, to address latency issues in scaling beyond bus-based architectures. The protocol introduced a novel "Forward" (F) state to enable efficient data sharing without directories, allowing a single cache to forward shared data to requesters in a single round-trip (two-hop) latency, mimicking broadcast snooping while leveraging high-bandwidth links.^[3]^[2] Building on the foundational MESI protocol from earlier work presented at ISCA in 1986, MESIF evolved by extending the states to include Forward, optimizing for unordered point-to-point networks. Initial details appeared in a 2004 technical report by Goodman and Hum, which described the protocol's mechanics for hierarchical scalability and low-latency operations. A refined version followed in a 2009 report, emphasizing its role as a precursor to Intel's QuickPath Interconnect (QPI), which facilitated non-uniform memory access (NUMA) in multi-socket systems. This development occurred amid Intel's shift from front-side bus topologies to integrated memory controllers and on-chip rings. Key intellectual property was formalized in U.S. Patent 6,922,756, granted in 2005 to Hum and Goodman, which detailed the Forward state's use in resolving coherence conflicts efficiently.^[5]^[3]^[2] A major milestone came with the integration of MESIF into Intel's Nehalem microarchitecture, launched in November 2008 with the Core i7 processors and Xeon 5500 series, marking the first commercial deployment of the protocol in production silicon. This enabled inclusive L3 caching and QPI links for multi-core coherence without excessive traffic. MESIF continued in successor architectures, including Westmere (launched March 2010), which refined Nehalem's 45 nm process while retaining the protocol for improved power efficiency, and Sandy Bridge (launched January 2011), which extended it to support AVX instructions and higher core counts via enhanced ring interconnects. These integrations solidified MESIF as a cornerstone of Intel's multi-socket scalability through the early 2010s. The protocol persisted beyond QPI with the introduction of Ultra Path Interconnect (UPI) in Skylake-SP (2017) and remains in use in the Xeon Scalable series, including the 5th generation (as of 2024).^[6]^[7]^[8]

Protocol States

State Descriptions

The MESIF protocol employs five distinct states for managing cache lines in a multiprocessor system, building upon the four states of the MESI protocol by introducing a Forward state to optimize shared data handling.^[2]^[3] The Modified (M) state indicates that a cache line has been altered by the local processor and holds the only valid copy in the system, differing from the main memory content. This state signifies exclusive ownership, requiring the modified data to be written back to memory upon eviction to maintain coherence.^[2]^[3] In the Exclusive (E) state, the cache line contains a clean copy that matches the main memory and is the sole valid instance across all caches. This exclusive access allows the local processor to modify the line without notifying other caches, as no other copies exist.^[2]^[3] The Shared (S) state denotes a clean cache line that matches main memory and can be present in multiple caches simultaneously. It permits read-only access by multiple processors, ensuring all copies remain consistent without modifications.^[2]^[3] The Invalid (I) state means the cache line holds no valid data and must be fetched from main memory or another cache if accessed. This state is used for lines that have been invalidated due to coherence actions, rendering them unusable until repopulated.^[2]^[3] The Forward (F) state represents a clean, shared copy akin to the Shared state, but designates this particular cache as the primary responder for future read requests to the line, enabling direct cache-to-cache data forwarding without involving the directory or memory. Unlike the Modified state, the Forward state is discardable—meaning the cache can drop the line or transition it to Shared without notifying other caches or writing back to memory—since it maintains consistency with main memory. This designation ensures a single point of response among shared copies, reducing protocol overhead in multi-cache environments.^[2]^[3]

Permitted State Combinations

The MESIF protocol enforces strict compatibility rules among cache states to maintain coherence while optimizing for shared data access in multi-core and multi-socket systems. These rules ensure that exclusive states like Modified (M) and Exclusive (E) cannot coexist with shared states, preventing multiple writable copies of the same cache line. Specifically, a cache line in the M state—indicating a unique, dirty copy—can only pair with I states in all other caches, as any shared presence would violate exclusivity. Similarly, the E state, representing a unique, clean copy, is compatible solely with I states elsewhere, allowing efficient upgrades to M without invalidations.^[3]^[9] In contrast, the Shared (S) state supports multiple copies across caches and can coexist with other S states, a single F state, or I states, enabling efficient read sharing without coherence overhead. The Forward (F) state, which designates a unique "forwarder" for read requests among shared copies, pairs with S or I states in other caches but is restricted to exactly one instance per cache line system-wide; multiple F states are prohibited to avoid conflicting responses and ensure ordered data forwarding. This uniqueness invariant for F, akin to exclusivity for M and E, maintains a single point of responsibility for servicing subsequent reads.^[2]^[3] The following table summarizes the permitted state combinations for a given cache line across multiple caches, focusing on pairwise compatibility while respecting global invariants like the single F rule:

Primary State	Compatible States in Other Caches	Notes
M	I	Exclusive dirty; no shared copies allowed.
E	I	Exclusive clean; no shared copies allowed.
S	S, F, I	Multiple S permitted; at most one F system-wide.
F	S, I	Unique forwarder; pairs with shared or invalid copies only.
I	Any (M, E, S, F, I)	Invalid state is compatible with all configurations.

These combinations prevent conflicts such as multiple modified copies, which could lead to data inconsistencies, while permitting efficient sharing of clean data in S or F states without forcing exclusivity or unnecessary invalidations. By limiting F to one cache line, the protocol ensures deterministic response ordering during snoops, reducing latency in point-to-point interconnects like Intel's QuickPath.^[2]^[9]^[3]

Operations and State Transitions

Read Operations

In the MESIF protocol, read operations primarily involve two types of requests: ReadShared (RS) and Read for Ownership (RFO). These requests are broadcast from the requester to the home agent and all peer caches to fetch data while maintaining coherence.^[5] The ReadShared (RS) request is used when a cache needs to obtain a shared copy of data without exclusive ownership. Upon broadcasting the RS request, peer caches in the Modified (M), Exclusive (E), or Forward (F) states respond with acknowledgments (IACK for invalid copies or SACK for shared copies), while a cache in the F state—the designated forwarder—supplies the data directly to the requester and transitions its own copy to the Shared (S) state. The requester then transitions to the F state if it receives data from the forwarder (becoming the new supplier for future requests) or to the S state if the data comes from the home agent or another shared source; if the data is uncached, the home agent provides it, allowing the requester to enter the E state. This mechanism ensures that uncontended RS requests complete in two hops without involving the home agent for data transfer, reducing latency.^[5] The Read for Ownership (RFO) request, while often associated with writes, is also employed for reads that require exclusive access to data, such as when a cache anticipates modifications. Similar to RS, the RFO is broadcast, prompting peers to acknowledge and invalidate their copies: caches in S or F states send IACK after invalidating (transitioning to Invalid, I), while a supplier in M, E, or F states forwards the data to the requester and invalidates its own copy. The requester acquires the data in the M state (if modified) or E state (if clean), ensuring no other cache holds a valid copy. This invalidation process distinguishes RFO from RS, as it clears all remote copies to grant ownership.^[5] Conflict handling in read operations occurs when multiple requests target the same cache line simultaneously. The home agent queues conflicting requests, resolves the order, and directs data forwarding using messages like Data Acknowledgment (DACK) to release the supplier and Transfer (XFR) to route data between requesters, ensuring only a single response carries the data to avoid bandwidth waste. This queuing and directed forwarding maintain protocol correctness in multi-request scenarios.^[5] Overall, MESIF read operations achieve low latency—typically two hops for cache hits via the F state forwarder—by leveraging point-to-point links and the single forwarder designation, which simplifies responses and prevents multiple data transmissions.^[5]

Write Operations

In the MESIF protocol, write operations are initiated through a Read For Ownership (RFO) transaction when a processor core requires exclusive access to a cache line for modification. The requester broadcasts an RFO request across the interconnect to acquire ownership, which inherently includes invalidating all other copies of the line in remote caches to ensure coherence.^[3] This broadcast targets all nodes, prompting responses that facilitate data transfer if necessary and confirm invalidations.^[3] If a supplier cache holds the line in the Modified (M), Exclusive (E), or Forward (F) state, it responds by forwarding the data to the requester and then invalidating its own copy, transitioning to the Invalid (I) state. For instance, in the M or E state, the supplier directly supplies the data and invalidates immediately upon acknowledgment from the requester. In the F state, the supplier forwards the data, temporarily transitions to Shared (S), and then invalidates to I after receiving a data acknowledgment (DACK) from the requester. Upon receiving the data, the requester transitions the line to the Modified (M) state, granting it permission to write without further coherence checks.^[3]^[2] Caches holding the line in the S state respond by invalidating to I without supplying data, sending an IACK to the requester, while the F cache supplies data as noted above. This ensures that non-owners do not interfere with ownership transfer, maintaining efficiency in multi-core environments. The home agent (typically the last-level cache or memory controller) then confirms ownership by sending an ACK to the requester after all responses are collected, completing the RFO transaction.^[3]^[10] Writebacks of modified lines occur exclusively during eviction events, not as part of the RFO process itself; the M-state data remains local until the line is displaced from the cache. This design avoids unnecessary memory traffic during active writes. The home agent serializes conflicting write requests using snoop filters to track sharers and resolve contention, queuing non-winning requests and directing data forwarding from the current owner when applicable.^[3]^[10]

Eviction and Writeback

In the MESIF protocol, the eviction of a cache line occurs when a cache needs to allocate space for a new line and selects an existing line for replacement. The actions taken during eviction depend on the line's current state to maintain coherence while minimizing unnecessary traffic. Cache lines in the Invalid (I) state contain no valid data and can be discarded freely without any notification to the home agent or other caches. Lines in the Shared (S) or Forward (F) states, which hold clean copies of data potentially present in other caches, are also discarded without notice or data transfer, as no modifications have occurred. Similarly, lines in the Exclusive (E) state, representing a unique clean copy, are simply removed from the cache without writing back to memory.^[3]^[11] The most involved eviction process applies to lines in the Modified (M) state, which hold dirty data that has not yet been propagated to memory. Upon eviction, the owning cache issues a Writeback (WB) request to the home agent, transferring the modified data along with ownership of the line.^[3] This WB transaction ensures that the dirty data is committed to memory at the home node before the line is invalidated in the originating cache. The home agent updates the memory copy and acknowledges the writeback, after which the originating cache transitions the line to the Invalid (I) state and can respond with an IACK (Invalid Acknowledge) to any pending snoops.^[3] Writeback operations integrate with ongoing protocol transactions but may introduce conflicts, particularly with concurrent ReadShared (RS) or ReadForOwnership (RFO) requests. During a WB, the originating cache stalls incoming snoops to prevent race conditions until the home agent provides acknowledgment, ensuring serializability.^[3] The home agent queues any conflicting requests and resolves them by prioritizing the writeback, directing data forwarding as needed, and notifying losers to retry after completion.^[3] This mechanism allows the WB to restore coherence without immediately establishing a shared state across all caches, reducing unnecessary snoop traffic in multi-node systems.^[11]

Implementation and Usage

In Intel Architectures

The MESIF protocol was first integrated into Intel architectures with the Nehalem microarchitecture in 2008, enabling multi-socket cache coherence through the QuickPath Interconnect (QPI). QPI, a point-to-point serial interconnect operating at up to 6.4 GT/s, facilitates source-snooping for efficient data sharing across sockets in systems like the Xeon 5500 series. In this implementation, each processor's uncore includes a home agent per node that manages snoop filters to track cache states without requiring a full directory structure, relying instead on inclusive last-level cache (LLC) policies to minimize inter-socket traffic.^[12]^[3] MESIF supports 2-hop data transfers via QPI's point-to-point links, where shared data in the Forward (F) state can be sourced directly from the most recent accessor, reducing latency compared to traditional 3-hop schemes. Snoop operations occur in modes such as home snoop, where the memory home resolves conflicts, and remote snoop, where requests broadcast to caching agents for responses like shared acknowledgments or data forwarding. This source-snooping approach, with per-core bits in the LLC tracking copies, cuts snoop disruptions by up to 30% in small-scale systems.^[3]^[12] The protocol was extended to subsequent architectures, including Skylake in 2017, where QPI evolved into the Ultra Path Interconnect (UPI) while retaining MESIF for coherence in multi-socket Xeon Scalable processors. UPI maintains similar source-snooping mechanics with home agents and snoop filters, supporting up to three links per socket at speeds reaching 10.4 GT/s, and integrates directory-based enhancements for larger configurations.^[13]^[14] This hybrid approach continued in the Sapphire Rapids microarchitecture in 2023, which employs MESIF with combined snooping and directory mechanisms over UPI 2.0 to scale for high-core-count systems supporting up to 60 cores per socket. The protocol was further extended in the Emerald Rapids refresh later in 2023 and the Granite Rapids architecture in 2024, maintaining MESIF coherence over UPI links (up to UPI 2.0 at 24 GT/s per link in Granite Rapids) for multi-socket configurations with up to 128 cores per socket as of 2025.^[15]^[16]^[17]

Advantages and Performance

The MESIF protocol provides key advantages in bandwidth efficiency and latency reduction for cache coherence in multi-socket systems. The Forward (F) state ensures that only one cache responds to read requests for shared clean data, eliminating redundant replies from multiple sharers that would occur in protocols like MESI, thereby reducing network traffic and on-chip bandwidth usage for shared reads.^[3]^[18] This optimization is particularly beneficial in point-to-point interconnects like Intel's QuickPath Interconnect (QPI), where it leverages higher link bandwidth compared to bus-based designs while maintaining snooping-like behavior. In terms of latency, MESIF achieves a 2-hop access for common operations involving hits in Exclusive, Modified, or Forward states, avoiding the 3-hop delay typical of directory-based protocols for cache-to-cache transfers.^[3] Measurements on Intel Haswell-EP processors using QPI show remote L3 cache latencies of approximately 113 ns for modified data hits and 146 ns for remote memory accesses, demonstrating effective low-latency coherence in multi-socket configurations.^[4] Performance evaluations indicate tangible gains in bandwidth-bound workloads. Simulations of four-node systems report 6-11% improvements in TPC-C transaction processing performance at 4 GHz, especially with 60-90% cache-to-cache hit rates.^[3] These benefits scale well for small-to-medium node counts up to 8 sockets, where MESIF's source-snooping approach outperforms pure directory methods in efficiency.^[3] However, in larger systems, the protocol's dependence on broadcast snoops incurs overhead, limiting its suitability for configurations beyond 16 cores without hybrid directory extensions.^[3]

Comparisons

With MESI Protocol

The MESIF protocol extends the four-state MESI (Modified, Exclusive, Shared, Invalid) coherence protocol by introducing a fifth Forward (F) state, which is absent in MESI and enables optimized handling of read-shared data.^[9] In MESI, shared reads typically require relaying requests through a home agent or directory, resulting in additional network hops and increased bandwidth consumption as multiple caches respond or the data is fetched from memory.^[9] This relay mechanism in MESI treats all shared cache lines uniformly in the S state, often leading to redundant responses from multiple sharers and higher latency in distributed systems.^[19] In contrast, MESIF's F state permits direct cache-to-cache transfers for shared data, where one designated forwarder cache supplies the line to the requester without home agent intervention, reducing both hops and bandwidth usage for frequent read-sharing scenarios.^[9] The F state, held exclusively by one agent, allows the forwarder to respond immediately while other sharers remain in S, streamlining coherence traffic compared to MESI's broadcast or directory-based snoop mechanisms that can generate multiple responses.^[19] These enhancements come with trade-offs: MESIF introduces greater protocol complexity through the additional state and associated state transitions, increasing design and verification overhead, but it is optimized for point-to-point interconnects in multi-socket systems.^[9] MESI, being simpler with fewer states, is better suited for bus-based architectures where snooping is efficient and shared data access patterns do not benefit as much from forwarding optimizations.^[19] MESIF is particularly advantageous in scalable non-uniform memory access (NUMA) environments, such as those using Intel's QuickPath Interconnect, where direct forwarding mitigates the latency penalties of distributed memory.^[9] Conversely, MESI remains preferable for uniform memory access (UMA) systems, like single-socket or bus-coherent multiprocessors, where its simplicity aligns with lower interconnect complexity and uniform access latencies.^[19]

With MOESI Protocol

The MESIF protocol differs from the MOESI protocol primarily in their additional states beyond the basic MESI framework. In MESIF, the Forward (F) state represents a clean, shared cache line where one cache is designated as the forwarder to supply data to other requesters, ensuring only a single response to snoop requests and reducing redundant traffic. In contrast, MOESI introduces the Owned (O) state, which permits a cache line to be both modified (dirty) and shared among multiple caches without requiring an immediate writeback to memory; this allows the owning cache to supply the dirty data directly to sharers while remaining responsible for eventual coherence with main memory. Unlike MOESI's O state, MESIF does not support dirty sharing, as its Modified (M) state remains exclusive to one cache, forcing writebacks for any sharing of modified data. Architecturally, both protocols are adapted for point-to-point interconnects rather than traditional shared-bus systems, with MESIF optimized for Intel's QuickPath Interconnect (QPI) in systems like Nehalem processors featuring inclusive L3 caches, where the F state leverages core valid bits in the L3 to minimize snoop broadcasts. MOESI, employed by AMD in architectures like Shanghai processors with non-inclusive L3 caches and HyperTransport links, uses the O state to handle coherence in victim-cache-like structures, avoiding full L3 inclusion to reduce tag storage but potentially increasing main memory accesses for line transfers. In terms of efficiency, MESIF's F state reduces clean shared traffic by designating a single forwarder, preventing multiple caches from responding to read requests and thus lowering interconnect bandwidth usage in scenarios with frequent clean data sharing. MOESI's O state optimizes dirty sharing by enabling direct transfers of modified data between caches without writebacks, which minimizes memory traffic in producer-consumer patterns but may require additional writebacks when ownership changes, potentially increasing latency compared to MESIF's cleaner handling of unmodified lines. Overall, MESIF avoids writebacks for owned clean data through forwarding, suiting its inclusive hierarchy, while MOESI's approach trades off more writebacks for the flexibility of dirty sharing. Performance-wise, MESIF demonstrates advantages in read-heavy workloads due to its lower latency for shared on-chip accesses (e.g., 13 ns in Nehalem vs. 15.2 ns in Shanghai, attributable to inclusive L3 design) and higher L3 read bandwidth (23.9 GB/s vs. 10.3 GB/s), benefiting from reduced snoop overhead.^[19] In write-heavy scenarios, MOESI's O state provides an edge for accesses to remote modified lines by allowing direct dirty sharing, though MESIF maintains competitive write bandwidth (19.3 GB/s L3 write in Nehalem vs. 9.4 GB/s in Shanghai) through efficient exclusive modifications.^[19] These differences highlight MESIF's suitability for read-dominated multi-core applications and MOESI's for workloads involving frequent dirty data propagation.