Descriptor Cache Eviction for Multi-Queue DMA Systems
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Solution Overview
Problem
The Multi-Queue Direct Memory Access (MQ-DMA) system faces challenges in efficiently managing descriptors, leading to increased system latency and reduced bandwidth due to the time-consuming process of fetching descriptors from host memory.
Innovation Solution
A method is introduced that involves using a global eviction timer and local eviction timers to determine candidate descriptor lists for eviction from the cache memory, ensuring efficient descriptor management and data movement between host systems and peripheral devices.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of time
If descriptors are kept in the MQ-DMA system for a long time, then the system can avoid re-fetching descriptors and reduce latency, but unused descriptors occupy cache memory and prevent the system from obtaining other needed descriptors
Solution Approach 1:
The system performs preliminary actions by prefetching descriptors into the cache memory before they are actually needed for data movement operations. This allows the descriptors to be readily available when needed, reducing fetch latency while managing cache usage proactively through eviction policies.
Solution Approach 2:
The system changes the state parameter of descriptors by implementing different states (such as active, inactive, or candidate for eviction) and transitioning between them based on usage patterns. This allows dynamic management of descriptor lifetime in the cache, balancing between keeping descriptors available and freeing cache space.
2Quantity of substance
If descriptors are evicted quickly from the MQ-DMA system, then cache memory is freed for other descriptors, but frequently used descriptors may be evicted and need re-fetching, increasing latency
Solution Approach 1:
The system implements feedback mechanisms through eviction policies that monitor descriptor usage patterns and cache occupancy. When cache memory becomes full or usage patterns indicate descriptors are no longer needed, the system provides feedback to evict appropriate descriptors, balancing cache availability with latency reduction.
3Quantity of substance
If the MQ-DMA system actively manages a large set of descriptors, then more descriptors are available for data movement, but the complexity of managing these descriptors increases
Solution Approach 1:
The system segments descriptor management by organizing descriptors into multiple descriptor lists or queues, each potentially managed with different eviction policies or priorities. This segmentation allows the system to handle large numbers of descriptors by dividing them into manageable groups, reducing the complexity of tracking and managing individual descriptors.
Data Source
AI summary
Evicting queues from a memory of a direct memory access system includes monitoring a global eviction timer. From a plurality of descriptor lists stored in a plurality of entries of a cache memory, a set of candidate descriptor lists is determined. The set of candidate descriptor lists includes one or more of the plurality of descriptor lists in a prefetch only state. An eviction event can be detected by detecting a first eviction condition including a state of the global eviction timer and a second eviction condition. In response to detecting the eviction event, a descriptor list from the set of candidate descriptor lists is selected for eviction. The selected descriptor list can be evicted from the cache memory.