Deferring DMA Memory Registration for Virtualized Hosts
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Solution Overview
Problem
In virtualized computer systems, pinning large amounts of host memory for Direct Memory Access (DMA) operations prevents memory overcommitment, which is undesirable for systems running multiple virtual machines, as it limits the ability to efficiently utilize memory resources.
Innovation Solution
The method of deferring memory registration and pinning for DMA operations, particularly for local memory access, by generating a local key for memory regions and queuing work requests until the total pinned memory falls below a threshold, allowing for dynamic management of memory usage and reducing the need for excessive pinning.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If memory regions are registered and pinned for DMA operations, then DMA operations can be performed reliably, but the amount of pinned memory increases preventing memory overcommitment
Solution Approach 1:
The system performs preliminary registration of memory regions with the RDMA adapter before actual DMA operations are needed, generating local keys in advance. This allows the memory to be registered and pinned beforehand, ensuring reliability when DMA operations are subsequently performed, while the preliminary nature of this action allows for better memory management planning.
Solution Approach 2:
The system dynamically manages memory registration by deferring the actual pinning operation until it is truly needed. Memory regions can be registered with the RDMA adapter without immediate pinning, and the pinning is dynamically activated only when local keys are issued and DMA operations are actually required, making the pinned memory quantity flexible rather than static.
2Productivity
If memory is overcommitted to support multiple virtual machines, then memory utilization efficiency improves, but the ability to perform DMA operations is limited
Solution Approach 1:
The system segments memory management into two distinct phases: registration phase where memory regions are registered with the RDMA adapter and local keys are generated, and execution phase where actual DMA operations occur. This segmentation allows memory to be allocated and registered without immediate pinning, enabling overcommitment for multiple VMs while ensuring DMA capability is available when needed.
Solution Approach 2:
The system introduces local keys as an intermediary mechanism between memory allocation and DMA operations. These local keys allow the RDMA adapter to identify and access memory regions without requiring the memory to be permanently pinned, serving as a mediator that enables both memory overcommitment and reliable DMA operations.
3Quantity of substance
If memory registration is deferred for local only access operations, then pinned memory quantity reduces, but additional key management complexity is introduced
Solution Approach 1:
The system implements self-service key management where the RDMA adapter automatically generates local keys for memory regions and manages their lifecycle. When memory regions are registered, the adapter autonomously creates and issues local keys without requiring complex external key management infrastructure, reducing the overall system complexity despite the deferred registration approach.
Data Source
AI summary
Systems and methods for deferring registration for Direct Memory Access (DMA) operations. An example method comprises: receiving a memory region registration request identifying a memory region for a direct memory access (DMA) operation; generating a local key for the memory region; deferring registration of the memory region until receiving a DMA work request referencing the local key; and responsive to receiving the DMA work request, registering the memory region for DMA transfer.