Lookaside

Lookaside refers to a small, fast cache or free-list used by software systems to speed up repeated operations by storing frequently used or fixed-size resources. The discipline is common in memory management and resource allocation, where a lookaside keeps preallocated blocks or objects ready for immediate reuse rather than requesting new ones from a general allocator each time.

In operating systems and kernels, lookaside lists are often implemented as per-size pools of fixed-length blocks. A component maintains a set of lookaside lists, each for a particular block size. When code needs a block, it takes one from the corresponding lookaside; when it finishes, it returns the block to the same list. If a list is empty, the allocator falls back to the general pool. This approach reduces locking, fragmentation, and allocation latency for small, frequent allocations, at the cost of extra memory to hold the cached blocks and added management logic.

Beyond the kernel, lookaside caches appear in various software domains, including network stacks, databases, and runtimes. They generally store buffers or small objects that are allocated and freed often, with the aim of improving throughput and cache locality. The exact implementation and policies—such as per-thread versus shared caches, depth limits, and eviction strategies—vary by system.

Design considerations for lookaside caches include memory overhead, hit rate, contention, and fragmentation. Proper sizing and lifecycle management are crucial to prevent wasted space or stale blocks, while providing fast paths for common allocation patterns.