Budget Fair Queueing (BFQ) storage I/O scheduler

Early Queue Merge

A set of processes may happen to perform interleaved reads, i.e., requests whose union would give rise to a sequential read pattern. There are two typical cases: in the first case, processes read fixed-size chunks of data at a fixed distance from each other, while in the second case processes may read variable-size chunks at variable distances. The latter case occurs for example with KVM, which splits the I/O generated by the guest into multiple chunks, and lets these chunks be served by a pool of cooperating processes, iteratively assigning the next chunk of I/O to the first available process. CFQ uses actual queue merging for the first type of processes, whereas it uses preemption to get a sequential read pattern out of the read requests performed by the second type of processes. In the end it uses two different mechanisms to achieve the same goal: boosting the throughput with interleaved I/O.

The attached patches introduce Early Queue Merge (EQM), a unified mechanism to get a sequential read pattern with both types of processes. The main idea is checking newly arrived requests against the next request of the active queue both in case of actual request insert and in case of request merge. By doing so, both the types of processes can be handled by just merging their queues. EQM is then simpler and more compact than the pair of mechanisms used in CFQ.

Finally, EQM also preserves the typical low-latency properties of BFQ, by properly restoring the weight-raising state of a queue when it gets back to a non-merged state.


The code for detecting whether two queues have to be merged is a slightly modified version of the CFQ code for detecting whether two queues belong to cooperating processes and whether the service of a queue should be preempted to boost the throughput.