Preventing Buffer-Credit Accumulations in Switches with Small, Shared Output Queues

摘要

We consider a switch with small output queues, shared among the input VOQ linecards. This has been shown to be a useful abstract model for realistic buffered switching fabrics. Cells are being scheduled by a central control unit, comprising independent, single resource schedulers, working in pipeline. This unit allocates output buffer credits to the requesting VOQs. We show how particular unbalanced transient VOQ states, produced by bursty traffic, affect credit reservations: when some input temporarily constitutes a bottleneck, too many credits may get reserved for it at once, leading to poor overall performance. We propose a threshold grant throttling method to control these credit accumulations. Then, we show how, under such grant throttling, typical round-robin credit schedulers can get synchronized, thus deteriorating performance. To avoid scheduler synchronization, we propose modified round-robin disciplines. Simulations under both smooth and bursty traffic demonstrate the effectiveness of the combined method: using only a 12-cell buffers per-output, for any switch size, N, and independently of the number of cells in transit between the linecards and the fabric, the performance achieved is very close to that of pure output queueing. We also discuss the operation of the independent input and output schedulers inside the control unit, their relation with PIM-like schedulers, and their relation with buffered crossbar schedulers.