Queue Length Limit.

In our implementation, each active download has an affiliated queue of limited size that is periodically filled up with data blocks by a server background process (Figure 6). A slot in the block queue becomes empty when a data block is passed to the client socket buffer. Small queue lengths make the system more capable to adjust to the variability of the client behavior, because the blocks sent to each client are chosen based on recent system conditions. On the other hand, large queue lengths can increase the throughput of the system by ensuring that each client has always outstanding block transfers. We examine the impact of the queue length limit on the performance of the system using 512MB download requests over equiprobable 10T and T3 links. With longer queue length limit, the miss ratio becomes higher, the disk bandwidth becomes bottleneck and the server network throughput drops (Figure 15). This is not surprising given the fact that longer queue lengths can lead to stale requests for blocks that have been flushed from the cache and incur extra disk activity to be fetched back into memory. In all the other experiments, the queue length limit is set equal to 5.