On Thu, May 29, 2014 at 11:05:42AM +0200, Paolo Valente wrote: > This patch boosts the throughput on NCQ-capable flash-based devices, > while still preserving latency guarantees for interactive and soft > real-time applications. The throughput is boosted by just not idling > the device when the in-service queue remains empty, even if the queue > is sync and has a non-null idle window. This helps to keep the drive's > internal queue full, which is necessary to achieve maximum > performance. This solution to boost the throughput is a port of > commits a68bbdd and f7d7b7a for CFQ. > > As already highlighted in patch 10, allowing the device to prefetch > and internally reorder requests trivially causes loss of control on > the request service order, and hence on service guarantees. > Fortunately, as discussed in detail in the comments to the function > bfq_bfqq_must_not_expire(), if every process has to receive the same > fraction of the throughput, then the service order enforced by the > internal scheduler of a flash-based device is relatively close to that > enforced by BFQ. In particular, it is close enough to let service > guarantees be substantially preserved. > > Things change in an asymmetric scenario, i.e., if not every process > has to receive the same fraction of the throughput. In this case, to > guarantee the desired throughput distribution, the device must be > prevented from prefetching requests. This is exactly what this patch > does in asymmetric scenarios.
Does it even make sense to use this type of heavy iosched on ssds? It's highly likely that ssds will soon be served through blk-mq bypassing all these. I don't feel too enthused about adding code to support ssds to ioscheds. A lot better approach would be just default to deadline for them anyway. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
