On Tuesday 28 June 2011, Per Forlin wrote: > How significant is the cache maintenance over head? > It depends, the eMMC are much faster now > compared to a few years ago and cache maintenance cost more due to > multiple cache levels and speculative cache pre-fetch. In relation the > cost for handling the caches have increased and is now a bottle neck > dealing with fast eMMC together with DMA. > > The intention for introducing non-blocking mmc requests is to minimize the > time between a mmc request ends and another mmc request starts. In the > current implementation the MMC controller is idle when dma_map_sg and > dma_unmap_sg is processing. Introducing non-blocking mmc request makes it > possible to prepare the caches for next job in parallel to an active > mmc request. > > This is done by making the issue_rw_rq() non-blocking. > The increase in throughput is proportional to the time it takes to > prepare (major part of preparations is dma_map_sg and dma_unmap_sg) > a request and how fast the memory is. The faster the MMC/SD is > the more significant the prepare request time becomes. Measurements on U5500 > and Panda on eMMC and SD shows significant performance gain for large > reads when running DMA mode. In the PIO case the performance is unchanged. > > There are two optional hooks pre_req() and post_req() that the host driver > may implement in order to move work to before and after the actual mmc_request > function is called. In the DMA case pre_req() may do dma_map_sg() and prepare > the dma descriptor and post_req runs the dma_unmap_sg. I think this looks good enough to merge into the linux-mmc tree, the code is clean and the benefits are clear. Acked-by: Arnd Bergmann <arnd@xxxxxxxx> One logical follow-up as both a cleanup and performance optimization would be to get rid of the mmc_queue_thread completely. When mmc_blk_issue_rq() is non-blocking always, you can call it directly from the mmc_request() function, instead of waking up another thread to do it for you. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
