On 10/24/2012 11:41 AM, Konstantin Dorfman wrote: > Hello Per, > > On Mon, October 22, 2012 1:02 am, Per Forlin wrote: >>> When mmcqt reports on completion of a request there should be >>> a context switch to allow the insertion of the next read ahead BIOs >>> to the block layer. Since the mmcqd tries to fetch another request >>> immediately after the completion of the previous request it gets NULL >>> and starts waiting for the completion of the previous request. >>> This wait on completion gives the FS the opportunity to insert the next >>> request but the MMC layer is already blocked on the previous request >>> completion and is not aware of the new request waiting to be fetched. >> I thought that I could trigger a context switch in order to give >> execution time for FS to add the new request to the MMC queue. >> I made a simple hack to call yield() in case the request gets NULL. I >> thought it may give the FS layer enough time to add a new request to >> the MMC queue. This would not delay the MMC transfer since the yield() >> is done in parallel with an ongoing transfer. Anyway it was just meant >> to be a simple test. >> >> One yield was not enough. Just for sanity check I added a msleep as >> well and that was enough to let FS add a new request, >> Would it be possible to gain throughput by delaying the fetch of new >> request? Too avoid unnecessary NULL requests >> >> If (ongoing request is read AND size is max read ahead AND new request >> is NULL) yield(); >> >> BR >> Per > We did the same experiment and it will not give maximum possible > performance. There is no guarantee that the context switch which was > manually caused by the MMC layer comes just in time: when it was early > then next fetch still results in NULL, when it was later, then we miss > possibility to fetch/prepare new request. > > Any delay in fetch of the new request that comes after the new request has > arrived hits throughput and latency. > > The solution we are talking about here will fix not only situation with FS > read ahead mechanism, but also it will remove penalty of the MMC context > waiting on completion while any new request arrives. > > Thanks, >
It seems strange that the block layer cannot keep up with relatively slow flash media devices. There must be a limitation on number of outstanding request towards MMC. I need to make up my mind if it's the best way to address this issue in the MMC framework or block layer. I have started to look into the block layer code but it will take some time to dig out the relevant parts. BR Per -- To unsubscribe from this list: send the line "unsubscribe linux-mmc" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
