On 17/02/17 15:22, Linus Walleij wrote: > On Fri, Feb 17, 2017 at 12:53 PM, Ziji Hu <huziji@xxxxxxxxxxx> wrote: > >> I would like to suggest that you should try the multiple thread >> test mode of iozone, since you are testing *Multi* Queue. > > Good point. This target has only 2 CPUs but still, maybe it performs! > >> Besides, it seems that your eMMC transfer speed is quite low. >> It is normal that read speed can reach more than 100MB/s in HS400. >> Could you try a higher speed mode? The test result might be >> limited by the bus clock frequency. > > The iozone tests are done on an SDcard. And I only did read tests on > the eMMC I have. > > It's because I'm afriad of wearing out my eMMC :( > > But OK I'll just take the risk and run iozone on the eMMC. > >> Actually I have been following your thread for some time. >> But currently I'm a little confused. >> May I know the purpose of your patch? > > Ulf describes it: we want to switch MMC/SD to MQ. > > To me, there are two reasons for that (no secret agendas...) > > 1. To get away from the legacy codebase in the old block layer. > Christoph and Jens have been very clear stating that the old block > layer is in maintenance mode and should be phased out, and they > asked explicitly for out help to do so. Currently > MMC/SD is a big fat roadblock to these plans so it is win-win for > MMC/SD and the block layer if we can just switch over to MQ. > > 2. My colleague Paolo Valente is working on the next generation > block scheduler BFQ which has very promising potential for > interactive loads. (Like taking a backup of your harddrive while > playing 1080p video let's say.) Since the > old block layer is no longer maintained, this scheduler will only > be merged and made available for systems deploying MQ. He's > already working full steam on that. > > I would like to make 1+2 happen in the next merge window > ultimately, but yeah, maybe I'm overly optimistic. But I will sure > try. > > Maybe I should add: > > 3. MQ is a better and future-proof fit for command queueing. MQ is not better - it is just different. Because mmc devices do not have multiple hardware queues, blk-mq essentially offers nothing but a different way of doing the same thing. And there are problems, such as blk-mq assumes that the primary arbiter of whether a request can be issued is the queue depth. As I wrote here: https://marc.info/?l=linux-mmc&m=148336571720463&w=2 that is not the case for mmc, even with command queuing. Also I wouldn't be surprise if BFQ needs some changes to work well with command queuing. It would be better if blk-mq support was experimental until we can see how well it works in practice.
