On Tue, Nov 22, 2016 at 9:49 AM, Ulf Hansson <ulf.hansson@xxxxxxxxxx> wrote: > On 22 November 2016 at 04:53, Jaehoon Chung <jh80.chung@xxxxxxxxxxx> wrote: >>> Correct, in general there is no value in using packed for Read. >>> But I can’t say this for all existing flash management solution. >>> The eMMC spec allows to use it for Read as well. >> >> As i know, when packed command had implemented, early eMMC had the firmware problem >> for Packed Read operation. but so I can't say Packed Read doesn't have the benefit for performance. >> But Packed Write command can see the benefit for performance. > > Regarding "performance", are you merely thinking about increased > throughput? With packed command we decrease the communication overhead > with the card so less commands becomes sent/received. > > Or, did you also observed an improved behaviour of the card from a > garbage collect point of view? In other words also a decreased latency > when the device is becoming more and more used? > > Finally, did you compare the packed command, towards using the > asynchronous request mechanisms (using the ->pre|post_req() mmc host > ops)? Packed write (and read, if we had implemented it) can only happen when we get a number of requests to different areas of the card. If they are to consecutive sectors (such as with a dd-test) it will not improve performance, as that case is already optimized by the block layer by front-merging of the requests into large chunks, I guess? Indeed commit ce39f9d17c14 mentions improvement on lmdd but not how it was invoked. I suspect it was invoked with random writes... It is important to do everything we can to improve random small writes though, and it seems packed command can do that. (...) >> Actually, some shipping Samsung devices with eMMC4.5 might be used packed command. >> (For Android/Tizen OS and ARTIK boards) > > Thanks for sharing this information! > > It seems like we need to run another round of performance > measurements, as to get some fresh number of the benefit of packed > command. > I would really appreciate if you could help out with that. I would add: do it on the upstream kernel and submit the stuff needed to make it work for you out-of-the box. This is on Exynos I guess? Can we have this flag set for the Exynos host controller, and/or proper DT bindings to mark it as packed command enabled? Hell I don't even know if this is a feature that needs anything special from the host controller. Does it? Should we rather just enable it for all host controllers if the card supports packed command? I'm lost. Yours, Linus Walleij -- 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
