On Sun, Oct 14, 2012 at 6:17 PM, Konstantin Dorfman <kdorfman@xxxxxxxxxxxxxx> wrote: > On Thu, 11 Oct 2012 17:19:01 +0200, Per Forlin <per.lkml@xxxxxxxxx> > wrote: > Hello Per, > >>I would like to start with some basic comments. >> >>1. Is this read sequential issue specific to MMC? >>2. Or is it common with all other block-drivers that gets data from >>the block layer (SCSI/SATA etc) ? >>If (#2) can the issue be addressed inside the block layer instead? >> >>BR >>Per > This issue specific to MMC, others block drivers probably not using > MMC mechanism for async request (or have more kernel threads for > processing incoming blk requests). > I think, since MMC actively fetches requests from block layer queue, > the solution has nothing to do with block layer context. > >> >>On Tue, Oct 2, 2012 at 5:39 PM, Konstantin Dorfman >><kdorfman@xxxxxxxxxxxxxx> wrote: >>> The main assumption of the async request design is that the file >>> system adds block requests to the block device queue asynchronously >>> without waiting for completion (see the Rationale section of >>> https://wiki.linaro.org/WorkingGroups/Kernel/Specs >>> /StoragePerfMMC-async-req). >>> >>> We found out that in case of sequential read operations this is not >>> the case, due to the read ahead mechanism. >>Would it be possible to improve this mechanism to achieve the same result? >>Allow an outstanding read ahead request on top of the current ongoing one. >> > > I need to look on this mechanism, but from first glance such > behaviour may be result of libc/vfs/fs decisions and too complex > comparing to the patch we are talking about. One observation I have made is that if setting the mmc_req_size to half READ_AHEAD changes the way block layer adds request to the MMC queue. Extract from https://wiki.linaro.org/WorkingGroups/Kernel/Specs/StoragePerfMMC-async-req#Unresolved_issues -------------------------------- Forcing mmc host driver to set mmc_req_size 64k results in this behaviour. dd if=/dev/mmcblk0 of=/dev/null bs=4k count=256 [mmc_queue_thread] req d955f9b0 blocks 32 [mmc_queue_thread] req (null) blocks 0 [mmc_queue_thread] req (null) blocks 0 [mmc_queue_thread] req d955f9b0 blocks 64 [mmc_queue_thread] req (null) blocks 0 [mmc_queue_thread] req d955f8d8 blocks 128 [mmc_queue_thread] req (null) blocks 0 [mmc_queue_thread] req d955f9b0 blocks 128 [mmc_queue_thread] req d955f800 blocks 128 [mmc_queue_thread] req d955f8d8 blocks 128 [mmc_queue_thread] req d955fec0 blocks 128 [mmc_queue_thread] req d955f800 blocks 128 [mmc_queue_thread] req d955f9b0 blocks 128 [mmc_queue_thread] req d967cd30 blocks 128 -------------------------------- This shows that the block layer can add request in a more asynchronous manner. I have not investigate that mechanism enough to say what can be done. Do you have an explanation to why the block layer behaves like this? BR Per > > > -- > Konstantin Dorfman, > QUALCOMM ISRAEL, on behalf of Qualcomm Innovation Center, Inc. is a member > of Code Aurora Forum, hosted by The Linux Foundation > -- 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
