On 02/21/2017 04:02 PM, Linus Torvalds wrote: > Hmm. The new config options are incomprehensible and their help > messages don't actually help. > > So can you fill in the blanks on what > > Default single-queue blk-mq I/O scheduler > Default multi-queue blk-mq I/O scheduler > > config options mean, and why they default to none? > > The config phase of the kernel is one of the worst parts of the whole > project, and adding these kinds of random and incomprehensible config > options does *not* help. I'll try and see if I can come up with some better sounding/reading explanations. But under a device managed by blk-mq, that device exposes a number of hardware queues. For older style devices, that number is typically 1 (single queue). This is true for most SCSI devices that are run by scsi-mq, which is often hosting rotational storage. Faster devices, like nvme, exposes a lot more hardware queues (multi-queue). Hence the distinction between having a scheduler attached for single-queue devices, and for multi-queue devices. For rotational devices, we'll want to default to something like mq-deadline, and I actually thought that was the default already. It should be (see below). For multi-queue devices, we'll want to initially default to "none", and then later attach a properly multiqueue scheduler, when we have it (it's still in development). "none" just means that we don't have a scheduler attached. In essence, we want to default to having a sane IO scheduler attached depending on device class. For single-queue devices, that's deadline for now. For multi-queue, we'll want to wait until we have something that scales really well. It's not that easy to present this information in a user grokkable fashion, since most people would not know the difference between the two. I'll send the below as a real patch, and also ponder how we can improve the Kconfig text. diff --git a/block/Kconfig.iosched b/block/Kconfig.iosched index 0715ce93daef..f6144c5d7c70 100644 --- a/block/Kconfig.iosched +++ b/block/Kconfig.iosched @@ -75,7 +75,7 @@ config MQ_IOSCHED_NONE choice prompt "Default single-queue blk-mq I/O scheduler" - default DEFAULT_SQ_NONE + default DEFAULT_SQ_DEADLINE if MQ_IOSCHED_DEADLINE=y help Select the I/O scheduler which will be used by default for blk-mq managed block devices with a single queue. -- Jens Axboe
