block: sqaushed commits to add FIOPS I/O scheduler [Shaohua Li]

block: fiops ioscheduler core

FIOPS (Fair IOPS) ioscheduler is IOPS based ioscheduler, so only targets for drive without I/O seek. It's quite similar like CFQ, but the dispatch decision is made according to IOPS instead of slice.

The algorithm is simple. Drive has a service tree, and each task lives in the tree. The key into the tree is called vios (virtual I/O). Every request has vios, which is calculated according to its ioprio, request size and so on. Task's vios is the sum of vios of all requests it dispatches. FIOPS always selects task with minimum vios in the service tree and let the task dispatch request. The dispatched request's vios is then added to the task's vios and the task is repositioned in the sevice tree.

Unlike CFQ, FIOPS doesn't have separate sync/async queues, because with I/O less writeback, usually a task can only dispatch either sync or async requests. Bias read or write request can still be done with read/write scale.

One issue is if workload iodepth is lower than drive queue_depth, IOPS share of a task might not be strictly according to its priority, request Bias read or write request can still be done with read/write scale.

One issue is if workload iodepth is lower than drive queue_depth, IOPS share of a task might not be strictly according to its priority, request size and so on. In this case, the drive is in idle actually. Solving the problem need make drive idle, so impact performance. I believe CFQ isn't completely fair between tasks in such case too.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

Signed-off-by: Paul Reioux <reioux@gmail.com>

Conflicts:
        block/Kconfig.iosched
        block/Makefile

block: fiops read/write request scale

read/write speed of Flash based storage usually is different. For example, in my SSD maxium thoughput of read is about 3 times faster than that of write. Add a scale to differenate read and write. Also add a tunable, so user can assign different scale for read and write.

By default, the scale is 1:1, which means the scale is a noop.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

block: fiops sync/async scale

CFQ gives 2.5 times more share to sync workload. This matches CFQ.

Note this is different with the read/write scale. We have 3 types of requests:
1. read
2. sync write
3. write
CFQ doesn't differentiate type 1 and 2, but request cost of 1 and 2 are usually different for flash based storage. So we have both sync/async and read/write scale here.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

block: fiops add ioprio support

Add CFQ-like ioprio support. Priority A will get 20% more share than priority A+1, which matches CFQ.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

block: fiops preserve vios key for deep queue depth workload

If the task has running request, even it's added into service tree newly, we preserve its vios key, so it will not lost its share. This should work for task driving big queue depth. For single depth task, there is no approach to preserve its vios key.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

block: fiops bias sync workload

If there are async requests running, delay async workload. Otherwise async workload (usually very deep iodepth) will use all queue iodepth and later sync requests will get long delayed. The idea is from CFQ.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>

block: fiops add some trace information

Add some trace information, which is helpful when I do debugging.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>