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I/O Scheduling

The I/O scheduler provides an interface with the Generic Block Layer, which is a kernel component that handles the requests for all block devices in the system.

A block device reads and writes 'blocks' of data to and from the hardware (usually a hard disk).

Both the Virtual Memory (VM) and Virtual File System(VFS) layers submit requests to block devices.

The scheduling scheme is loaded at kernel boot.

To view the available schedulers:

# Selected is cfq scheduler
cat /sys/block/sda/queue/scheduler
noop deadline [cfq]

To change the value:

echo deadline > /sys/block/sda/queue/scheduler

Types of Schedulers​

  1. Completely Fair Queue (cfq)

    • spreads i/o bandwidth equally across all processes submitting requests.
    • there are 64 dispatch queues which receive requests.
    • requests are read from all queues in round-robin and in FIFO per queue.

  2. deadline

    • aggressively reorders requests to prevent large latencies and improve overall performance.
    • the kernel associates a deadline for every request.
    • has 5 queues:
    • 2 sorted lists (1 for reading, 1 for writing) and are arranged by the starting block.
    • 2 FIFO lists (1 for r, 1 for w) and are sorted by submission time.
    • dispatch queue which contains requests to be sent into the device driver.

    schedulers

We can tune the specifics of the scheduler by modifying different configurations in /sys/block/sda/queue/iosched.

To check whether the device is SSD (0 is SSD):

cat /sys/block/$device/queue/rotational