BUFQ(9)

BUFQ(9) | Kernel Developer's Manual | BUFQ(9)

# NAME

`bufq`, `bufq_init`, `bufq_register`, `bufq_unregister`, `bufq_state`, `bufq_alloc`, `bufq_drain`, `bufq_free`, `bufq_getstrategyname`, `bufq_move`, `bufq_put`, `bufq_get`, `bufq_peek`, `bufq_cancel` —
device buffer queues

# SYNOPSIS

`#include <sys/bufq.h>`

void
`bufq_init`(void);

int
`bufq_register`(struct bufq_strat *);

int
`bufq_unregister`(struct bufq_strat *);

int
`bufq_alloc`(struct bufq_state **bufq, const char *strategy, int flags);

void
`bufq_drain`(struct bufq_state *bufq);

void
`bufq_free`(struct bufq_state *bufq);

const char *
`bufq_getstrategyname`(struct bufq_state *bufq);

void
`bufq_move`(struct bufq_state *dst, struct bufq_state *src);

void
`bufq_put`(struct bufq_state *bufq, struct buf *bp);

struct buf *
`bufq_get`(struct bufq_state *bufq);

struct buf *
`bufq_peek`(struct bufq_state *bufq);

struct buf *
`bufq_cancel`(struct bufq_state *bufq, struct buf *bp);

# DESCRIPTION

The `bufq` subsystem is a set of operations for the management of device buffer queues. Various strategies for ordering of entries in the buffer queues can be provided by loadable kernel modules (see module(9)). By default, the `BUFQ_FCFS` and `BUFQ_DISKSORT` strategies are included in the kernel; see options(4) for more details.

The primary data type for using the operations is the bufq_state structure, which is opaque for users. Each buffer queue strategy module is defined by the bufq_strat structure, which is also opaque for users.

# FUNCTIONS

`bufq_init`(void)
  Initialize the `bufq` subsystem. This routine must be called before any other `bufq` routines.
`bufq_register`(bs)
  Registers the specified buffer queue strategy module so it can be used in subsequent operations.
`bufq_unregister`(bs)
  Unregisters the specified buffer queue strategy module. The routine will fail if any buffer queues for the specified strategy are in use (see `bufq_alloc`() below).
`bufq_alloc`(bufq, strategy, flags)
  Allocate and initialize a bufq_state descriptor.

  The argument strategy specifies a buffer queue strategy to be used for this buffer queue. The following special values can be used:

  `BUFQ_STRAT_ANY`
    Let `bufq_alloc`() select a strategy.
  `BUFQ_DISK_DEFAULT_STRAT`
    Let `bufq_alloc`() select a strategy, assuming it will be used for a normal disk device.

  Valid bits for the flags are:

  `BUFQ_SORT_RAWBLOCK`
    sort by b_rawblkno
  `BUFQ_SORT_CYLINDER`
    sort by b_cylinder and then by b_rawblkno
  `BUFQ_EXACT`
    Fail if a strategy specified by strategy is not available. In that case, bufq_alloc returns `ENOENT`. If this flag is not specified, `bufq_alloc`() will silently use one of available strategies.

  If a specific strategy is specified but not currently available, the `bufq` subsystem will attempt to auto-load the corresponding kernel module using module_autoload(9).

`bufq_drain`(bufq)
  Drain a bufq_state descriptor.
`bufq_free`(bufq)
  Destroy a bufq_state descriptor.
`bufq_getstrategyname`(bufq)
  Get a strategy identifier of a buffer queue, the string returned will be NUL-terminated and it always will be a valid strategy name.
`bufq_move`(dst, src)
  Move all requests from the buffer queue src to the buffer queue dst.
`bufq_put`(bufq, bp)
  Put the buf bp in the queue.
`bufq_get`(bufq)
  Get the next buf from the queue and remove it from the queue. Returns `NULL` if the queue is empty.
`bufq_peek`(bufq)
  Get the next buf from the queue without removal. The next buf will remain the same until `bufq_get`(), `bufq_put`(), or `bufq_drain`() is called. Returns `NULL` if the queue is empty.
`bufq_cancel`(bufq, bp)
  Cancel the buf bp issued earlier on the queue. Returns `NULL` if the element can not be found on the queue or bp if it has been found and removed. This operation can be computationally expensive if there are a lot of buffers queued.

# CODE REFERENCES

The actual code implementing the device buffer queues can be found in the file sys/kern/subr_bufq.c. The code implementing specific buffer queue strategies can be found in the files sys/kern/bufq_*.c.

# NOTES

A list of currently available buffer queue strategies is available via the “kern.bufq.strategies” sysctl(7) variables.

# HISTORY

The `bufq` subsystem appeared in NetBSD 2.0.

# AUTHORS

The `bufq` subsystem was written by Jürgen Hannken-Illjes ⟨hannken@NetBSD.org⟩.

November 17, 2016 | NetBSD 10.0