UBC(9)

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

# NAME

ubc — unified buffer cache

# SYNOPSIS

#include <uvm/uvm.h>

void *
ubc_alloc(struct uvm_object *uobj, voff_t offset, vsize_t *lenp, int advice, int flags);

void
ubc_release(void *va, int flags);

int
ubc_uiomove(struct uvm_object *uobj, struct uio *uio, vsize_t todo, int advice, int flags);

void
ubc_purge(struct uvm_object *uobj);

# DESCRIPTION

ubc_alloc() creates a kernel mapping of uobj starting at offset offset. The desired length of the mapping is pointed to by lenp, but the actual mapping may be smaller than this. lenp is updated to contain the actual length mapped. advice is the access pattern hint, which must be one of

UVM_ADV_NORMAL
  No hint
UVM_ADV_RANDOM
  Random access hint
UVM_ADV_SEQUENTIAL
  Sequential access hint (from lower offset to higher offset)

The possible flags are

UBC_READ
  Mapping will be accessed for read.
UBC_WRITE
  Mapping will be accessed for write.
UBC_FAULTBUSY
  Fault in window's pages already during mapping operation. Makes sense only for write.

Once the mapping is created, it must be accessed only by methods that can handle faults, such as uiomove() or kcopy(). Page faults on the mapping will result in the object's pager method being called to resolve the fault.

ubc_release() frees the mapping at va for reuse. The mapping may be cached to speed future accesses to the same region of the object. The flags can be any of

UBC_UNMAP
  Do not cache mapping.

ubc_uiomove() allocates an UBC memory window, performs I/O on it and unmaps the window. The advice parameter takes the same values as the respective parameter in ubc_alloc() and the flags parameter takes the same arguments as ubc_alloc() and ubc_release(). Additionally, the flag UBC_PARTIALOK can be provided to indicate that it is acceptable to return if an error occurs mid-transfer.

ubc_purge() disassociates all UBC structures from an empty UVM object, specified by uobj.

# CODE REFERENCES

The ubc subsystem is implemented within the file sys/uvm/uvm_bio.c.

# SEE ALSO

pmap(9), uiomove(9), uvm(9), vnode(9), vnodeops(9)

Chuck Silvers, UBC: An Efficient Unified I/O and Memory Caching Subsystem for NetBSD, Proceedings of the FREENIX Track: 2000 USENIX Annual Technical Conference, USENIX Association, http://www.usenix.org/event/usenix2000/freenix/full_papers/silvers/silvers.pdf, 285-290, June 18-23, 2000.

# HISTORY

UBC first appeared in NetBSD 1.6.

# AUTHORS

Chuck Silvers <chuq@chuq.com> designed and implemented the UBC part of UVM, which uses UVM pages to cache vnode data rather than the traditional buffer cache buffers.

June 14, 2011 | NetBSD 7.1