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_zerorange(struct
uvm_bject *uobj, off_t
off, size_t len,
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(9) or kcopy(9). 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_zerorange()
sets a range of bytes in a UVM object to zero. The
flags parameter takes the same arguments as
ubc_release().
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
kcopy(9), 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.