NAME
ubc —
unified buffer cache
SYNOPSIS
#include
<uvm/uvm.h>
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_uiomove()
allocates an UBC memory window, performs I/O on it and unmaps the window. The
advice parameter 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)
and the flags parameter is
- 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.
- UBC_UNMAP
- Do not cache mapping.
- UBC_PARTIALOK
- Indicate that it is acceptable to return if an error occurs mid-transfer.
UBC memory window is a kernel mapping of uobj starting at offset offset.
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.
Size of individual UBC memory window is limited
to ubc_winsize.
ubc_uiomove()
sequentially creates the UBC memory windows to eventually process the whole
range according to offset and
len parameters.
The mappings may be cached to speed future accesses to the same
region of the object, unless UBC_UNMAP was specified
in flags parameter.
ubc_zerorange()
sets a range of bytes in a UVM object to zero. The
flags parameter takes the same arguments as
ubc_uiomove().
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.