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UBC(9) NetBSD Kernel Developer's Manual UBC(9)
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ubc -- unified buffer cache
ubc_alloc(struct uvm_object *uobj, voff_t offset, vsize_t *lenp,
int advice, int flags);
ubc_release(void *va, int flags);
ubc_uiomove(struct uvm_object *uobj, struct uio *uio, vsize_t todo,
int advice, int flags);
ubc_zerorange(struct uvm_bject *uobj, off_t off, size_t len, int flags);
ubc_purge(struct uvm_object *uobj);
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
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 map-
ping will result in the object's pager method being called to resolve the
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.
The ubc subsystem is implemented within the file sys/uvm/uvm_bio.c.
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,
285-290, June 18-23, 2000.
UBC first appeared in NetBSD 1.6.
Chuck Silvers <firstname.lastname@example.org> designed and implemented the UBC part of
UVM, which uses UVM pages to cache vnode data rather than the traditional
buffer cache buffers.
NetBSD 8.1 April 6, 2017 NetBSD 8.1