UBC: An Efficient Unified I/O and Memory Caching

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UBC: An Efficient Unified I/O and
Memory Caching Subsystem for NetBSD
Chuck Silvers
The NetBSD Project
[email protected], http://www.netbsd.org/
This paper introduces UBC (“Unified Buffer Cache”), a
design for unifying the filesystem and virtual memory
caches of file data, thereby providing increased system
performance. In this paper we discuss both the traditional BSD caching interfaces and new UBC interfaces,
concentrating on the design decisions that were made as
the design progressed. We also discuss the designs used
by other operating systems to solve the same problems
that UBC solves, with emphasis on the practical implications of the differences between these designs. This
project is still in progress, and once completed will be
part of a future release of NetBSD.
1 Introduction
Modern operating systems allow filesystem data to be
accessed using two mechanisms: memory mapping, and
I/O system calls such as read() and write(). In
traditional UNIX-like operating systems, memory mapping requests are handled by the virtual memory subsystem while I/O calls are handled by the I/O subsystem. Traditionally these two subsystems were developed
separately and were not tightly integrated. For example,
in the NetBSD operating system[1], the VM subsystem
(“UVM”[2]) and I/O subsystem each have their own data
caching mechanisms that operate semi-independently of
each other. This lack of integration leads to inefficient
overall system performance and a lack of flexibility. To
achieve good performance it is important for the virtual memory and I/O subsystems to be highly integrated.
This integration is the function of UBC.
2 Background
In order to understand the improvements made in UBC,
it is important to first understand how things work without UBC. First, some terms:
“buffer cache”:
A pool of memory allocated during system startup
which is dedicated to caching filesystem data and is
managed by special-purpose routines.
The memory is organized into “buffers,” which are
variable-sized chunks of file data that are mapped to
kernel virtual addresses as long as they retain their
“page cache”:
The portion of available system memory which is
used for cached file data and is managed by the VM
The amount of memory used by the page cache can
vary from nearly 0% to nearly 100% of the physical
memory that isn’t locked into some other use.
The kernel abstraction which represents a file.
Most manipulation of vnodes and their associated
data are performed via “VOPs” (short for “vnode
The major interfaces for accessing file data are:
read() and write():
The read() system call reads data from disk into
the kernel’s cache if necessary, then copies data
from the kernel’s cached copy to the application’s
address space. The write() system call moves
data the opposite direction, copying from the application’s address space into the kernel’s cache and
eventually writing the data from the cache to disk.
These interfaces can be implemented using either
the buffer cache or the page cache to store the data
in the kernel.
The mmap() system call gives the application direct memory-mapped access to the kernel’s page
cache data. File data is read into the page cache
lazily as processes attempt to access the mappings
created with mmap() and generate page faults.
In NetBSD without UBC, read() and write() are
implemented using the buffer cache. The read() system call reads file data into a buffer cache buffer and then
copies it to the application. The mmap() system call,
however, has to use the page cache to store its data since
the buffer cache memory is not managed by the VM system and thus not cannot be mapped into an application
address space. Therefore the file data in the buffer cache
is copied into page cache pages, which are then used
to satisfy page faults on the application mappings. To
write modified data in page cache pages back to disk,
the new version is copied back to the buffer cache and
from there is written to disk. Figure 1 shows the flow of
data between the disk and the application with a traditional buffer cache.
page cache
extra data
buffer cache
raw device
This double-caching of data is a major source of inefficiency. Having two copies of file data means that twice
as much memory is used, which reduces the amount of
memory available for applications. Copying the data
back and forth between the buffer cache and the page
cache wastes CPU cycles, clobbers CPU caches and is
generally bad for performance. Having two copies of
the data also allows the possibility that the two copies
will become inconsistent, which can lead to application
problems which are difficult to debug.
The use of the buffer cache for large amounts of data is
generally bad, since the static sizing of the buffer cache
means that the buffer cache is often either too small (resulting in excessive cache misses), or too large (resulting
in too little memory left for other uses).
The buffer cache also has the limitation that cached data
must always be mapped into kernel virtual space, which
puts an additional artificial limit on the amount of data
which can be cached since modern hardware can easily
have more RAM than kernel virtual address space.
To solve these problems, many operating systems have
changed their usage of the page cache and the buffer
cache. Each system has its own variation, so we will
describe UBC first and then some other popular operating systems.
Figure 1: NetBSD before UBC.
3 So what is UBC anyway?
UBC is a new subsystem which solves the problems with
the two-cache model. In the UBC model, we store file
data in the page cache for both read()/write() and
mmap() accesses. File data is read directly into the
page cache without going through the buffer cache by
creating two new VOPs which return page cache pages
with the desired data, calling into the device driver to
read the data from disk if necessary. Since page cache
pages aren’t always mapped, we created a new mechanism for providing temporary mappings of page cache
pages, which is used by read() and write() while
copying the file data to the application’s address space.
Figure 2 shows the changed data flow with UBC.
UBC introduces these new interfaces:
These new VOPs are provided by the filesystems
to allow the VM system to request ranges of pages
page cache
raw device
Figure 2: NetBSD with UBC.
to be read into memory from disk or written from
memory back to disk. VOP GETPAGES() must allocate pages from the VM system for data which is
not already cached and then initiate device I/O operations to read all the disk blocks which contain
the data for those pages. VOP PUTPAGES() must
initiate device I/Os to write dirty pages back to disk.
ubc alloc(), ubc release()
These functions allocate and free temporary mappings of page cache file data. These are the page
cache equivalents of the buffer cache functions
getblk() and brelse()[3]. These temporary mappings are not wired, but they are cached
to speed repeated access to the same file. The
selection of which virtual addresses to use for
these temporary mappings is important on hardware which has a virtually-addressed CPU data
cache, so the addresses are carefully chosen to be
correctly aligned with the preferred addresses for
user file mappings, so that both kinds of mappings
can be present at the same time without creating coherency problems in the CPU cache. It is still possible for applications to create unaligned file mappings, but if the application lets the operating system choose the mapping address then all mappings
will always be aligned.
ubc pager
This is a UVM pager which handles page faults on
the mappings created by ubc alloc(). (A UVM
pager is an abstraction which embodies knowledge
of page-fault resolution and other VM data management. See the UVM paper[2] for more information on pagers.) Since its only purpose is to handle those page faults, the only action performed by
ubc pager is to call the new VOP GETPAGES()
operation to get pages as needed to resolve the
In addition to these new interfaces, several changes were
made to the existing UVM design to fix problems which
were glossed over in the original design.
Previously in UVM, vnodes and uvm objects were
not interchangeable, and in fact several fields were duplicated and maintained separately in each. These duplicate fields were combined. At this time there’s still a
bit of extra initialization the first time a struct vnode is used as a struct uvm object, but that will
be removed eventually.
Previously UVM only supported 32-bit offsets into
uvm objects, which meant that data could only be
stored in the page cache for the first 4 GB of a file.
This wasn’t much of a problem before since the number of programs which wanted to access file offsets past
4 GB via mmap() was small, but now that read() and
write() also use the page cache interfaces to access
data, we had to support 64-bit uvm object offsets in
order to continue to allow any access to file offsets past
4 GB.
4 What do other operating systems do?
The problems addressed by UBC have been around for
a long time, ever since memory-mapped access to files
was first introduced in the late 1980’s. Most UNIX-like
operating systems have addressed this issue one way or
another, but there are considerable differences in how
they go about it.
The first operating system to address these problems
was SunOS[4, 5], and UBC is largely modeled after
this design. The main differences in the design of the
SunOS cache and UBC result from the differences between the SunOS VM system and UVM. Since UVM’s
pager abstraction and SunOS’s segment-driver abstraction are similar, this didn’t change the design much at
When work on UBC first began over two years ago, the
other design that we examined was that of FreeBSD[6],
which had also already dealt with this problem. The
model in FreeBSD was to keep the same buffer cache interfaces to access file data, but to use page cache pages as
the memory for a buffer’s data rather than memory from
a separate pool. The result is that the same physical page
is accessed to retrieve a given range of the file regardless
of whether the access is made via the buffer cache interface or the page cache interface. This had the advantage
that filesystems did not need to be changed in order to
take benefit from the changes. However, the glue to do
the translation between the interfaces was just as complicated as the glue in the SunOS design and failed to
address certain deadlock problems (such as an application calling write() with a buffer which was a memory mapping of the same file being written to), so we
chose the SunOS approach over this one.
The approach taken by Linux[7] (as of kernel version
2.3.44, the latest version at the time this paper was written) is actually fairly similar to the SunOS design also.
File data is stored only in the page cache. Temporary
mappings of page cache pages to support read() and
write() usually aren’t needed since Linux usually
maps all of physical memory into the kernel’s virtual address space all the time. One interesting twist that Linux
adds is that the device block numbers where a page is
stored on disk are cached with the page in the form of a
list of buffer head structures. When a modified page
is to be written back to disk, the I/O requests can be sent
to the device driver right away, without needing to read
any indirect blocks to determine where the page’s data
should be written.
The last of the operating systems we examined, HPUX, takes a completely different stance on the issue of
how to cache filesystem data. HP-UX continues to store
file data in both the buffer cache and the page cache,
though it does avoid the extra of copying of data that is
present in pre-UBC NetBSD by reading data from disk
directly into the page cache. The reasoning behind this
is apparently that most files are only accessed by either
read()/write() or mmap(), but not both, so as long
as both mechanisms perform well individually, there’s
no need to redesign HP-UX just to fix the coherency issue. There is some attempt made to avoid incoherency
between the two caches, but locking constraints prevent
this from being completely effective.
There are other operating systems which have implemented a unified cache (eg. Compaq’s Tru64 UNIX and
IBM’s AIX), but we were unable to find information on
the design of these operating systems for comparison.
5 Performance
Since UBC is unfortunately not yet finished, a detailed
performance analysis would be premature. However, we
have made some simple comparisons just to see where
we stand. The hardware used for this test was a 333MHz
Pentium II with 64MB of RAM and a 12GB IDE disk.
The operations performed were a series of “dd” commands designed to expose the behaviour of sequential
reads and writes. We create a 1GB file (which is much
larger than the physical memory available for caching),
then overwrite this file to see the speed at which the
data modifications caused by the write() are flushed
to disk without the overhead of allocating blocks to the
file. Then we read back the entire file to get an idea of
how fast the filesystem can get data from the disk. Finally, we read the first 50MB of the file (which should
fit entirely in physical memory) several times to determine the speed of access to cached data. See Table 1 for
the results of these tests.
The great disparity in the results of the first four tests
on the three non-UBC operating systems is due to differences in performance of their IDE disk drivers. All of
the operating systems tested except NetBSD with UBC
do sequential buffered reads from a large file at the same
speed as reads from the raw device, so all we can really
say from this is that the other caching designs don’t add
any noticable overhead. For reads, the UBC system is
not yet running at device speed, so there’s still room for
improvement. Further analysis is required to determine
the cause of the slowdown.
UBC obviously needs much improvement in the area of
write performance. This is partly due to UVM not being
very efficient about flushing modified pages when memory is low and partly because the filesystem code currently doesn’t trigger any asynchronous writes to disk
during a big sequence of writes, so the writes to disk
are all started by the inefficient UVM code. We’ve been
concentrating on read performance so far, so this poor
write performance is not surprising.
The interesting part of this test series is the set of tests
where we read the same 50MB file five times. This
clearly shows the benefit of the increased memory available for caching in the UBC system over NetBSD without UBC. In NetBSD 1.4.2, all five reads occured at the
speed of the device, whereas in all the other systems
the reads were completed at memory speed after several runs. We have no explanation for why FreeBSD and
Linux didn’t complete the second 50MB read at memory
speed, or why Linux didn’t complete even the third read
raw device
non-resident file
non-resident file
resident file
repeat above
repeat above
repeat above
new file
overwrite file
Run Time (seconds)
with UBC
Table 1: UBC performance comparison.
at memory speed.
6 Conclusion
In this paper we introduced UBC, a improved design for
filesystem and virtual memory caching in NetBSD. This
design includes many improvements over the previous
design used in NetBSD by:
Eliminating double caching of file data in the kernel
(and the possibility of cache incoherency that goes
with it) when the same file is accessed via different
Allowing more flexibility in how physical memory
is used, which can greatly improve performance for
applications whose data fits in physical memory.
7 Availability
much larger amount of page-cache data which will
be dirty.
Elimination of the data copying in read() and
write() via UVM page loanout when possible.
This could be done without UBC too, but with
UBC it will be zero-copy instead of one-copy (from
buffer cache to page cache).
Elimination of the need to map pages to do I/O to
them by adding a page list to struct buf and
adding glue in bus dma to map pages temporarily
for hardware that actually needs that.
Adding support for “XIP” (eXecute In Place). This
will allow zero-copy access to filesystem images
stored in flash roms or other memory-mapped storage devices.
Adding support for cache coherency in layered
filesystems. (The current UBC design does not address caching in layered filesystems.)
This work will be part of a future release of NetBSD
once it is completed. Until then, the source code is
available in the “chs-ubc2” branch in the NetBSD
CVS tree, accessible via anonymous CVS. See
for details.
This being a work-in-progress, there is naturally much
more work to do! Planned work includes:
We would like to thank everyone who helped review
drafts of this paper. Special thanks to Chuck Cranor!
[1] The NetBSD Project. The NetBSD Operating System. See
http://www.netbsd.org/ for more information.
Integration of UBC into the NetBSD development
source tree and performance improvement. The
pagedaemon needs to be enhanced to deal with the
[2] C. Cranor and G. Parulkar. The UVM Virtual Memory
System. In Proceedings of the 1999 USENIX Technical
Conference, June 1999.
[3] Marice J. Bach. The Design of the UNIX Operating System. Prentice Hall, February 1987.
[4] J. Moran, R. Gingell and W. Shannon. Virtual Memory
Architecture in SunOS. In Proceedings of USENIX Summer Conference, pages 81-94. USENIX, June 1987.
[5] J. Moran. SunOS Virtual Memory Implementation. In
Proceedings of the Spring 1988 European UNIX Users
Group Conference, April 1988.
[6] The FreeBSD Project. The FreeBSD Operating System.
See http://www.freebsd.org/ for more information.
[7] L. Torvalds, et al. The Linux Operating System. See
http://www.linux.org/ for more information.

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