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| Copyright
© 1996 Mark
Russinovich and Bryce
Cogswell |
Last
Updated July 9, 1997
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| Introduction |
TVCACHE
is a replacement for VCACHE that implements compression on cached
blocks in an attempt to improve performance (our benchmarks show that it
doesn't fare too great). In any case, we are posting the complete source
for TVCACHE because the VCACHE interface is undocumented. The
source presented here is the only complete specification for the VCACHE
interface that we are aware of. Note that the source is presented solely
for educational purposes and is not licensed for commercial use. |
| How
it Works |
In yet
another brilliant design move, the current implementation of Windows 95
does not provide any official way to replace the default VCACHE with an
alternate VxD. However, TVCACHE overcomes this limitation by replacing
the function pointers in the VCACHE service table with pointers to its
own routines before VCACHE is activated for the first time. Thus, using
VCACHE's Device Declaration Block, TVCACHE subsumes VCACHE's role as the
cache manager for Windows.
Once
active, TVCACHE actually does much more work than VCACHE needs to do.
This extra work involves fragmented memory management and compressed
buffer management. VCACHE only has to deal with page sized (4K)
chunks, whereas TVCACHE must deal with almost arbitrary sized chunks
of data (in 128 byte increments). TVCACHE mimics VCACHE's exact policy
of cache growing and shrinking and our benchmarks show VCACHE and
TVCACHE (with compression disabled) have identical performance
characteristics.
Whether
you are writing your own Windows 95 file system driver, just curious
about how VCACHE works, or want to experiment with VCACHE grow/shrink
policies, TVCACHE serves as the only source of precise and accurate
information on how VCACHE actually functions internally.
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TVCACHE
is a Windows 95 VxD that is written in C, and it requires VtoolsD
Version 2.0 or greater to build. Defining the TESTING flag has the build
produce a stand-alone Windows console program that stress tests the
TVCACHE code by simulating multi-threaded cache accesses. Setting
COMPRESSON to 0 disables compression so that TVCACHE functions in a
manner similar to VCACHE. |
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To
install TVCACHE, add the following line to the [386enh] section of
system.ini:
device=<path>\tvcache.vxd
where <path>
is the directory in which tvache.vxd is located. You must reboot your
machine for TVCACHE to start. |
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The
VCACHE interface is only partially documented in the Windows 95 DDK, and
there are no guidelines for using it. While we're not taking the time to
write up a document on the VCACHE interface specification, the source to
TVCACHE indirectly documents everything you need to know. In its stress
testing of TVCACHE code, the main() test function at the bottom of
vcache.c serves as an example of how the VCACHE interface is used.
Let us
know if you find TVCACHE useful. |
Download
TVCACHE (72KB) |
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