Friday, August 17, 2007

ZIP and RAPI - Is it worth it?

After a few more days working on Lucian Wischik's Zip Utils code I finally managed to build a working RAPI ZIP decompression server. The architecture of this RAPI extension is quite simple: it exports a single method (ZipServer) that implements a streamed RAPI call. Streaming (non-blocking) RAPI calls are very nice because they allow you to keep an open connection with the desktop which is especially useful when expanding very large compressed files: you can slice the compressed stream and have it decompress on the device. For more implementation details please see the source VS 2005 project in the above link.

To make this work I changed the CeUnzip project in order to support calling a RAPI extension and added an extra mandatory command line argument: -i for expanding on the PC from a ZIP file on the device and -e for the exact opposite operation. It's here that I make the remote call to the RAPI extension DLL. Please note that the two projects use different versions of the unzip.cpp file. I will correct this before publishing an article that describes in greater detail the inner workings of the code.

How about results? Is this really faster than using an on-the-fly decompression algorithm on the PC and using CeWriteFile to write the decompressed data on the device?

My first results were very disappointing. When compared to an installer application I wrote for a customer that decompresses on the PC and writes to the device, I consistently got worse timings. The only remedy that made the device DLL run as fast as the PC was to increase the IRAPIStream read buffer size to 128 or 256 KB. As a matter of fact, RAPI read latency is a big issue here. Before giving up I looked at the particular ZIP file I was using to test: it was quite large and had a relatively small compression ratio. Maybe this explains something?

My next test was to use a very high compression ratio ZIP file and run the same tests (expanding it to the device using the two different approaces). This ZIP file was built from a very large SDF file which almost disappeared when compressed: 193 KB to only 3 KB. Now the results were a bit different...

Depending on the device, the DLL expansion was from 2 to 4.5 times faster than the PC's. This means that for compression ratios closer to 1 it is (almost) indifferent which approach to use, but when the ZIP compression ratios get larger, it's far better to use the server DLL.

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