Sunday, August 19, 2007

ZIP and RAPI - Yes, it's faster

I really was not pleased with the performance results I got from the RAPI Unzip extension I published in my last post. My reasoning was that if you send a compressed data stream to a device and decompress it you should get a better performance than decompressing the data on the PC and sending it to the device. The results I got were not very satisfying: the RAPI extension DLL was not clearly faster than the PC decompression, especially for ZIP files with small compression rates.

So I decided to take a harder look at the code and found the culprit: WriteFile latency. The function was being called way too often and sometimes with small sets of data. If you look at the code, you will see that WriteFile is called when either the decompression input or output buffers are emptied. This is wrong. The call must only be made when the output buffer is empty and not when the input buffer is. When the input buffer is emptied it must be refilled from the RAPI stream and the output buffer should not be flushed, otherwise we risk flushing a small amout of data expending one WriteFile call.

The final result is now ready and is consistently faster than the PC decompression approach. Try it!

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