On Thu, 20 Nov 2003, Oliver Neukum wrote: ... > But neither sg nor st currently honor the 512 byte alignment requirement. > Whether this really hurts st in terms of performance is another question. > It depends on hardware :-)
Before implementing direct i/o, I made some measurements with the hardware available. The tape drive was HP DDS-4 and computer dual PIII 700 MHz. The results with a test program using well-compressible data (zeroes): Buffer length 32768 bytes, 32768 buffers written (1024.000 MB, 1073741824 bytes) Variable block mode (writing 32768 byte blocks). dio: write: wall 60.436 user 0.015 ( 0.0 %) system 0.485 ( 0.8 %) speed 16.944 MB/s read: wall 64.580 user 0.014 ( 0.0 %) system 0.500 ( 0.8 %) speed 15.856 MB/s no dio: write: wall 61.373 user 0.024 ( 0.0 %) system 2.897 ( 4.7 %) speed 16.685 MB/s read: wall 66.435 user 0.055 ( 0.1 %) system 6.347 ( 9.6 %) speed 15.413 MB/s The common high-end drives at that time streamed at 30-40 MB/s assuming 2:1 compression. It meant that about 20 % of cpu was needed for extra copies when reading. Other people reported even higher percentages with other hardware. I just repeated the tests using the same drive but a 2.6 GHz PIV on a Intel D875 motherboard and dual channel memory. The results were: dio: write: wall 58.668 user 0.013 ( 0.0 %) system 0.217 ( 0.4 %) speed 17.045 MB/s read: wall 63.425 user 0.011 ( 0.0 %) system 0.245 ( 0.4 %) speed 15.767 MB/s no dio: write: wall 58.713 user 0.017 ( 0.0 %) system 0.534 ( 0.9 %) speed 17.032 MB/s read: wall 63.265 user 0.020 ( 0.0 %) system 0.805 ( 1.3 %) speed 15.807 MB/s Current ordinary drives stream up to 70 MB/s (assuming 2:1 compression). This would lead to over 5 % overhead per drive when not doing dio. -- Kai ------------------------------------------------------- This SF.net email is sponsored by: SF.net Giveback Program. Does SourceForge.net help you be more productive? Does it help you create better code? SHARE THE LOVE, and help us help YOU! Click Here: http://sourceforge.net/donate/ _______________________________________________ [EMAIL PROTECTED] To unsubscribe, use the last form field at: https://lists.sourceforge.net/lists/listinfo/linux-usb-devel
