23 Apr 2000 axboe   » (Master)

Analyzed heaps of blklog data resulting from dbench runs with various different elevator settings. As suspected, the max_bomb_segments setting of 4 causes a huge performance loss. I've put the results that blklog generates from a dbench 48 run with the various elevator settings up at kernel.dk/elevator along with a small util to tweak the elevator with (elvset.c) and my blklog app (blklog.c). The latter isn't really useful, since I haven't put up the blklog driver just yet...

The (8192, 131072, 128, 2) (read_latency, write_latency, max_bomb_segments, and max_read_p) elevator setting gives good bench results while still maintaining decent interactive feel. Please experiment and let me know what your findings are! The last entry of the settings above is an addition that I made, read the former three. I mainly run the tests on a SCSI disk that has tagged command queueing, so elevator sorting is basically a non-issue for me. People with IDE drives are the ones that should feel the biggest impact when playing with the elevator.

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