iosched: parallel streaming reads

Here we see how well the scheduler can cope with multiple processes reading
multiple large files.  We read ten well laid out 100 megabyte files in
parallel (ten readers):

	for i in $(seq 0 9)
	do
		time cat 100-meg-file-$i > /dev/null &
	done

2.4.21-pre4:

	0.00s user 0.18s system 2% cpu 6.115 total
	0.02s user 0.22s system 1% cpu 14.312 total
	0.01s user 0.19s system 1% cpu 14.812 total
	0.00s user 0.14s system 0% cpu 20.462 total
	0.02s user 0.19s system 0% cpu 23.887 total
	0.06s user 0.14s system 0% cpu 27.085 total
	0.01s user 0.26s system 0% cpu 32.367 total
	0.00s user 0.22s system 0% cpu 34.844 total
	0.01s user 0.21s system 0% cpu 35.233 total
	0.01s user 0.16s system 0% cpu 37.007 total

2.5.61+hacks:

	0.01s user 0.16s system 0% cpu 2:12.00 total
	0.01s user 0.15s system 0% cpu 2:12.12 total
	0.00s user 0.14s system 0% cpu 2:12.34 total
	0.01s user 0.15s system 0% cpu 2:12.68 total
	0.00s user 0.15s system 0% cpu 2:12.93 total
	0.01s user 0.17s system 0% cpu 2:13.06 total
	0.01s user 0.14s system 0% cpu 2:13.18 total
	0.01s user 0.17s system 0% cpu 2:13.31 total
	0.01s user 0.16s system 0% cpu 2:13.49 total
	0.01s user 0.19s system 0% cpu 2:13.51 total

2.5.61+CFQ:

	0.01s user 0.16s system 0% cpu 50.778 total
	0.01s user 0.16s system 0% cpu 51.067 total
	0.01s user 0.16s system 0% cpu 52.854 total
	0.01s user 0.17s system 0% cpu 53.303 total
	0.01s user 0.17s system 0% cpu 54.565 total
	0.01s user 0.18s system 0% cpu 1:07.39 total
	0.01s user 0.17s system 0% cpu 1:19.96 total
	0.00s user 0.17s system 0% cpu 1:28.74 total
	0.01s user 0.18s system 0% cpu 1:31.28 total
	0.01s user 0.18s system 0% cpu 1:32.34 total

2.5.61+AS

	0.01s user 0.17s system 0% cpu 27.995 total
	0.01s user 0.18s system 0% cpu 30.550 total
	0.00s user 0.17s system 0% cpu 31.413 total
	0.00s user 0.18s system 0% cpu 32.381 total
	0.01s user 0.17s system 0% cpu 33.273 total
	0.01s user 0.18s system 0% cpu 33.389 total
	0.01s user 0.15s system 0% cpu 34.534 total
	0.01s user 0.17s system 0% cpu 34.481 total
	0.00s user 0.17s system 0% cpu 34.694 total
	0.01s user 0.16s system 0% cpu 34.832 total


AS and 2.4 almost achieved full disk bandwidth.  2.4 does quite well here,
although it was unfair.

As an aside, I reran this test with the VM readahead wound down from the
usual 128k to just 8k:

2.5.61+CFQ:

	0.01s user 0.25s system 0% cpu 7:48.39 total
	0.01s user 0.23s system 0% cpu 7:48.72 total
	0.02s user 0.26s system 0% cpu 7:48.93 total
	0.02s user 0.25s system 0% cpu 7:48.93 total
	0.01s user 0.26s system 0% cpu 7:49.08 total
	0.02s user 0.25s system 0% cpu 7:49.22 total
	0.02s user 0.26s system 0% cpu 7:49.25 total
	0.02s user 0.25s system 0% cpu 7:50.35 total
	0.02s user 0.26s system 0% cpu 8:19.82 total
	0.02s user 0.28s system 0% cpu 8:19.83 total

2.5.61 base:

	0.01s user 0.25s system 0% cpu 8:10.53 total
	0.01s user 0.27s system 0% cpu 8:11.96 total
	0.02s user 0.26s system 0% cpu 8:14.95 total
	0.02s user 0.26s system 0% cpu 8:17.33 total
	0.02s user 0.25s system 0% cpu 8:18.05 total
	0.01s user 0.24s system 0% cpu 8:19.03 total
	0.02s user 0.27s system 0% cpu 8:19.66 total
	0.02s user 0.25s system 0% cpu 8:20.00 total
	0.02s user 0.26s system 0% cpu 8:20.10 total
	0.02s user 0.25s system 0% cpu 8:20.11 total

2.5.61+AS

	0.02s user 0.23s system 0% cpu 28.640 total
	0.01s user 0.23s system 0% cpu 28.066 total
	0.02s user 0.23s system 0% cpu 28.525 total
	0.01s user 0.20s system 0% cpu 28.925 total
	0.01s user 0.22s system 0% cpu 28.835 total
	0.02s user 0.21s system 0% cpu 29.014 total
	0.02s user 0.23s system 0% cpu 29.093 total
	0.01s user 0.20s system 0% cpu 29.175 total
	0.01s user 0.23s system 0% cpu 29.233 total
	0.01s user 0.21s system 0% cpu 29.285 total

We see here that the anticipatory scheduler is not dependent upon large
readahead to get good performance.



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