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Subjectiosched: effect of streaming read on streaming write

Here we look at how much damage a streaming read can do to writeout
performance. Start a streaming read with:

while true
do
cat 512M-file > /dev/null
done

and measure how long it takes to write out and fsync a 100 megabyte file:

time write-and-fsync -f -m 100 outfile

2.4.21-pre4: 6.4 seconds
2.5.61+hacks: 7.7 seconds
2.5.61+CFQ: 8.4 seconds
2.5.61+AS: 11.9 seconds

This is the one where the anticipatory scheduler could show its downside.
It's actually not too bad - the read stream steals 2/3rds of the disk
bandwidth. Dirty memory will reach the vm threshold and writers will
throttle. This is usually what we want to happen.

Here is the vmstat 1 trace for the anticipatory scheduler:

r b swpd free buff cache si so bi bo in cs us sy id wa
1 1 8728 2268 2620 233412 0 0 40360 0 1658 802 0 4 0 96
0 2 8728 3780 2508 231924 0 0 40616 4 1668 874 0 5 0 95
0 2 8728 3668 2276 232416 0 0 40740 20 1668 978 0 4 0 96
0 3 8728 3660 2192 232668 40 0 35296 12 1603 904 0 4 0 95
0 5 8728 3612 1964 231672 0 0 26220 18572 1497 1381 0 15 0 85
0 5 8728 2100 1732 233584 0 0 25232 8696 1497 867 0 3 16 81
0 5 8728 3664 1204 232424 0 0 27668 8696 1533 787 0 3 0 97
1 4 8728 2432 792 234108 0 0 27160 8696 1527 965 0 3 0 97
0 6 8728 2208 760 234436 0 0 25904 9584 1513 856 0 3 0 97
2 6 8728 3776 760 233148 0 0 27776 8716 1537 880 0 3 0 97
0 6 8728 2204 624 234968 0 0 27924 8812 1541 991 0 4 0 96
0 4 8716 2508 600 234740 0 0 28188 8216 1537 1038 0 4 0 96
0 4 8716 4072 532 233316 0 16 25624 9644 1515 896 0 3 0 97
0 4 8716 3740 548 233624 0 0 27548 8696 1528 908 0 3 0 97



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