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| author |   Eric Dumazet <edumazet@google.com> | 2012-05-12 03:32:13 +0000 |
|---|---|---|
| committer |   David S. Miller <davem@davemloft.net> | 2012-05-12 15:50:49 -0400 |
| commit | 536edd67109df5e0cdb2c4ee759e9bade7976367 (patch) | |
| tree | b253ee5ce32fdc37346120c9ebbfd1f187ad6b95 /net/sched/Makefile | |
| parent | rndis_wlan: cleanup: change oid from __le32 to u32 in various places (diff) | |
| download | linux-dev-536edd67109df5e0cdb2c4ee759e9bade7976367.tar.xz linux-dev-536edd67109df5e0cdb2c4ee759e9bade7976367.zip | |
codel: use Newton method instead of sqrt() and divides
As Van pointed out, interval/sqrt(count) can be implemented using
multiplies only.
http://en.wikipedia.org/wiki/Methods_of_computing_square_roots#Iterative_methods_for_reciprocal_square_roots
This patch implements the Newton method and reciprocal divide.
Total cost is 15 cycles instead of 120 on my Corei5 machine (64bit
kernel).
There is a small 'error' for count values < 5, but we don't really care.
I reuse a hole in struct codel_vars :
- pack the dropping boolean into one bit
- use 31bit to store the reciprocal value of sqrt(count).
Suggested-by: Van Jacobson <van@pollere.net>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Dave Taht <dave.taht@bufferbloat.net>
Cc: Kathleen Nichols <nichols@pollere.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Matt Mathis <mattmathis@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
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