contrib/timer-wheel: import linux kernel timer-wheel

This patch imports timer-wheel[1] algorithm from the linux
kernel (~/kernel/time/timer.c) with some modifications.

Timer-wheel is an efficent way to track millions of timers for
expiry. This is a variant of the simple but RAM heavy approach
of having a list (timer bucket) for every future second.
Timer-wheel categorizes every future second into a logarithmic
array of arrays. This is done by splitting the 32 bit "timeout"
value into fixed "sliced" bits, thereby each category has a
fixed size array to which buckets are assigned.

A classic split would be 8+6+6+6 (used in this patch) which
results in 256+64+64+64 == 512 buckets. Therefore, the entire
32 bit futuristic timeouts have been mapped into 512 buckets.

[
   NOTE:
     There are other possible splits, such as "8+8+8+8", but
     this patch sticks to the widely used and tested default.
]

Therfore, the first category "holds" timers whose expiry range
is between 1..256, the next cateogry holds 257..16384, third
category 16385..1048576 and so on. When timers are added,
unless it's in the first category, timers with different
timeouts could end up in the same bucket. This means that the
timers are "partially sorted" -- sorted in their highest bits.

The expiry code walks the first array of buckets and exprires
any pending timers (1..256). Next, at time value 257, timers
in the first bucket of the second array is "cascaded" onto
the first category and timers are placed into respective
buckets according to the thier timeout values. Cascading
"brings down" the timers timeout to the coorect bucket
of their respective category. Therefore, timers are sorted
by their highest bits of the timeout value and then by the
lower bits too.

[1] https://lwn.net/Articles/152436/

Change-Id: I1219abf69290961ae9a3d483e11c107c5f49c4e3
BUG: 1170075
Signed-off-by: Venky Shankar <vshankar@redhat.com>
Reviewed-on: http://review.gluster.org/9707
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>

0 files changed, 0 insertions, 0 deletions