path: root/xlators
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authorVenky Shankar <>2015-02-03 19:05:28 +0530
committerVijay Bellur <>2015-03-18 22:05:51 -0700
commit5394f3cf60b0815d2919d24e9945ba47e3bb1f9b (patch)
tree8c3aedc4c5b3777c693c6ffbdcf0aa1150580669 /xlators
parent2bf23b5b9be9ad600d71474a8f94b62765344b7c (diff)
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] Change-Id: I1219abf69290961ae9a3d483e11c107c5f49c4e3 BUG: 1170075 Signed-off-by: Venky Shankar <> Reviewed-on: Reviewed-by: Vijay Bellur <> Tested-by: Vijay Bellur <>
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