94e44a9c8e
When allocating a non-small array of buckets for a map, also preallocate some overflow buckets. The estimate of the number of overflow buckets is based on a simulation of putting mid=(low+high)/2 elements into a map, where low is the minimum number of elements needed to reach this value of b (according to overLoadFactor), and high is the maximum number of elements possible to put in this value of b (according to overLoadFactor). This estimate is surprisingly reliable and accurate. The number of overflow buckets needed is quadratic, for a fixed value of b. Using this mid estimate means that we will overallocate a few too many overflow buckets when the actual number of elements is near low, and underallocate significantly too few overflow buckets when the actual number of elements is near high. The mechanism introduced in this CL can be re-used for other overflow bucket optimizations. For example, given an initial size hint, we could estimate quite precisely the number of overflow buckets. This is #19931. We could also change from "non-nil means end-of-list" to "pointer-to-hmap.buckets means end-of-list", and then create a linked list of reusable overflow buckets when they are freed by map growth. That is #19992. We could also use a similar mechanism to do bulk allocation of overflow buckets. All these uses can co-exist with only the one additional pointer in mapextra, given a little care. name old time/op new time/op delta MapPopulate/1-8 60.1ns ± 2% 60.3ns ± 2% ~ (p=0.278 n=19+20) MapPopulate/10-8 577ns ± 1% 578ns ± 1% ~ (p=0.140 n=20+20) MapPopulate/100-8 8.06µs ± 1% 8.19µs ± 1% +1.67% (p=0.000 n=20+20) MapPopulate/1000-8 104µs ± 1% 104µs ± 1% ~ (p=0.317 n=20+20) MapPopulate/10000-8 891µs ± 1% 888µs ± 1% ~ (p=0.101 n=19+20) MapPopulate/100000-8 8.61ms ± 1% 8.58ms ± 0% -0.34% (p=0.009 n=20+17) name old alloc/op new alloc/op delta MapPopulate/1-8 0.00B 0.00B ~ (all equal) MapPopulate/10-8 179B ± 0% 179B ± 0% ~ (all equal) MapPopulate/100-8 3.33kB ± 0% 3.38kB ± 0% +1.48% (p=0.000 n=20+16) MapPopulate/1000-8 55.5kB ± 0% 53.4kB ± 0% -3.84% (p=0.000 n=19+20) MapPopulate/10000-8 432kB ± 0% 428kB ± 0% -1.06% (p=0.000 n=19+20) MapPopulate/100000-8 3.65MB ± 0% 3.62MB ± 0% -0.70% (p=0.000 n=20+20) name old allocs/op new allocs/op delta MapPopulate/1-8 0.00 0.00 ~ (all equal) MapPopulate/10-8 1.00 ± 0% 1.00 ± 0% ~ (all equal) MapPopulate/100-8 18.0 ± 0% 17.0 ± 0% -5.56% (p=0.000 n=20+20) MapPopulate/1000-8 96.0 ± 0% 72.6 ± 1% -24.38% (p=0.000 n=20+20) MapPopulate/10000-8 625 ± 0% 319 ± 0% -48.86% (p=0.000 n=20+20) MapPopulate/100000-8 6.23k ± 0% 4.00k ± 0% -35.79% (p=0.000 n=20+20) Change-Id: I01f41cb1374bdb99ccedbc00d04fb9ae43daa204 Reviewed-on: https://go-review.googlesource.com/40979 Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Keith Randall <khr@golang.org> |
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