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runtime: reuse evacuated map overflow buckets #19992

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josharian opened this Issue Apr 15, 2017 · 6 comments

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josharian commented Apr 15, 2017

runtime/hashmap.go contains this TODO:

		// TODO: reuse overflow buckets instead of using new ones, if there
		// is no iterator using the old buckets.  (If !oldIterator.)

This issue is to track this TODO, and make it easier to refer to in commit messages, etc., since I am looking into some related optimizations.

@josharian josharian added this to the Unplanned milestone Apr 15, 2017

gopherbot pushed a commit that referenced this issue Apr 19, 2017

runtime: refactor hmap setoverflow into newoverflow
This simplifies the code, as well as providing
a single place to modify to change the
allocation of new overflow buckets.

Updates #19931
Updates #19992

Change-Id: I77070619f5c8fe449bbc35278278bca5eda780f2
Reviewed-on: https://go-review.googlesource.com/40975
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

gopherbot pushed a commit that referenced this issue Apr 19, 2017

runtime: convert hmap.overflow into hmap.extra
Any change to how we allocate overflow buckets
will require some extra hmap storage,
but we don't want hmap to grow,
particular as small maps usually don't need overflow buckets.

This CL converts the existing hmap overflow field,
which is usually used for pointer-free maps,
into a generic extra field.

This extra field can be used to hold data that is optional.
If it is valuable enough to do have special
handling of overflow buckets, which are medium-sized,
it is valuable enough to pay an extra alloc and two extra words for.

Adding fields to extra would entail adding overhead to pointer-free maps;
any mapextra fields added would need to be weighed against that.
This CL is just rearrangement, though.

Updates #19931
Updates #19992

Change-Id: If8537a206905b9d4dc6cd9d886184ece671b3f80
Reviewed-on: https://go-review.googlesource.com/40976
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

gopherbot pushed a commit that referenced this issue Apr 19, 2017

runtime: add bmap.setoverflow
bmap already has a overflow (getter) method.
Add a setoverflow (setter) method, for readability.

Updates #19931
Updates #19992

Change-Id: I00b3d94037c0d75508a7ebd51085c5c3857fb764
Reviewed-on: https://go-review.googlesource.com/40977
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

gopherbot pushed a commit that referenced this issue Apr 19, 2017

runtime: add a map growth benchmark
Updates #19931
Updates #19992

Change-Id: Ib2d4e6b9b89a49caa443310d896dce8d6db06050
Reviewed-on: https://go-review.googlesource.com/40978
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>

gopherbot pushed a commit that referenced this issue Apr 19, 2017

runtime: preallocate some overflow buckets
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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josharian commented Apr 19, 2017

CL 40979 and its associated patches laid some groundwork for this, if anyone wants to investigate further. I don't plan to, during this cycle at least. See in particular Keith's comment: https://go-review.googlesource.com/c/40979/1/src/runtime/hashmap.go#215.

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gopherbot commented Apr 21, 2017

CL https://golang.org/cl/40976 mentions this issue.

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gopherbot commented Apr 21, 2017

CL https://golang.org/cl/40975 mentions this issue.

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gopherbot commented Apr 21, 2017

CL https://golang.org/cl/40977 mentions this issue.

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gopherbot commented Apr 21, 2017

CL https://golang.org/cl/40979 mentions this issue.

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gopherbot commented Apr 21, 2017

CL https://golang.org/cl/40978 mentions this issue.

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