Queue data structure for Go

Package queue implements a double-ended queue (aka "deque") data structure on top of a slice. All operations run in (amortized) constant time. Benchmarks compare favorably to container/list as well as to Go's channels. These queues are not safe for concurrent use.

I tried to stick to the conventions established by container/list even though I disagree with them (see RANT.md for details). In other words, this data structure is ready for the standard library (hah!).

Background

In 2013 I was hacking a breadth-first search in Go and needed a queue, but all I could find in the standard library was container/list.

Now in principle there's nothing wrong with container/list, but I had just admonished my students to always think carefully about the number of memory allocations their programs make. In other words, it felt wrong for me to use a data structure that allocates memory for every single vertex we visit during a breadth-first search.

After I got done with my project, I decided to clean up the queue code a little and to push it here to give everybody else what I really wanted to find in the standard library: A queue abstraction that doesn't allocate memory on every single insertion.

Performance

Please read BENCH.md for some perspective. Some numbers below are most likely "contaminated" in a way that makes our queues appear worse than they are.

Here are the numbers for my (ancient) home machine:

$ go test -bench=. -benchmem -count=10 >bench.txt
$ benchstat bench.txt
name               time/op
PushFrontQueue-2   82.8µs ± 1%
PushFrontList-2     162µs ± 1%
PushBackQueue-2    83.4µs ± 1%
PushBackList-2      158µs ± 3%
PushBackChannel-2   110µs ± 2%
RandomQueue-2       161µs ± 2%
RandomList-2        281µs ± 4%
GrowShrinkQueue-2   110µs ± 1%
GrowShrinkList-2    170µs ± 5%

name               alloc/op
PushFrontQueue-2   40.9kB ± 0%
PushFrontList-2    57.4kB ± 0%
PushBackQueue-2    40.9kB ± 0%
PushBackList-2     57.4kB ± 0%
PushBackChannel-2  24.7kB ± 0%
RandomQueue-2      45.7kB ± 0%
RandomList-2       90.8kB ± 0%
GrowShrinkQueue-2  57.2kB ± 0%
GrowShrinkList-2   57.4kB ± 0%

name               allocs/op
PushFrontQueue-2    1.03k ± 0%
PushFrontList-2     2.05k ± 0%
PushBackQueue-2     1.03k ± 0%
PushBackList-2      2.05k ± 0%
PushBackChannel-2   1.03k ± 0%
RandomQueue-2       1.63k ± 0%
RandomList-2        3.24k ± 0%
GrowShrinkQueue-2   1.04k ± 0%
GrowShrinkList-2    2.05k ± 0%
$ go version
go version go1.8.1 linux/amd64
$ cat /proc/cpuinfo | grep "model name" | uniq
model name	: AMD Athlon(tm) 64 X2 Dual Core Processor 6000+

That's a speedup of 1.55-1.96 over container/list and a speedup of 1.31 over Go's channels. We also consistently allocate less memory in fewer allocations than container/list. (Note that the number of allocations seems off: since we grow by doubling we should only allocate memory O(log n) times.)

The same benchmarks on one of our department's servers:

$ go test -bench=. -benchmem -count=10 >bench.txt
$ benchstat bench.txt
name               time/op
PushFrontQueue-8   88.8µs ± 3%
PushFrontList-8     156µs ± 5%
PushBackQueue-8    88.3µs ± 1%
PushBackList-8      159µs ± 2%
PushBackChannel-8   132µs ± 2%
RandomQueue-8       156µs ± 7%
RandomList-8        279µs ±10%
GrowShrinkQueue-8   117µs ± 0%
GrowShrinkList-8    164µs ± 4%

name               alloc/op
PushFrontQueue-8   40.9kB ± 0%
PushFrontList-8    57.4kB ± 0%
PushBackQueue-8    40.9kB ± 0%
PushBackList-8     57.4kB ± 0%
PushBackChannel-8  24.7kB ± 0%
RandomQueue-8      45.7kB ± 0%
RandomList-8       90.8kB ± 0%
GrowShrinkQueue-8  57.2kB ± 0%
GrowShrinkList-8   57.4kB ± 0%

name               allocs/op
PushFrontQueue-8    1.03k ± 0%
PushFrontList-8     2.05k ± 0%
PushBackQueue-8     1.03k ± 0%
PushBackList-8      2.05k ± 0%
PushBackChannel-8   1.03k ± 0%
RandomQueue-8       1.63k ± 0%
RandomList-8        3.24k ± 0%
GrowShrinkQueue-8   1.04k ± 0%
GrowShrinkList-8    2.05k ± 0%
$ go version
go version go1.7.5 linux/amd64
$ cat /proc/cpuinfo | grep "model name" |uniq
model name	: Intel(R) Xeon(R) CPU           E5440  @ 2.83GHz

That's a speedup of 1.76-1.80 over container/list and a speedup of 1.49 over Go's channels.

The same benchmarks on a different department server:

$ go test -bench=. -benchmem -count=10 >bench.txt
$ benchstat bench.txt
name                time/op
PushFrontQueue-24   89.1µs ± 8%
PushFrontList-24     176µs ± 8%
PushBackQueue-24    86.8µs ± 5%
PushBackList-24      178µs ± 6%
PushBackChannel-24   151µs ±12%
RandomQueue-24       180µs ±24%
RandomList-24        334µs ± 7%
GrowShrinkQueue-24   117µs ± 3%
GrowShrinkList-24    187µs ± 6%

name                alloc/op
PushFrontQueue-24   40.9kB ± 0%
PushFrontList-24    57.4kB ± 0%
PushBackQueue-24    40.9kB ± 0%
PushBackList-24     57.4kB ± 0%
PushBackChannel-24  24.7kB ± 0%
RandomQueue-24      45.7kB ± 0%
RandomList-24       90.8kB ± 0%
GrowShrinkQueue-24  57.2kB ± 0%
GrowShrinkList-24   57.4kB ± 0%

name                allocs/op
PushFrontQueue-24    1.03k ± 0%
PushFrontList-24     2.05k ± 0%
PushBackQueue-24     1.03k ± 0%
PushBackList-24      2.05k ± 0%
PushBackChannel-24   1.03k ± 0%
RandomQueue-24       1.63k ± 0%
RandomList-24        3.24k ± 0%
GrowShrinkQueue-24   1.04k ± 0%
GrowShrinkList-24    2.05k ± 0%
$ go version
go version go1.7.4 linux/amd64
$ cat /proc/cpuinfo | grep "model name" |uniq
model name	: Intel(R) Xeon(R) CPU E5-2420 0 @ 1.90GHz

That's a speedup of 1.86-2.05 over container/list and a speedup of 1.74 over Go's channels.

The same benchmarks on an old Raspberry Pi Model B Rev 1:

$ benchstat bench.txt
name             time/op
PushFrontQueue    788µs ±24%
PushFrontList    2.74ms ±14%
PushBackQueue    1.11ms ± 3%
PushBackList     2.73ms ±14%
PushBackChannel  1.25ms ± 3%
RandomQueue      1.50ms ± 1%
RandomList       4.92ms ± 6%
GrowShrinkQueue  1.26ms ± 0%
GrowShrinkList   2.88ms ± 2%

name             alloc/op
PushFrontQueue   16.5kB ± 0%
PushFrontList    33.9kB ± 0%
PushBackQueue    16.5kB ± 0%
PushBackList     33.9kB ± 0%
PushBackChannel  8.45kB ± 0%
RandomQueue      16.5kB ± 0%
RandomList       53.4kB ± 0%
GrowShrinkQueue  24.6kB ± 0%
GrowShrinkList   33.9kB ± 0%

name             allocs/op
PushFrontQueue     12.0 ± 0%
PushFrontList     1.03k ± 0%
PushBackQueue      12.0 ± 0%
PushBackList      1.03k ± 0%
PushBackChannel    1.00 ± 0%
RandomQueue        12.0 ± 0%
RandomList        1.63k ± 0%
GrowShrinkQueue    20.0 ± 0%
GrowShrinkList    1.03k ± 0%
$ go version
go version go1.3.3 linux/arm
$ cat /proc/cpuinfo |grep "model name"
model name	: ARMv6-compatible processor rev 7 (v6l)

That's a speedup of 2.46-3.48 over container/list but only a speedup of 1.13 over Go's channels. (Note that I had to manually repeat the benchmarks and then run benchtest elsewhere since those features/tools are not available for Go 1.3; however, the number of allocations seems to be correct here for the first time, maybe there's some breakage in the more recent benchmarking framework?)

Go's channels as queues

Go's channels used to beat our queue implementation by about 22% for PushBack. That seemed sensible considering that channels are built into the language and offer a lot less functionality: We have to size them correctly if we want to use them as a simple queue in an otherwise non-concurrent setting, they are not double-ended, and they don't support "peeking" at the next element without removing it.

That all changed with two commits in which I replaced the "manual" loop when a queue has to grow with copy and the % operations to wrap indices around the slice with equivalent & operations. (The code was originally written without these "hacks" because I wanted to show it to my "innocent" Java students.) Those two changes really paid off.

(I used to call channels "ridiculously fast" before and recommended their use in situations where nothing but performance matters. Alas that may no longer be good advice. Either that, or I am just benchmarking incorrectly.)

Kudos

Hacking queue data structures in Go seems to be a popular way to spend an evening. Kudos to...

Rodrigo Moraes for posting this gist which reminded me of Go's copy builtin and a similar trick I had previously used in Java.
Evan Huus for sharing his queue which reminded me of the old "replace % by &" trick I had used many times before.
Dariusz Górecki for his commit to Evan's queue that simplified Rodrigo's snippet and hence mine.

If you find something in my code that helps you improve yours, feel free to run with it!

Why use this queue?

Looking around at other people's queues shows some "common problems" that this implementation tries to avoid:

Most queues out there are just that, they are not double-ended. Deques are more general but their implementation is not significantly more complex. Why go for anything less as a general-purpose data structure?
Some queues panic. Personally I approve of this if it's done right (see RANT.md), but not using panic is a better fit with the rest of the library/language.
Many queues use linked lists with one element per node, leading to the same performance problems container/list has.
Some queues offer "strange" operations that don't fit the queue/deque abstraction. Call me a purist, but I prefer my interfaces complete yet minimal.
Some queues are safe for concurrent use, but Go's philosophy seems to be to leave this up to the programmer, not the library designer. The overhead of locking doesn't "disappear" when you don't need it.
Some queues based on slices use % instead of & for wrapping indices. That shouldn't matter for performance but in fact it still does (as of Go 1.7.5 anyway).
Some queues based on slices never shrink. In specific applications that may be fine, for a general-purpose data structure it's not.

With that in mind, here's a list of queue/deque implementations that I wouldn't recommend:

https://github.com/eapache/queue not double-ended, panics, "strange" Get operation
https://github.com/emnl/goods/tree/master/queue not double-ended, linked list
https://github.com/oleiade/lane linked list, safe for concurrent use
https://github.com/Workiva/go-datastructures/tree/master/queue full of semi-arcane concurrency stuff
https://github.com/pbberlin/tools/blob/master/util/util-fifo-queue.go not double-ended, uses % instead of &, requires starting size not less than 5???
https://gist.github.com/moraes/2141121 not double-ended, doesn't shrink, extra Node type
https://github.com/ErikDubbelboer/ringqueue/ not double-ended, uses % instead of &, seems to shrink too early (if I am reading the code correctly)
https://github.com/iNamik/go_container/tree/master/queue not double-ended, panics, "strange" Peek and AtCapacity operations, doesn't wrap around in slice

Of course you could always roll your own. I spent a reasonable amount of time on this one, making sure that it works well as a general-purpose queue/deque data structure. But go ahead, you can probably do better.

Where's the actual competition?

https://github.com/juju/utils/tree/master/deque seems pretty good. It uses a viable alternative representation: a list of blocks. That should "waste" less memory in some scenarios, but of course the code is more complicated than ours. Sadly it doesn't have Front or Back operations, so the interface isn't complete. But that would be an easy fix...