Add ReplicationSet.ZoneCount method.

[charleskorn]

What this PR does:

This PR adds a ZoneCount method to ReplicationSet. This is useful for consumers that need to allocate a slice or map with enough space for an entry per zone.

This is an initial naïve implementation. A more complex implementation could take advantage of the fact that many places that create a ReplicationSet know how many zones it contains and so this could be stored in the ReplicationSet. I'm open to feedback on whether this is worth implementing.

Which issue(s) this PR fixes:

(none)

Checklist

• Tests updated
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[pracucci]

I suspect it's faster if you store the unique set of zones as a slice, because we don't expect more than 3 zones (and iterating a slice 3 times over and over should be faster than lookup up a map). Could you add a microbenchmark and try it, please?

[charleskorn]

Good idea, the slice lookup is faster for the larger sets we expect to see, and the absolute difference for smaller sets is not substantial:

goos: darwin
goarch: arm64
pkg: github.com/grafana/dskit/ring
                                                           │   map.txt    │              slice.txt               │
                                                           │    sec/op    │    sec/op     vs base                │
ReplicationSetZoneCount/1_instances_per_zone,_1_zones-10     12.83n ±  1%    25.11n ± 0%   +95.71% (p=0.002 n=6)
ReplicationSetZoneCount/1_instances_per_zone,_2_zones-10     24.29n ±  7%    58.20n ± 1%  +139.65% (p=0.002 n=6)
ReplicationSetZoneCount/1_instances_per_zone,_3_zones-10     31.23n ±  4%    97.15n ± 1%  +211.03% (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_1_zones-10     20.98n ±  6%    28.89n ± 0%   +37.67% (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_2_zones-10     42.05n ±  8%    70.84n ± 0%   +68.47% (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_3_zones-10     56.70n ± 11%   119.00n ± 0%  +109.90% (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_1_zones-10     42.93n ± 21%    40.18n ± 0%    -6.42% (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_2_zones-10     88.44n ± 10%   102.60n ± 2%   +16.02% (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_3_zones-10     136.2n ±  2%    183.8n ± 0%   +34.90% (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_1_zones-10    92.34n ±  3%    59.22n ± 0%   -35.87% (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_2_zones-10    201.3n ±  1%    161.4n ± 0%   -19.82% (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_3_zones-10    322.3n ±  3%    297.9n ± 0%    -7.59% (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_1_zones-10   815.9n ±  4%    408.8n ± 0%   -49.90% (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_2_zones-10   1.693µ ±  5%    1.144µ ± 0%   -32.41% (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_3_zones-10   2.777µ ±  7%    2.113µ ± 0%   -23.90% (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_1_zones-10   2.442µ ±  9%    1.172µ ± 0%   -52.01% (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_2_zones-10   5.053µ ± 11%    3.301µ ± 0%   -34.67% (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_3_zones-10   7.953µ ±  2%    6.135µ ± 1%   -22.86% (p=0.002 n=6)
geomean                                                      208.8n          222.2n         +6.45%

                                                           │   map.txt   │          slice.txt          │
                                                           │    B/op     │    B/op     vs base         │
ReplicationSetZoneCount/1_instances_per_zone,_1_zones-10     0.00 ± 0%     16.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/1_instances_per_zone,_2_zones-10     0.00 ± 0%     48.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/1_instances_per_zone,_3_zones-10      0.0 ± 0%     112.0 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_1_zones-10     0.00 ± 0%     16.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_2_zones-10     0.00 ± 0%     48.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_3_zones-10      0.0 ± 0%     112.0 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_1_zones-10     0.00 ± 0%     16.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_2_zones-10     0.00 ± 0%     48.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_3_zones-10      0.0 ± 0%     112.0 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_1_zones-10    0.00 ± 0%     16.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_2_zones-10    0.00 ± 0%     48.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_3_zones-10     0.0 ± 0%     112.0 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_1_zones-10   0.00 ± 0%     16.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_2_zones-10   0.00 ± 0%     48.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_3_zones-10    0.0 ± 0%     112.0 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_1_zones-10   0.00 ± 0%     16.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_2_zones-10   0.00 ± 0%     48.00 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_3_zones-10    0.0 ± 0%     112.0 ± 0%  ? (p=0.002 n=6)
geomean                                                                ¹   44.14       ?
¹ summaries must be >0 to compute geomean

                                                           │   map.txt    │          slice.txt          │
                                                           │  allocs/op   │ allocs/op   vs base         │
ReplicationSetZoneCount/1_instances_per_zone,_1_zones-10     0.000 ± 0%     1.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/1_instances_per_zone,_2_zones-10     0.000 ± 0%     2.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/1_instances_per_zone,_3_zones-10     0.000 ± 0%     3.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_1_zones-10     0.000 ± 0%     1.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_2_zones-10     0.000 ± 0%     2.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/2_instances_per_zone,_3_zones-10     0.000 ± 0%     3.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_1_zones-10     0.000 ± 0%     1.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_2_zones-10     0.000 ± 0%     2.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/5_instances_per_zone,_3_zones-10     0.000 ± 0%     3.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_1_zones-10    0.000 ± 0%     1.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_2_zones-10    0.000 ± 0%     2.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/10_instances_per_zone,_3_zones-10    0.000 ± 0%     3.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_1_zones-10   0.000 ± 0%     1.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_2_zones-10   0.000 ± 0%     2.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/100_instances_per_zone,_3_zones-10   0.000 ± 0%     3.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_1_zones-10   0.000 ± 0%     1.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_2_zones-10   0.000 ± 0%     2.000 ± 0%  ? (p=0.002 n=6)
ReplicationSetZoneCount/300_instances_per_zone,_3_zones-10   0.000 ± 0%     3.000 ± 0%  ? (p=0.002 n=6)
geomean                                                                 ¹   1.817       ?
¹ summaries must be >0 to compute geomean

[charleskorn]

Is it worth the complexity of passing the zone count through from places that create ReplicationSets that know how many zones there are? eg. in Ring.Get() we know the number of zones in the set if zone awareness is enabled

[pracucci]

I would wait to see the actual impact on performance before doing any other optimization. Typically operations on the ring in the read path are neglibigle (while they may be significative on the write path).

[dimitarvdimitrov]

LGTM

Another option to reducing the impact of this counting is to cache it within ReplicationsSet. Maybe an unexported field on the struct numZones which is returned if non-zero; otherwise, the loop you already have runs and stores the result in numZones. But I'm also not sure about the usage pattern of ZoneCount() and whether it will be called multiple times.

[charleskorn]

LGTM

Another option to reducing the impact of this counting is to cache it within ReplicationsSet. Maybe an unexported field on the struct numZones which is returned if non-zero; otherwise, the loop you already have runs and stores the result in numZones. But I'm also not sure about the usage pattern of ZoneCount() and whether it will be called multiple times.

The one place I'm planning on using this (Mimir's read path) will only call the method once per request, so caching isn't required for my use case at least.