Micro optimization to parallelStorageShards for the case there's only 1 shard

[pracucci]

What this PR does

This PR is a follow up of #13924. In this PR I'm doing a micro optimization to parallelStorageShards for the case there's only 1 shard. The impact of this optimization is minimal, but I think doesn't make the code harder to follow, so it may be worth to keep it.

My local benchmarks are not super stable, but here you can get an idea of the impact (minimal). Keep in mind PusherConsumer_ParallelPusher_MultiTenant uses a mocked backend. In the real world, where metrics are actually ingested in TSDB, the impact is much smaller:

goos: darwin
goarch: arm64
pkg: github.com/grafana/mimir/pkg/storage/ingest
cpu: Apple M3 Pro
                                                                       │ BenchmarkPusherConsumer_ParallelPusher_MultiTenant-before.txt │ BenchmarkPusherConsumer_ParallelPusher_MultiTenant-after.txt │
                                                                       │                            sec/op                             │                sec/op                  vs base               │
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=1-11                                                          8.421µ ±  5%                            8.862µ ±  2%   +5.24% (p=0.009 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=10-11                                                         8.578µ ± 81%                            8.971µ ± 46%        ~ (p=0.240 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=100-11                                                        8.394µ ±  3%                            9.005µ ±  1%   +7.28% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=1000-11                                                       8.308µ ±  2%                            8.998µ ±  0%   +8.31% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=1-11                                                         21.22µ ±  4%                            21.35µ ±  2%        ~ (p=0.937 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=10-11                                                        43.02µ ±  6%                            40.24µ ±  0%   -6.46% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=100-11                                                       43.24µ ±  1%                            40.50µ ±  0%   -6.33% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=1000-11                                                      43.37µ ±  2%                            40.73µ ±  1%   -6.10% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=1-11                                                        126.3µ ±  0%                            120.2µ ±  1%   -4.85% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=10-11                                                       137.7µ ±  0%                            122.0µ ±  3%  -11.40% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=100-11                                                      304.7µ ±  3%                            275.2µ ±  0%   -9.69% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=1000-11                                                     303.5µ ±  2%                            275.1µ ±  0%   -9.35% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=1-11                                                       1.142m ±  1%                            1.124m ±  1%   -1.58% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=10-11                                                      1.051m ±  1%                            1.115m ±  0%   +6.10% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=100-11                                                     1.264m ±  1%                            1.301m ±  0%   +2.91% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=1000-11                                                    3.060m ±  3%                            2.996m ±  4%        ~ (p=0.240 n=6)
geomean                                                                                                                   97.29µ                                  95.70µ         -1.63%

                                                                       │ BenchmarkPusherConsumer_ParallelPusher_MultiTenant-before.txt │ BenchmarkPusherConsumer_ParallelPusher_MultiTenant-after.txt │
                                                                       │                             B/op                              │                  B/op                   vs base              │
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=1-11                                                          17.40Ki ± 0%                             17.44Ki ± 0%  +0.19% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=10-11                                                         17.40Ki ± 0%                             17.43Ki ± 0%  +0.19% (p=0.035 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=100-11                                                        17.39Ki ± 0%                             17.43Ki ± 0%  +0.21% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=1000-11                                                       17.39Ki ± 0%                             17.43Ki ± 0%  +0.20% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=1-11                                                         25.15Ki ± 0%                             25.21Ki ± 0%  +0.25% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=10-11                                                        78.70Ki ± 0%                             78.70Ki ± 0%       ~ (p=0.223 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=100-11                                                       78.70Ki ± 0%                             78.70Ki ± 0%  +0.01% (p=0.037 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=1000-11                                                      78.69Ki ± 0%                             78.70Ki ± 0%       ~ (p=0.058 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=1-11                                                        120.9Ki ± 0%                             121.7Ki ± 0%  +0.68% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=10-11                                                       158.2Ki ± 0%                             159.4Ki ± 0%  +0.80% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=100-11                                                      707.4Ki ± 0%                             708.3Ki ± 0%  +0.13% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=1000-11                                                     707.3Ki ± 0%                             708.3Ki ± 0%  +0.14% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=1-11                                                       985.3Ki ± 0%                             978.7Ki ± 1%  -0.66% (p=0.041 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=10-11                                                      1.064Mi ± 0%                             1.091Mi ± 0%  +2.54% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=100-11                                                     1.479Mi ± 0%                             1.547Mi ± 0%  +4.62% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=1000-11                                                    6.935Mi ± 0%                             6.947Mi ± 0%  +0.17% (p=0.002 n=6)
geomean                                                                                                                   156.1Ki                                  157.0Ki       +0.59%

                                                                       │ BenchmarkPusherConsumer_ParallelPusher_MultiTenant-before.txt │ BenchmarkPusherConsumer_ParallelPusher_MultiTenant-after.txt │
                                                                       │                           allocs/op                           │              allocs/op                vs base                │
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=1-11                                                            48.00 ± 0%                             48.00 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=10-11                                                           48.00 ± 0%                             48.00 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=100-11                                                          48.00 ± 0%                             48.00 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=1,tenants=1000-11                                                         48.00 ± 0%                             48.00 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=1-11                                                           166.0 ± 0%                             166.0 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=10-11                                                          291.0 ± 0%                             291.0 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=100-11                                                         291.0 ± 0%                             291.0 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=10,tenants=1000-11                                                        291.0 ± 0%                             291.0 ± 0%       ~ (p=1.000 n=6) ¹
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=1-11                                                         1.364k ± 0%                            1.366k ± 0%  +0.15% (p=0.004 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=10-11                                                        1.466k ± 0%                            1.467k ± 0%  +0.07% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=100-11                                                       2.664k ± 0%                            2.664k ± 0%       ~ (p=0.545 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=100,tenants=1000-11                                                      2.663k ± 0%                            2.664k ± 0%       ~ (p=0.182 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=1-11                                                        13.28k ± 0%                            13.27k ± 0%       ~ (p=0.093 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=10-11                                                       13.38k ± 0%                            13.41k ± 0%  +0.19% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=100-11                                                      14.36k ± 0%                            14.37k ± 0%  +0.08% (p=0.002 n=6)
PusherConsumer_ParallelPusher_MultiTenant/records=1000,tenants=1000-11                                                     26.23k ± 0%                            26.22k ± 0%  -0.07% (p=0.002 n=6)
geomean                                                                                                                     784.6                                  784.8       +0.02%
¹ all samples are equal

No changelog because I will add one at the end of this work, once -ingest-storage.kafka.ingestion-concurrency-sequential-pusher-enabled will be removed and the new behaviour will be the default. With the default config parallelStorageShards is never used with only 1 shard (you need to set -ingest-storage.kafka.ingestion-concurrency-sequential-pusher-enabled=false to use parallelStorageShards even when there's only 1 shard).

Which issue(s) this PR fixes or relates to

N/A

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]. If changelog entry is not needed, please add the changelog-not-needed label to the PR.
• about-versioning.md updated with experimental features.

---

Note

Optimizes shard routing when numShards == 1 and refines performance benchmarks.

• In parallelStorageShards.PushToStorageAndReleaseRequest, bypasses label hashing/modulo and always targets shard 0 when only one shard; metadata routing now skips round-robin and random start when single-shard.
• BenchmarkPusherConsumer_ParallelPusher_MultiTenant: replaces single-parameter sweep with nested records×tenants cases, prebuilds records per case, and sets IngestionConcurrencySequentialPusherEnabled=false to exercise parallel pusher consistently.

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[narqo]

🔥 Code-wise looks good to me. The CI isn't happy, though

[pracucci]

The CI isn't happy, though

It was an unrelated flaky test.