concurrency-limit initial logical size calculation [v2]

[problame]

Problem

Before this PR, there was no concurrency limit on initial logical size computations.

While logical size computations are lazy in theory, in practice (production), they happen in a short timeframe after restart.

This means that on a PS with 20k tenants, we'd have up to 20k concurrent initial logical size calculation requests.

This is self-inflicted needless overload.

This hasn't been a problem so far because the .await points on the logical size calculation path never return Pending, hence we have a natural concurrency limit of the number of executor threads.
But, as soon as we return Pending somewhere in the logical size calculation path, other concurrent tasks get scheduled by tokio.
If these other tasks are also logical size calculations, they eventually pound on the same bottleneck.

For example, in #5479, we want to switch the VirtualFile descriptor cache to a tokio::sync::RwLock, which makes us return Pending, and without measures like this patch, after PS restart, VirtualFile descriptor cache thrashes heavily for 2 hours until all the logical size calculations have been computed and the degree of concurrency / concurrent VirtualFile operations is down to regular levels.
See the Experiment section below for details.

Background

Before this PR, initial logical size calculation was spawned lazily on first call to Timeline::get_current_logical_size().

In practice (prod), the lazy calculation is triggered by WalReceiverConnectionHandler if the timeline is active according to storage broker, or by the first iteration of consumption metrics worker after restart (MetricsCollection).

The spawns by walreceiver are high-priority because logical size is needed by Safekeepers (via walreceiver PageserverFeedback) to enforce the project logical size limit.
The spawns by metrics collection are not on the user-critical path and hence low-priority. ¹

The ratio of walreceiver-initiated spawns vs consumption-metrics-initiated spawns can be reconstructed from logs (spawning logical size computation from context of task kind {:?}").
PR #5995 and #6018 adds metrics for this.

First investigation of the ratio lead to the discovery that walreceiver spawns 75% of init logical size computations.
That's because of two bugs:

• In Safekeepers: Safekeeper timeline is deactivated without persisting state #5993
• In interaction between Pageservers and Safekeepers: WAL records that are no-ops cause timelines to remain active #5962

The safekeeper bug is likely primarily responsible but we don't have the data yet. The metrics will hopefully provide some insights.

When assessing production-readiness of this PR, please assume that neither of these bugs are fixed yet.

Changes In This PR

With this PR, initial logical size calculation is reworked as follows:

First, all initial logical size calculation task_mgr tasks are started early, as part of timeline activation, and run a retry loop with long back-off until success. This removes the lazy computation; it was needless complexity because in practice, we compute all logical sizes anyways, because consumption metrics collects it.

Second, within the initial logical size calculation task, each attempt queues behind the background loop concurrency limiter semaphore. This fixes the performance issue that we pointed out in the "Problem" section earlier.

Third, there is a twist to queuing behind the background loop concurrency limiter semaphore. Logical size is needed by Safekeepers (via walreceiver PageserverFeedback) to enforce the project logical size limit. However, we currently do open walreceiver connections even before we have an exact logical size. That's bad, and I'll build on top of this PR to fix that (#5963). But, for the purposes of this PR, we don't want to introduce a regression, i.e., we don't want to provide an exact value later than before this PR. The solution is to introduce a priority-boosting mechanism (GetLogicalSizePriority), allowing callers of Timeline::get_current_logical_size to specify how urgently they need an exact value. The effect of specifying high urgency is that the initial logical size calculation task for the timeline will skip the concurrency limiting semaphore. This should yield effectively the same behavior as we had before this PR with lazy spawning.

Last, the priority-boosting mechanism obsoletes the init_order's grace period for initial logical size calculations. It's a separate commit to reduce the churn during review. We can drop that commit if people think it's too much churn, and commit it later once we know this PR here worked as intended.

Experiment With #5479

I validated this PR combined with #5479 to assess whether we're making forward progress towards asyncification.

The setup is an i3en.3xlarge instance with 20k tenants, each with one timeline that has 9 layers.
All tenants are inactive, i.e., not known to SKs nor storage broker.
This means all initial logical size calculations are spawned by consumption metrics MetricsCollection task kind.
The consumption metrics worker starts requesting logical sizes at low priority immediately after restart. This is achieved by deleting the consumption metrics cache file on disk before starting PS.[^consumption_metrics_cache_file]

[^consumption_metrics_cache_file] Consumption metrics worker persists its interval across restarts to achieve persistent reporting intervals across PS restarts; delete the state file on disk to get predictable (and I believe worst-case in terms of concurrency during PS restart) behavior.

Before this patch, all of these timelines would all do their initial logical size calculation in parallel, leading to extreme thrashing in page cache and virtual file cache.

With this patch, the virtual file cache thrashing is reduced significantly (from 80k open-system-calls/second to ~500 open-system-calls/second during loading).

Critique

The obvious critique with above experiment is that there's no skipping of the semaphore, i.e., the priority-boosting aspect of this PR is not exercised.

If even just 1% of our 20k tenants in the setup were active in SK/storage_broker, then 200 logical size calculations would skip the limiting semaphore immediately after restart and run concurrently.

Further critique: given the two bugs wrt timeline inactive vs active state that were mentioned in the Background section, we could have 75% of our 20k tenants being (falsely) active on restart.

So... (next section)

This Doesn't Make Us Ready For Async VirtualFile

This PR is a step towards asynchronous VirtualFile, aka, #5479 or even #4744.

But it doesn't yet enable us to ship #5479.

The reason is that this PR doesn't limit the amount of high-priority logical size computations.
If there are many high-priority logical size calculations requested, we'll fall over like we did if #5479 is applied without this PR.
And currently, at very least due to the bugs mentioned in the Background section, we run thousands of high-priority logical size calculations on PS startup in prod.

So, at a minimum, we need to fix these bugs.

Then we can ship #5479 and #4744, and things will likely be fine under normal operation.

But in high-traffic situations, overload problems will still be more likely to happen, e.g., VirtualFile cache descriptor thrashing.
The solution candidates for that are orthogonal to this PR though:

• global concurrency limiting
• per-tenant rate limiting => Epic: per-tenant read path throttling #5899
• load shedding
• scaling bottleneck resources (fd cache size (neondatabase/cloud#8351), page cache size(neondatabase/cloud#8351), spread load across more PSes, etc)

Conclusion

Even with the remarks from in the previous section, we should merge this PR because:

1. it's an improvement over the status quo (esp. if the aforementioned bugs wrt timeline active / inactive are fixed)
2. it prepares the way for WIP: fix: logical size limit is broken during PS restart #6010
3. it gets us close to shipping revert "revert recent VirtualFile asyncification changes #5291" #5479 and Epic: scalable async disk IO (tokio-epoll-uring) #4744

Footnotes

1. We can't delay metrics collection indefintely because there are TBD internal SLOs tied to metrics collection happening in a timeline manner (https://github.com/neondatabase/cloud/issues/7408). But let's ignore that in this issue. ↩

[github-actions]

2436 tests run: 2337 passed, 0 failed, 99 skipped (full report)

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Flaky tests (4)

Postgres 16

• test_branching_with_pgbench[flat-1-10]: debug

Postgres 15

• test_statvfs_pressure_min_avail_bytes: debug
• test_timetravel: debug

Postgres 14

• test_gc_cutoff: debug

Code coverage (full report)

• functions: 54.5% (9281 of 17022 functions)
• lines: 82.0% (53944 of 65781 lines)

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_{The comment gets automatically updated with the latest test results
5b0e7a7 at 2023-12-04T17:07:34.152Z :recycle:}

[problame]

@koivunej I polished up the PR description, re-ran the experiment to make sure this is a step forward with regard to #5479 , and added two sections explaining what exactly that means.

The remaining comment is to make sure that logical size computation impl is cancel safe: #6000 (comment)

I think it is, and added a commit that adds cancel-safety comments to the methods that I audited. I'm treating the Layer::get_reconstruct_data as a black box.

Would appreciate you to comment there, and resolve your other conversation.

Then let's get this shipped so we maximize soak time.