pageserver: use a single tokio runtime (#6555)

Before this PR, each core had 3 executor threads from 3 different
runtimes. With this PR, we just have one runtime, with one thread per
core. Switching to a single tokio runtime should reduce that effective
over-commit of CPU and in theory help with tail latencies -- iff all
tokio tasks are well-behaved and yield to the runtime regularly.

Are All Tasks Well-Behaved? Are We Ready?
-----------------------------------------

Sadly there doesn't seem to be good out-of-the box tokio tooling to
answer this question.

We *believe* all tasks are well behaved in today's code base, as of the
switch to `virtual_file_io_engine = "tokio-epoll-uring"` in production
(https://github.com/neondatabase/aws/pull/1121).

The only remaining executor-thread-blocking code is walredo and some
filesystem namespace operations.

Filesystem namespace operations work is being tracked in #6663 and not
considered likely to actually block at this time.

Regarding walredo, it currently does a blocking `poll` for read/write to
the pipe file descriptors we use for IPC with the walredo process.
There is an ongoing experiment to make walredo async (#6628), but it
needs more time because there are surprisingly tricky trade-offs that
are articulated in that PR's description (which itself is still WIP).
What's relevant for *this* PR is that
1. walredo is always CPU-bound
2. production tail latencies for walredo request-response
(`pageserver_wal_redo_seconds_bucket`) are
  - p90: with few exceptions, low hundreds of micro-seconds
  - p95: except on very packed pageservers, below 1ms
  - p99: all below 50ms, vast majority below 1ms
  - p99.9: almost all around 50ms, rarely at >= 70ms
- [Dashboard
Link](https://neonprod.grafana.net/d/edgggcrmki3uof/2024-03-walredo-latency?orgId=1&var-ds=ZNX49CDVz&var-pXX_by_instance=0.9&var-pXX_by_instance=0.99&var-pXX_by_instance=0.95&var-adhoc=instance%7C%21%3D%7Cpageserver-30.us-west-2.aws.neon.tech&var-per_instance_pXX_max_seconds=0.0005&from=1711049688777&to=1711136088777)

The ones below 1ms are below our current threshold for when we start
thinking about yielding to the executor.
The tens of milliseconds stalls aren't great, but, not least because of
the implicit overcommit of CPU by the three runtimes, we can't be sure
whether these tens of milliseconds are inherently necessary to do the
walredo work or whether we could be faster if there was less contention
for CPU.

On the first item (walredo being always CPU-bound work): it means that
walredo processes will always compete with the executor threads.
We could yield, using async walredo, but then we hit the trade-offs
explained in that PR.

tl;dr: the risk of stalling executor threads through blocking walredo
seems low, and switching to one runtime cleans up one potential source
for higher-than-necessary stall times (explained in the previous
paragraphs).


Code Changes
------------

- Remove the 3 different runtime definitions.
- Add a new definition called `THE_RUNTIME`.
- Use it in all places that previously used one of the 3 removed
runtimes.
- Remove the argument from `task_mgr`.
- Fix failpoint usage where `pausable_failpoint!` should have been used.
We encountered some actual failures because of this, e.g., hung
`get_metric()` calls during test teardown that would client-timeout
after 300s.

As indicated by the comment above `THE_RUNTIME`, we could take this
clean-up further.
But before we create so much churn, let's first validate that there's no
perf regression.


Performance
-----------

We will test this in staging using the various nightly benchmark runs.

However, the worst-case impact of this change is likely compaction
(=>image layer creation) competing with compute requests.
Image layer creation work can't be easily generated & repeated quickly
by pagebench.
So, we'll simply watch getpage & basebackup tail latencies in staging.

Additionally, I have done manual benchmarking using pagebench.
Report:
https://neondatabase.notion.site/2024-03-23-oneruntime-change-benchmarking-22a399c411e24399a73311115fb703ec?pvs=4
Tail latencies and throughput are marginally better (no regression =
good).
Except in a workload with 128 clients against one tenant.
There, the p99.9 and p99.99 getpage latency is about 2x worse (at
slightly lower throughput).
A dip in throughput every 20s (compaction_period_ is clearly visible,
and probably responsible for that worse tail latency.
This has potential to improve with async walredo, and is an edge case
workload anyway.


Future Work
-----------

1. Once this change has shown satisfying results in production, change
the codebase to use the ambient runtime instead of explicitly
referencing `THE_RUNTIME`.
2. Have a mode where we run with a single-threaded runtime, so we
uncover executor stalls more quickly.
3. Switch or write our own failpoints library that is async-native:
https://github.com/neondatabase/neon/issues/7216

pageserver/src/task_mgr.rs

@@ -98,42 +98,22 @@ use utils::id::TimelineId;
 // other operations, if the upload tasks e.g. get blocked on locks. It shouldn't
 // happen, but still.
 //
-pub static COMPUTE_REQUEST_RUNTIME: Lazy<Runtime> = Lazy::new(|| {
-    tokio::runtime::Builder::new_multi_thread()
-        .thread_name("compute request worker")
-        .enable_all()
-        .build()
-        .expect("Failed to create compute request runtime")
-});
 
-pub static MGMT_REQUEST_RUNTIME: Lazy<Runtime> = Lazy::new(|| {
+/// The single tokio runtime used by all pageserver code.
+/// In the past, we had multiple runtimes, and in the future we should weed out
+/// remaining references to this global field and rely on ambient runtime instead,
+/// i.e., use `tokio::spawn` instead of `THE_RUNTIME.spawn()`, etc.
+pub static THE_RUNTIME: Lazy<Runtime> = Lazy::new(|| {
     tokio::runtime::Builder::new_multi_thread()
-        .thread_name("mgmt request worker")
-        .enable_all()
-        .build()
-        .expect("Failed to create mgmt request runtime")
-});
-
-pub static WALRECEIVER_RUNTIME: Lazy<Runtime> = Lazy::new(|| {
-    tokio::runtime::Builder::new_multi_thread()
-        .thread_name("walreceiver worker")
-        .enable_all()
-        .build()
-        .expect("Failed to create walreceiver runtime")
-});
-
-pub static BACKGROUND_RUNTIME: Lazy<Runtime> = Lazy::new(|| {
-    tokio::runtime::Builder::new_multi_thread()
-        .thread_name("background op worker")
         // if you change the number of worker threads please change the constant below
         .enable_all()
         .build()
         .expect("Failed to create background op runtime")
 });
 
-pub(crate) static BACKGROUND_RUNTIME_WORKER_THREADS: Lazy<usize> = Lazy::new(|| {
+pub(crate) static THE_RUNTIME_WORKER_THREADS: Lazy<usize> = Lazy::new(|| {
     // force init and thus panics
-    let _ = BACKGROUND_RUNTIME.handle();
+    let _ = THE_RUNTIME.handle();
     // replicates tokio-1.28.1::loom::sys::num_cpus which is not available publicly
     // tokio would had already panicked for parsing errors or NotUnicode
     //
@@ -325,7 +305,6 @@ struct PageServerTask {
 /// Note: if shutdown_process_on_error is set to true failure
 ///   of the task will lead to shutdown of entire process
 pub fn spawn<F>(
-    runtime: &tokio::runtime::Handle,
     kind: TaskKind,
     tenant_shard_id: Option<TenantShardId>,
     timeline_id: Option<TimelineId>,
@@ -354,7 +333,7 @@ where
 
     let task_name = name.to_string();
     let task_cloned = Arc::clone(&task);
-    let join_handle = runtime.spawn(task_wrapper(
+    let join_handle = THE_RUNTIME.spawn(task_wrapper(
         task_name,
         task_id,
         task_cloned,