For all high-rooted long-lived IoConcurrency's, IoConcurrency::drop will
never run.
What we actually care about is that we leave no dangling IOs after
get_vectored_impl, which lives much shorter than a high-rooted
IoConcurrency.
However, lifetime of `ValuesReconstructData` is generally == lifetime of
get_vectored_impl.
Reproduced by
test_runner/regress/test_branching.py::test_branching_with_pgbench[debug-pg16-flat-1-10]'
It kinda makes sense that this deadlocks in `sequential` mode.
However, it also deadlocks in `sidecar-task` mode.
I don't understand why.
Otherwise we might hit ERRORs in otherwise safe situations (such as user
queries), which isn't a great user experience.
## Problem
https://github.com/neondatabase/neon/pull/10376
## Summary of changes
Instead of accepting internal errors as acceptable, we ensure we don't
exceed our allocated usage.
## Refs
- fixes https://github.com/neondatabase/neon/issues/10444
## Problem
We're seeing a panic `handles are only shut down once in their lifetime`
in our performance testbed.
## Hypothesis
Annotated code in
https://github.com/neondatabase/neon/issues/10444#issuecomment-2602286415.
```
T1: drop Cache, executes up to (1)
=> HandleInner is now in state ShutDown
T2: Timeline::shutdown => PerTimelineState::shutdown executes shutdown() again => panics
```
Likely this snuck in the final touches of #10386 where I narrowed down
the locking rules.
## Summary of changes
Make duplicate shutdowns a no-op.
## Summary
Whereas currently we send all WAL to all pageserver shards, and each
shard filters out the data that it needs,
in this RFC we add a mechanism to filter the WAL on the safekeeper, so
that each shard receives
only the data it needs.
This will place some extra CPU load on the safekeepers, in exchange for
reducing the network bandwidth
for ingesting WAL back to scaling as O(1) with shard count, rather than
O(N_shards).
Touches #9329.
## Checklist before requesting a review
- [ ] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.
## Checklist before merging
- [ ] Do not forget to reformat commit message to not include the above
checklist
---------
Co-authored-by: Vlad Lazar <vlalazar.vlad@gmail.com>
Co-authored-by: Vlad Lazar <vlad@neon.tech>
The other test focus on the external interface usage while the tests
added in this PR add some testing around HandleInner's lifecycle,
ensuring we don't leak it once either connection gets dropped or
per-timeline-state is shut down explicitly.
It was requested by review in #10305 to use an enum or something like it
for distinguishing the different modes instead of two parameters,
because two flags allow four combinations, and two of them don't really
make sense/ aren't used.
follow-up of #10305
## Problem
Since #9916 , the chaos code is actively fighting the optimizer: tenants
tend to be attached in their preferred AZ, so most chaos migrations were
moving them to a non-preferred AZ.
## Summary of changes
- When picking migrations, prefer to migrate things _toward_ their
preferred AZ when possible. Then pick shards to move the other way when
necessary.
The resulting behavior should be an alternating "back and forth" where
the chaos code migrates thiings away from home, and then migrates them
back on the next iteration.
The side effect will be that the chaos code actively helps to push
things into their home AZ. That's not contrary to its purpose though: we
mainly just want it to continuously migrate things to exercise
migration+notification code.
## Problem
Occasionally, we encounter bugs in test environments that can be
detected at the point of uploading an index, but we proceed to upload it
anyway and leave a tenant in a broken state that's awkward to handle.
## Summary of changes
- Validate index when submitting it for upload, so that we can see the
issue quickly e.g. in an API invoking compaction
- Validate index before executing the upload, so that we have a hard
enforcement that any code path that tries to upload an index will not
overwrite a valid index with an invalid one.
Add an endpoint to obtain the utilization of a safekeeper. Future
changes to the storage controller can use this endpoint to find the most
suitable safekeepers for newly created timelines, analogously to how
it's done for pageservers already.
Initially we just want to assign by timeline count, then we can iterate
from there.
Part of https://github.com/neondatabase/neon/issues/9011
## Problem
871e8b325f failed CI on main because a job
ran to soon. This was caused by
ea84ec357f. While `promote-images-dev`
does not inherently need `neon-image`, a few jobs depending on
`promote-images-dev` do need it, and previously had it when it was
`promote-images`, which depended on `test-images`, which in turn
depended on `neon-image`.
## Summary of changes
To ensure jobs depending `docker.io/neondatabase/neon` images get them,
`promote-images-dev` gets the dependency to `neon-image` back which it
previously had transitively through `test-images`.
Instead of generating our own request ID, we can just use the one
provided by the control plane. In the event, we get a request from a
client which doesn't set X-Request-ID, then we just generate one which
is useful for tracing purposes.
Signed-off-by: Tristan Partin <tristan@neon.tech>
# Refs
- fixes https://github.com/neondatabase/neon/issues/10309
- fixup of batching design, first introduced in
https://github.com/neondatabase/neon/pull/9851
- refinement of https://github.com/neondatabase/neon/pull/8339
# Problem
`Tenant::shutdown` was occasionally taking many minutes (sometimes up to
20) in staging and prod if the
`page_service_pipelining.mode="concurrent-futures"` is enabled.
# Symptoms
The issue happens during shard migration between pageservers.
There is page_service unavailability and hence effectively downtime for
customers in the following case:
1. The source (state `AttachedStale`) gets stuck in `Tenant::shutdown`,
waiting for the gate to close.
2. Cplane/Storcon decides to transition the target `AttachedMulti` to
`AttachedSingle`.
3. That transition comes with a bump of the generation number, causing
the `PUT .../location_config` endpoint to do a full `Tenant::shutdown` /
`Tenant::attach` cycle for the target location.
4. That `Tenant::shutdown` on the target gets stuck, waiting for the
gate to close.
5. Eventually the gate closes (`close completed`), correlating with a
`page_service` connection handler logging that it's exiting because of a
network error (`Connection reset by peer` or `Broken pipe`).
While in (4):
- `Tenant::shutdown` is stuck waiting for all `Timeline::shutdown` calls
to complete.
So, really, this is a `Timeline::shutdown` bug.
- retries from Cplane/Storcon to complete above state transitions, fail
with errors related to the tenant mgr slot being in state
`TenantSlot::InProgress`, the tenant state being
`TenantState::Stopping`, and the timelines being in
`TimelineState::Stopping`, and the `Timeline::cancel` being cancelled.
- Existing (and/or new?) page_service connections log errors `error
reading relation or page version: Not found: Timed out waiting 30s for
tenant active state. Latest state: None`
# Root-Cause
After a lengthy investigation ([internal
write-up](https://www.notion.so/neondatabase/2025-01-09-batching-deadlock-Slow-Log-Analysis-in-Staging-176f189e00478050bc21c1a072157ca4?pvs=4))
I arrived at the following root cause.
The `spsc_fold` channel (`batch_tx`/`batch_rx`) that connects the
Batcher and Executor stages of the pipelined mode was storing a `Handle`
and thus `GateGuard` of the Timeline that was not shutting down.
The design assumption with pipelining was that this would always be a
short transient state.
However, that was incorrect: the Executor was stuck on writing/flushing
an earlier response into the connection to the client, i.e., socket
write being slow because of TCP backpressure.
The probable scenario of how we end up in that case:
1. Compute backend process sends a continuous stream of getpage prefetch
requests into the connection, but never reads the responses (why this
happens: see Appendix section).
2. Batch N is processed by Batcher and Executor, up to the point where
Executor starts flushing the response.
3. Batch N+1 is procssed by Batcher and queued in the `spsc_fold`.
4. Executor is still waiting for batch N flush to finish.
5. Batcher eventually hits the `TimeoutReader` error (10min).
From here on it waits on the
`spsc_fold.send(Err(QueryError(TimeoutReader_error)))`
which will never finish because the batch already inside the `spsc_fold`
is not
being read by the Executor, because the Executor is still stuck in the
flush.
(This state is not observable at our default `info` log level)
6. Eventually, Compute backend process is killed (`close()` on the
socket) or Compute as a whole gets killed (probably no clean TCP
shutdown happening in that case).
7. Eventually, Pageserver TCP stack learns about (6) through RST packets
and the Executor's flush() call fails with an error.
8. The Executor exits, dropping `cancel_batcher` and its end of the
spsc_fold.
This wakes Batcher, causing the `spsc_fold.send` to fail.
Batcher exits.
The pipeline shuts down as intended.
We return from `process_query` and log the `Connection reset by peer` or
`Broken pipe` error.
The following diagram visualizes the wait-for graph at (5)
```mermaid
flowchart TD
Batcher --spsc_fold.send(TimeoutReader_error)--> Executor
Executor --flush batch N responses--> socket.write_end
socket.write_end --wait for TCP window to move forward--> Compute
```
# Analysis
By holding the GateGuard inside the `spsc_fold` open, the pipelining
implementation
violated the principle established in
(https://github.com/neondatabase/neon/pull/8339).
That is, that `Handle`s must only be held across an await point if that
await point
is sensitive to the `<Handle as Deref<Target=Timeline>>::cancel` token.
In this case, we were holding the Handle inside the `spsc_fold` while
awaiting the
`pgb_writer.flush()` future.
One may jump to the conclusion that we should simply peek into the
spsc_fold to get
that Timeline cancel token and be sensitive to it during flush, then.
But that violates another principle of the design from
https://github.com/neondatabase/neon/pull/8339.
That is, that the page_service connection lifecycle and the Timeline
lifecycles must be completely decoupled.
Tt must be possible to shut down one shard without shutting down the
page_service connection, because on that single connection we might be
serving other shards attached to this pageserver.
(The current compute client opens separate connections per shard, but,
there are plans to change that.)
# Solution
This PR adds a `handle::WeakHandle` struct that does _not_ hold the
timeline gate open.
It must be `upgrade()`d to get a `handle::Handle`.
That `handle::Handle` _does_ hold the timeline gate open.
The batch queued inside the `spsc_fold` only holds a `WeakHandle`.
We only upgrade it while calling into the various `handle_` methods,
i.e., while interacting with the `Timeline` via `<Handle as
Deref<Target=Timeline>>`.
All that code has always been required to be (and is!) sensitive to
`Timeline::cancel`, and therefore we're guaranteed to bail from it
quickly when `Timeline::shutdown` starts.
We will drop the `Handle` immediately, before we start
`pgb_writer.flush()`ing the responses.
Thereby letting go of our hold on the `GateGuard`, allowing the timeline
shutdown to complete while the page_service handler remains intact.
# Code Changes
* Reproducer & Regression Test
* Developed and proven to reproduce the issue in
https://github.com/neondatabase/neon/pull/10399
* Add a `Test` message to the pagestream protocol (`cfg(feature =
"testing")`).
* Drive-by minimal improvement to the parsing code, we now have a
`PagestreamFeMessageTag`.
* Refactor `pageserver/client` to allow sending and receiving
`page_service` requests independently.
* Add a Rust helper binary to produce situation (4) from above
* Rationale: (4) and (5) are the same bug class, we're holding a gate
open while `flush()`ing.
* Add a Python regression test that uses the helper binary to
demonstrate the problem.
* Fix
* Introduce and use `WeakHandle` as explained earlier.
* Replace the `shut_down` atomic with two enum states for `HandleInner`,
wrapped in a `Mutex`.
* To make `WeakHandle::upgrade()` and `Handle::downgrade()`
cache-efficient:
* Wrap the `Types::Timeline` in an `Arc`
* Wrap the `GateGuard` in an `Arc`
* The separate `Arc`s enable uncontended cloning of the timeline
reference in `upgrade()` and `downgrade()`.
If instead we were `Arc<Timeline>::clone`, different connection handlers
would be hitting the same cache line on every upgrade()/downgrade(),
causing contention.
* Please read the udpated module-level comment in `mod handle`
module-level comment for details.
# Testing & Performance
The reproducer test that failed before the changes now passes, and
obviously other tests are passing as well.
We'll do more testing in staging, where the issue happens every ~4h if
chaos migrations are enabled in storcon.
Existing perf testing will be sufficient, no perf degradation is
expected.
It's a few more alloctations due to the added Arc's, but, they're low
frequency.
# Appendix: Why Compute Sometimes Doesn't Read Responses
Remember, the whole problem surfaced because flush() was slow because
Compute was not reading responses. Why is that?
In short, the way the compute works, it only advances the page_service
protocol processing when it has an interest in data, i.e., when the
pagestore smgr is called to return pages.
Thus, if compute issues a bunch of requests as part of prefetch but then
it turns out it can service the query without reading those pages, it
may very well happen that these messages stay in the TCP until the next
smgr read happens, either in that session, or possibly in another
session.
If there’s too many unread responses in the TCP, the pageserver kernel
is going to backpressure into userspace, resulting in our stuck flush().
All of this stems from the way vanilla Postgres does prefetching and
"async IO":
it issues `fadvise()` to make the kernel do the IO in the background,
buffering results in the kernel page cache.
It then consumes the results through synchronous `read()` system calls,
which hopefully will be fast because of the `fadvise()`.
If it turns out that some / all of the prefetch results are not needed,
Postgres will not be issuing those `read()` system calls.
The kernel will eventually react to that by reusing page cache pages
that hold completed prefetched data.
Uncompleted prefetch requests may or may not be processed -- it's up to
the kernel.
In Neon, the smgr + Pageserver together take on the role of the kernel
in above paragraphs.
In the current implementation, all prefetches are sent as GetPage
requests to Pageserver.
The responses are only processed in the places where vanilla Postgres
would do the synchronous `read()` system call.
If we never get to that, the responses are queued inside the TCP
connection, which, once buffers run full, will backpressure into
Pageserver's sending code, i.e., the `pgb_writer.flush()` that was the
root cause of the problems we're fixing in this PR.
The extension now supports Postgres 17. The release also seems to be
binary compatible with the previous version.
Signed-off-by: Tristan Partin <tristan@neon.tech>
## Problem
`test_storage_controller_node_deletion` sometimes failed because shards
were moving around during timeline creation, and neon_local isn't
tolerant of that. The movements were unexpected because the shards had
only just been created.
This was a regression from #9916Closes: #10383
## Summary of changes
- Make this test use multiple AZs -- this makes the storage controller's
scheduling reliably stable
Why this works: in #9916 , I made a simplifying assumption that we would
have multiple AZs to get nice stable scheduling -- it's much easier,
because each tenant has a well defined primary+secondary location when
they have an AZ preference and nodes have different AZs. Everything
still works if you don't have multiple AZs, but you just have this quirk
that sometimes the optimizer can disagree with initial scheduling, so
once in a while a shard moves after being created -- annoying for tests,
harmless IRL.
## Problem
All pageserver have the same application name which makes it hard to
distinguish them.
## Summary of changes
Include the node id in the application name sent to the safekeeper. This
should gives us
more visibility in logs. There's a few metrics that will increase in
cardinality by `pageserver_count`,
but that's fine.
## Problem
Node fills were limited to moving (total shards / node_count) shards. In
systems that aren't perfectly balanced already, that leads us to skip
migrating some of the shards that belong on this node, generating work
for the optimizer later to gradually move them back.
## Summary of changes
- Where a shard has a preferred AZ and is currently attached outside
this AZ, then always promote it during fill, irrespective of target fill
count
## Problem
We were comparing serialized configs from the database with serialized
configs from memory. If fields have been added/removed to TenantConfig,
this generates spurious consistency errors. This is fine in test
environments, but limits the usefulness of this debug API in the field.
Closes: https://github.com/neondatabase/neon/issues/10369
## Summary of changes
- Do a decode/encode cycle on the config before comparing it, so that it
will have exactly the expected fields.
