## Problem
The k-merge in pageserver compaction currently relies on iterators over
the keys and also over the values. This approach does not support async
code because we are using iterators and those don't support async in
general. Also, the k-merge implementation we use doesn't support async
either. Instead, as we already load all the keys into memory, the plan
is to just do the sorting in-memory for now, switch to async, and then
once we want to support workloads that don't have all keys stored in
memory, we can look into switching to a k-merge implementation that
supports async instead.
## Summary of changes
The core of this PR is the move from functions on the `PersistentLayer`
trait to return custom iterator types to inherent functions on `DeltaLayer`
that return buffers with all keys or value references.
Value references are a type we created in this PR, containing a
`BlobRef` as well as an `Arc` pointer to the `DeltaLayerInner`, so that
we can lazily load the values during compaction. This preserves the
property of the current code.
This PR does not switch us to doing the k-merge via sort on slices, but
with this PR, doing such a switch is relatively easy and only requires
changes of the compaction code itself.
Part of https://github.com/neondatabase/neon/issues/4743
In #4743, we'd like to convert the read path to use `async` rust. In
preparation of that, this PR switches some functions that are calling
lower level functions like `BlockReader::read_blk`,
`BlockCursor::read_blob`, etc into `async`. The PR does not switch all
functions however, and only focuses on the ones which are easy to
switch.
This leaves around some async functions that are (currently)
unnecessarily `async`, but on the other hand it makes future changes
smaller in diff.
Part of #4743 (but does not completely address it).
Does three things:
* add a `Display` impl for `LayerFileName` equal to the `short_id`
* based on that, replace the `Layer::short_id` function by a requirement
for a `Display` impl
* use that `Display` impl in the places where the `short_id` and `file_name()` functions were used instead
Fixes#4145
This PR concludes the "async `Layer::get_value_reconstruct_data`"
project.
The problem we're solving is that, before this patch, we'd execute
`Layer::get_value_reconstruct_data` on the tokio executor threads.
This function is IO- and/or CPU-intensive.
The IO is using VirtualFile / std::fs; hence it's blocking.
This results in unfairness towards other tokio tasks, especially under
(disk) load.
Some context can be found at
https://github.com/neondatabase/neon/issues/4154
where I suspect (but can't prove) load spikes of logical size
calculation to
cause heavy eviction skew.
Sadly we don't have tokio runtime/scheduler metrics to quantify the
unfairness.
But generally, we know blocking the executor threads on std::fs IO is
bad.
So, let's have this change and watch out for severe perf regressions in
staging & during rollout.
## Changes
* rename `Layer::get_value_reconstruct_data` to
`Layer::get_value_reconstruct_data_blocking`
* add a new blanket impl'd `Layer::get_value_reconstruct_data`
`async_trait` method that runs `get_value_reconstruct_data_blocking`
inside `spawn_blocking`.
* The `spawn_blocking` requires `'static` lifetime of the captured
variables; hence I had to change the data flow to _move_ the
`ValueReconstructState` into and back out of get_value_reconstruct_data
instead of passing a reference. It's a small struct, so I don't expect a
big performance penalty.
## Performance
Fundamentally, the code changes cause the following performance-relevant
changes:
* Latency & allocations: each `get_value_reconstruct_data` call now
makes a short-lived allocation because `async_trait` is just sugar for
boxed futures under the hood
* Latency: `spawn_blocking` adds some latency because it needs to move
the work to a thread pool
* using `spawn_blocking` plus the existing synchronous code inside is
probably more efficient better than switching all the synchronous code
to tokio::fs because _each_ tokio::fs call does `spawn_blocking` under
the hood.
* Throughput: the `spawn_blocking` thread pool is much larger than the
async executor thread pool. Hence, as long as the disks can keep up,
which they should according to AWS specs, we will be able to deliver
higher `get_value_reconstruct_data` throughput.
* Disk IOPS utilization: we will see higher disk utilization if we get
more throughput. Not a problem because the disks in prod are currently
under-utilized, according to node_exporter metrics & the AWS specs.
* CPU utilization: at higher throughput, CPU utilization will be higher.
Slightly higher latency under regular load is acceptable given the
throughput gains and expected better fairness during disk load peaks,
such as logical size calculation peaks uncovered in #4154.
## Full Stack Of Preliminary PRs
This PR builds on top of the following preliminary PRs
1. Clean-ups
* https://github.com/neondatabase/neon/pull/4316
* https://github.com/neondatabase/neon/pull/4317
* https://github.com/neondatabase/neon/pull/4318
* https://github.com/neondatabase/neon/pull/4319
* https://github.com/neondatabase/neon/pull/4321
* Note: these were mostly to find an alternative to #4291, which I
thought we'd need in my original plan where we would need to convert
`Tenant::timelines` into an async locking primitive (#4333). In reviews,
we walked away from that, but these cleanups were still quite useful.
2. https://github.com/neondatabase/neon/pull/4364
3. https://github.com/neondatabase/neon/pull/4472
4. https://github.com/neondatabase/neon/pull/4476
5. https://github.com/neondatabase/neon/pull/4477
6. https://github.com/neondatabase/neon/pull/4485
7. https://github.com/neondatabase/neon/pull/4441
This is preliminary work for/from #4220 (async `Layer::get_value_reconstruct_data`).
# Full Stack Of Preliminary PRs
Thanks to the countless preliminary PRs, this conversion is relatively
straight-forward.
1. Clean-ups
* https://github.com/neondatabase/neon/pull/4316
* https://github.com/neondatabase/neon/pull/4317
* https://github.com/neondatabase/neon/pull/4318
* https://github.com/neondatabase/neon/pull/4319
* https://github.com/neondatabase/neon/pull/4321
* Note: these were mostly to find an alternative to #4291, which I
thought we'd need in my original plan where we would need to convert
`Tenant::timelines` into an async locking primitive (#4333). In reviews,
we walked away from that, but these cleanups were still quite useful.
2. https://github.com/neondatabase/neon/pull/4364
3. https://github.com/neondatabase/neon/pull/4472
4. https://github.com/neondatabase/neon/pull/4476
5. https://github.com/neondatabase/neon/pull/4477
6. https://github.com/neondatabase/neon/pull/4485
# Significant Changes In This PR
## `compact_level0_phase1` & `create_delta_layer`
This commit partially reverts
"pgserver: spawn_blocking in compaction (#4265)"
4e359db4c7.
Specifically, it reverts the `spawn_blocking`-ificiation of
`compact_level0_phase1`.
If we didn't revert it, we'd have to use `Timeline::layers.blocking_read()`
inside `compact_level0_phase1`. That would use up a thread in the
`spawn_blocking` thread pool, which is hard-capped.
I considered wrapping the code that follows the second
`layers.read().await` into `spawn_blocking`, but there are lifetime
issues with `deltas_to_compact`.
Also, this PR switches the `create_delta_layer` _function_ back to
async, and uses `spawn_blocking` inside to run the code that does sync
IO, while keeping the code that needs to lock `Timeline::layers` async.
## `LayerIter` and `LayerKeyIter` `Send` bounds
I had to add a `Send` bound on the `dyn` type that `LayerIter`
and `LayerKeyIter` wrap. Why? Because we now have the second
`layers.read().await` inside `compact_level0_phase`, and these
iterator instances are held across that await-point.
More background:
https://github.com/neondatabase/neon/pull/4462#issuecomment-1587376960
## `DatadirModification::flush`
Needed to replace the `HashMap::retain` with a hand-rolled variant
because `TimelineWriter::put` is now async.
Follow-up to #3513.
This removes the old blanket `std::fmt::Debug` impl on `dyn Layer` which
did not seem to be used from anywhere (no compilation errors after
removing).
Adds `std::fmt::Debug` requirement and implementations for `trait Layer`
implementors:
- LayerDescriptor (derived)
- RemoteLayer (manual)
- DeltaLayer (manual)
- ImageLayer (manual)
Manual implementations are used to skip PageserverConf, tenant and
timeline ids, large collections.
Adds and adjusts some doc comments to be more rustdoc alike.
Closes https://github.com/neondatabase/neon/issues/3439
Adds a set of commands to manipulate the layer map:
* dump the layer map contents
* evict the layer form the layer map (remove the local file, put the
remote layer instead in the layer map)
* download the layer (operation, reversing the eviction)
The commands will change later, when the statistics is added on top, so
the swagger schema is not adjusted.
The commands might have issues with big amount of layers: no pagination
is done for the dump command, eviction and download commands look for
the layer to evict/download by iterating all layers sequentially and
comparing the layer names.
For now, that seems to be tolerable ("big" number of layers is ~2_000)
and further experiments are needed.
---------
Co-authored-by: Christian Schwarz <christian@neon.tech>
