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pageserver: add bench_ingest #7409

Merged
merged 12 commits into from
Aug 6, 2024
Merged

pageserver: add bench_ingest #7409

merged 12 commits into from
Aug 6, 2024

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jcsp
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@jcsp jcsp commented Apr 17, 2024

Problem

We lack a rust bench for the inmemory layer and delta layer write paths: it is useful to benchmark these components independent of postgres & WAL decoding.

Related: #8452

Summary of changes

  • Refactor DeltaLayerWriter to avoid carrying a Timeline, so that it can be cleanly tested + benched without a Tenant/Timeline test harness. It only needed the Timeline for building Layer, so this can be done in a separate step.
  • Add bench_ingest, which exercises a variety of workload "shapes" (big values, small values, sequential keys, random keys)
  • Include a small uncontroversial optimization: in freeze, only exhaustively walk values to assert ordering relative to end_lsn in debug mode.

These benches are limited by drive performance on a lot of machines, but still useful as a local tool for iterating on CPU/memory improvements around this code path.

Anecdotal measurements on Hetzner AX102 (Ryzen 7950xd):


ingest-small-values/ingest 128MB/100b seq
                        time:   [1.1160 s 1.1230 s 1.1289 s]
                        thrpt:  [113.38 MiB/s 113.98 MiB/s 114.70 MiB/s]
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) low mild
Benchmarking ingest-small-values/ingest 128MB/100b rand: Warming up for 3.0000 s
Warning: Unable to complete 10 samples in 10.0s. You may wish to increase target time to 18.9s.
ingest-small-values/ingest 128MB/100b rand
                        time:   [1.9001 s 1.9056 s 1.9110 s]
                        thrpt:  [66.982 MiB/s 67.171 MiB/s 67.365 MiB/s]
Benchmarking ingest-small-values/ingest 128MB/100b rand-1024keys: Warming up for 3.0000 s
Warning: Unable to complete 10 samples in 10.0s. You may wish to increase target time to 11.0s.
ingest-small-values/ingest 128MB/100b rand-1024keys
                        time:   [1.0715 s 1.0828 s 1.0937 s]
                        thrpt:  [117.04 MiB/s 118.21 MiB/s 119.46 MiB/s]
ingest-small-values/ingest 128MB/100b seq, no delta
                        time:   [425.49 ms 429.07 ms 432.04 ms]
                        thrpt:  [296.27 MiB/s 298.32 MiB/s 300.83 MiB/s]
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) low mild

ingest-big-values/ingest 128MB/8k seq
                        time:   [373.03 ms 375.84 ms 379.17 ms]
                        thrpt:  [337.58 MiB/s 340.57 MiB/s 343.13 MiB/s]
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) high mild
ingest-big-values/ingest 128MB/8k seq, no delta
                        time:   [81.534 ms 82.811 ms 83.364 ms]
                        thrpt:  [1.4994 GiB/s 1.5095 GiB/s 1.5331 GiB/s]
Found 1 outliers among 10 measurements (10.00%)


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

Copy link

github-actions bot commented Apr 17, 2024

2138 tests run: 2069 passed, 0 failed, 69 skipped (full report)


Flaky tests (2)

Postgres 16

Postgres 15

Code coverage* (full report)

  • functions: 32.8% (7153 of 21803 functions)
  • lines: 50.5% (57735 of 114292 lines)

* collected from Rust tests only


The comment gets automatically updated with the latest test results
da223bc at 2024-08-06T16:48:12.791Z :recycle:

problame added a commit that referenced this pull request Apr 26, 2024
part of #7124

# Problem

(Re-stating the problem from #7124 for posterity)

The `test_bulk_ingest` benchmark shows about 2x lower throughput with
`tokio-epoll-uring` compared to `std-fs`.
That's why we temporarily disabled it in #7238.

The reason for this regression is that the benchmark runs on a system
without memory pressure and thus std-fs writes don't block on disk IO
but only copy the data into the kernel page cache.
`tokio-epoll-uring` cannot beat that at this time, and possibly never.
(However, under memory pressure, std-fs would stall the executor thread
on kernel page cache writeback disk IO. That's why we want to use
`tokio-epoll-uring`. And we likely want to use O_DIRECT in the future,
at which point std-fs becomes an absolute show-stopper.)

More elaborate analysis:
https://neondatabase.notion.site/Why-test_bulk_ingest-is-slower-with-tokio-epoll-uring-918c5e619df045a7bd7b5f806cfbd53f?pvs=4

# Changes

This PR increases the buffer size of `blob_io` and `EphemeralFile` from
PAGE_SZ=8k to 64k.

Longer-term, we probably want to do double-buffering / pipelined IO.

# Resource Usage

We currently do not flush the buffer when freezing the InMemoryLayer.
That means a single Timeline can have multiple 64k buffers alive, esp if
flushing is slow.
This poses an OOM risk.

We should either bound the number of frozen layers
(#7317).

Or we should change the freezing code to flush the buffer and drop the
allocation.

However, that's future work.

# Performance

(Measurements done on i3en.3xlarge.)

The `test_bulk_insert.py` is too noisy, even with instance storage. It
varies by 30-40%. I suspect that's due to compaction. Raising amount of
data by 10x doesn't help with the noisiness.)

So, I used the `bench_ingest` from @jcsp 's #7409  .
Specifically, the `ingest-small-values/ingest 128MB/100b seq` and
`ingest-small-values/ingest 128MB/100b seq, no delta` benchmarks.

|     |                   | seq | seq, no delta |
|-----|-------------------|-----|---------------|
| 8k  | std-fs            | 55  | 165           |
| 8k  | tokio-epoll-uring | 37  | 107           |
| 64k | std-fs            | 55  | 180           |
| 64k | tokio-epoll-uring | 48  | 164           |

The `8k` is from before this PR, the `64k` is with this PR.
The values are the throughput reported by the benchmark (MiB/s).

We see that this PR gets `tokio-epoll-uring` from 67% to 87% of `std-fs`
performance in the `seq` benchmark. Notably, `seq` appears to hit some
other bottleneck at `55 MiB/s`. CC'ing #7418 due to the apparent
bottlenecks in writing delta layers.

For `seq, no delta`, this PR gets `tokio-epoll-uring` from 64% to 91% of
`std-fs` performance.
problame added a commit that referenced this pull request Jul 2, 2024
part of #7418

# Motivation

(reproducing #7418)

When we do an `InMemoryLayer::write_to_disk`, there is a tremendous
amount of random read I/O, as deltas from the ephemeral file (written in
LSN order) are written out to the delta layer in key order.

In benchmarks (#7409) we can
see that this delta layer writing phase is substantially more expensive
than the initial ingest of data, and that within the delta layer write a
significant amount of the CPU time is spent traversing the page cache.

# High-Level Changes

Add a new mode for L0 flush that works as follows:

* Read the full ephemeral file into memory -- layers are much smaller
than total memory, so this is afforable
* Do all the random reads directly from this in memory buffer instead of
using blob IO/page cache/disk reads.
* Add a semaphore to limit how many timelines may concurrently do this
(limit peak memory).
* Make the semaphore configurable via PS config.

# Implementation Details

The new `BlobReaderRef::Slice` is a temporary hack until we can ditch
`blob_io` for `InMemoryLayer` => Plan for this is laid out in
#8183

# Correctness

The correctness of this change is quite obvious to me: we do what we did
before (`blob_io`) but read from memory instead of going to disk.

The highest bug potential is in doing owned-buffers IO. I refactored the
API a bit in preliminary PR
#8186 to make it less
error-prone, but still, careful review is requested.

# Performance

I manually measured single-client ingest performance from `pgbench -i
...`.

Full report:
https://neondatabase.notion.site/2024-06-28-benchmarking-l0-flush-performance-e98cff3807f94cb38f2054d8c818fe84?pvs=4

tl;dr:

* no speed improvements during ingest,  but
* significantly lower pressure on PS PageCache (eviction rate drops to
1/3)
  * (that's why I'm working on this)
* noticable but modestly lower CPU time

This is good enough for merging this PR because the changes require
opt-in.

We'll do more testing in staging & pre-prod.

# Stability / Monitoring

**memory consumption**: there's no _hard_ limit on max `InMemoryLayer`
size (aka "checkpoint distance") , hence there's no hard limit on the
memory allocation we do for flushing. In practice, we a) [log a
warning](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L5741-L5743)
when we flush oversized layers, so we'd know which tenant is to blame
and b) if we were to put a hard limit in place, we would have to decide
what to do if there is an InMemoryLayer that exceeds the limit.
It seems like a better option to guarantee a max size for frozen layer,
dependent on `checkpoint_distance`. Then limit concurrency based on
that.

**metrics**: we do have the
[flush_time_histo](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L3725-L3726),
but that includes the wait time for the semaphore. We could add a
separate metric for the time spent after acquiring the semaphore, so one
can infer the wait time. Seems unnecessary at this point, though.
VladLazar pushed a commit that referenced this pull request Jul 8, 2024
part of #7418

# Motivation

(reproducing #7418)

When we do an `InMemoryLayer::write_to_disk`, there is a tremendous
amount of random read I/O, as deltas from the ephemeral file (written in
LSN order) are written out to the delta layer in key order.

In benchmarks (#7409) we can
see that this delta layer writing phase is substantially more expensive
than the initial ingest of data, and that within the delta layer write a
significant amount of the CPU time is spent traversing the page cache.

# High-Level Changes

Add a new mode for L0 flush that works as follows:

* Read the full ephemeral file into memory -- layers are much smaller
than total memory, so this is afforable
* Do all the random reads directly from this in memory buffer instead of
using blob IO/page cache/disk reads.
* Add a semaphore to limit how many timelines may concurrently do this
(limit peak memory).
* Make the semaphore configurable via PS config.

# Implementation Details

The new `BlobReaderRef::Slice` is a temporary hack until we can ditch
`blob_io` for `InMemoryLayer` => Plan for this is laid out in
#8183

# Correctness

The correctness of this change is quite obvious to me: we do what we did
before (`blob_io`) but read from memory instead of going to disk.

The highest bug potential is in doing owned-buffers IO. I refactored the
API a bit in preliminary PR
#8186 to make it less
error-prone, but still, careful review is requested.

# Performance

I manually measured single-client ingest performance from `pgbench -i
...`.

Full report:
https://neondatabase.notion.site/2024-06-28-benchmarking-l0-flush-performance-e98cff3807f94cb38f2054d8c818fe84?pvs=4

tl;dr:

* no speed improvements during ingest,  but
* significantly lower pressure on PS PageCache (eviction rate drops to
1/3)
  * (that's why I'm working on this)
* noticable but modestly lower CPU time

This is good enough for merging this PR because the changes require
opt-in.

We'll do more testing in staging & pre-prod.

# Stability / Monitoring

**memory consumption**: there's no _hard_ limit on max `InMemoryLayer`
size (aka "checkpoint distance") , hence there's no hard limit on the
memory allocation we do for flushing. In practice, we a) [log a
warning](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L5741-L5743)
when we flush oversized layers, so we'd know which tenant is to blame
and b) if we were to put a hard limit in place, we would have to decide
what to do if there is an InMemoryLayer that exceeds the limit.
It seems like a better option to guarantee a max size for frozen layer,
dependent on `checkpoint_distance`. Then limit concurrency based on
that.

**metrics**: we do have the
[flush_time_histo](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L3725-L3726),
but that includes the wait time for the semaphore. We could add a
separate metric for the time spent after acquiring the semaphore, so one
can infer the wait time. Seems unnecessary at this point, though.
VladLazar pushed a commit that referenced this pull request Jul 8, 2024
part of #7418

# Motivation

(reproducing #7418)

When we do an `InMemoryLayer::write_to_disk`, there is a tremendous
amount of random read I/O, as deltas from the ephemeral file (written in
LSN order) are written out to the delta layer in key order.

In benchmarks (#7409) we can
see that this delta layer writing phase is substantially more expensive
than the initial ingest of data, and that within the delta layer write a
significant amount of the CPU time is spent traversing the page cache.

# High-Level Changes

Add a new mode for L0 flush that works as follows:

* Read the full ephemeral file into memory -- layers are much smaller
than total memory, so this is afforable
* Do all the random reads directly from this in memory buffer instead of
using blob IO/page cache/disk reads.
* Add a semaphore to limit how many timelines may concurrently do this
(limit peak memory).
* Make the semaphore configurable via PS config.

# Implementation Details

The new `BlobReaderRef::Slice` is a temporary hack until we can ditch
`blob_io` for `InMemoryLayer` => Plan for this is laid out in
#8183

# Correctness

The correctness of this change is quite obvious to me: we do what we did
before (`blob_io`) but read from memory instead of going to disk.

The highest bug potential is in doing owned-buffers IO. I refactored the
API a bit in preliminary PR
#8186 to make it less
error-prone, but still, careful review is requested.

# Performance

I manually measured single-client ingest performance from `pgbench -i
...`.

Full report:
https://neondatabase.notion.site/2024-06-28-benchmarking-l0-flush-performance-e98cff3807f94cb38f2054d8c818fe84?pvs=4

tl;dr:

* no speed improvements during ingest,  but
* significantly lower pressure on PS PageCache (eviction rate drops to
1/3)
  * (that's why I'm working on this)
* noticable but modestly lower CPU time

This is good enough for merging this PR because the changes require
opt-in.

We'll do more testing in staging & pre-prod.

# Stability / Monitoring

**memory consumption**: there's no _hard_ limit on max `InMemoryLayer`
size (aka "checkpoint distance") , hence there's no hard limit on the
memory allocation we do for flushing. In practice, we a) [log a
warning](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L5741-L5743)
when we flush oversized layers, so we'd know which tenant is to blame
and b) if we were to put a hard limit in place, we would have to decide
what to do if there is an InMemoryLayer that exceeds the limit.
It seems like a better option to guarantee a max size for frozen layer,
dependent on `checkpoint_distance`. Then limit concurrency based on
that.

**metrics**: we do have the
[flush_time_histo](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L3725-L3726),
but that includes the wait time for the semaphore. We could add a
separate metric for the time spent after acquiring the semaphore, so one
can infer the wait time. Seems unnecessary at this point, though.
VladLazar pushed a commit that referenced this pull request Jul 8, 2024
part of #7418

# Motivation

(reproducing #7418)

When we do an `InMemoryLayer::write_to_disk`, there is a tremendous
amount of random read I/O, as deltas from the ephemeral file (written in
LSN order) are written out to the delta layer in key order.

In benchmarks (#7409) we can
see that this delta layer writing phase is substantially more expensive
than the initial ingest of data, and that within the delta layer write a
significant amount of the CPU time is spent traversing the page cache.

# High-Level Changes

Add a new mode for L0 flush that works as follows:

* Read the full ephemeral file into memory -- layers are much smaller
than total memory, so this is afforable
* Do all the random reads directly from this in memory buffer instead of
using blob IO/page cache/disk reads.
* Add a semaphore to limit how many timelines may concurrently do this
(limit peak memory).
* Make the semaphore configurable via PS config.

# Implementation Details

The new `BlobReaderRef::Slice` is a temporary hack until we can ditch
`blob_io` for `InMemoryLayer` => Plan for this is laid out in
#8183

# Correctness

The correctness of this change is quite obvious to me: we do what we did
before (`blob_io`) but read from memory instead of going to disk.

The highest bug potential is in doing owned-buffers IO. I refactored the
API a bit in preliminary PR
#8186 to make it less
error-prone, but still, careful review is requested.

# Performance

I manually measured single-client ingest performance from `pgbench -i
...`.

Full report:
https://neondatabase.notion.site/2024-06-28-benchmarking-l0-flush-performance-e98cff3807f94cb38f2054d8c818fe84?pvs=4

tl;dr:

* no speed improvements during ingest,  but
* significantly lower pressure on PS PageCache (eviction rate drops to
1/3)
  * (that's why I'm working on this)
* noticable but modestly lower CPU time

This is good enough for merging this PR because the changes require
opt-in.

We'll do more testing in staging & pre-prod.

# Stability / Monitoring

**memory consumption**: there's no _hard_ limit on max `InMemoryLayer`
size (aka "checkpoint distance") , hence there's no hard limit on the
memory allocation we do for flushing. In practice, we a) [log a
warning](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L5741-L5743)
when we flush oversized layers, so we'd know which tenant is to blame
and b) if we were to put a hard limit in place, we would have to decide
what to do if there is an InMemoryLayer that exceeds the limit.
It seems like a better option to guarantee a max size for frozen layer,
dependent on `checkpoint_distance`. Then limit concurrency based on
that.

**metrics**: we do have the
[flush_time_histo](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L3725-L3726),
but that includes the wait time for the semaphore. We could add a
separate metric for the time spent after acquiring the semaphore, so one
can infer the wait time. Seems unnecessary at this point, though.
VladLazar pushed a commit that referenced this pull request Jul 8, 2024
part of #7418

# Motivation

(reproducing #7418)

When we do an `InMemoryLayer::write_to_disk`, there is a tremendous
amount of random read I/O, as deltas from the ephemeral file (written in
LSN order) are written out to the delta layer in key order.

In benchmarks (#7409) we can
see that this delta layer writing phase is substantially more expensive
than the initial ingest of data, and that within the delta layer write a
significant amount of the CPU time is spent traversing the page cache.

# High-Level Changes

Add a new mode for L0 flush that works as follows:

* Read the full ephemeral file into memory -- layers are much smaller
than total memory, so this is afforable
* Do all the random reads directly from this in memory buffer instead of
using blob IO/page cache/disk reads.
* Add a semaphore to limit how many timelines may concurrently do this
(limit peak memory).
* Make the semaphore configurable via PS config.

# Implementation Details

The new `BlobReaderRef::Slice` is a temporary hack until we can ditch
`blob_io` for `InMemoryLayer` => Plan for this is laid out in
#8183

# Correctness

The correctness of this change is quite obvious to me: we do what we did
before (`blob_io`) but read from memory instead of going to disk.

The highest bug potential is in doing owned-buffers IO. I refactored the
API a bit in preliminary PR
#8186 to make it less
error-prone, but still, careful review is requested.

# Performance

I manually measured single-client ingest performance from `pgbench -i
...`.

Full report:
https://neondatabase.notion.site/2024-06-28-benchmarking-l0-flush-performance-e98cff3807f94cb38f2054d8c818fe84?pvs=4

tl;dr:

* no speed improvements during ingest,  but
* significantly lower pressure on PS PageCache (eviction rate drops to
1/3)
  * (that's why I'm working on this)
* noticable but modestly lower CPU time

This is good enough for merging this PR because the changes require
opt-in.

We'll do more testing in staging & pre-prod.

# Stability / Monitoring

**memory consumption**: there's no _hard_ limit on max `InMemoryLayer`
size (aka "checkpoint distance") , hence there's no hard limit on the
memory allocation we do for flushing. In practice, we a) [log a
warning](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L5741-L5743)
when we flush oversized layers, so we'd know which tenant is to blame
and b) if we were to put a hard limit in place, we would have to decide
what to do if there is an InMemoryLayer that exceeds the limit.
It seems like a better option to guarantee a max size for frozen layer,
dependent on `checkpoint_distance`. Then limit concurrency based on
that.

**metrics**: we do have the
[flush_time_histo](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L3725-L3726),
but that includes the wait time for the semaphore. We could add a
separate metric for the time spent after acquiring the semaphore, so one
can infer the wait time. Seems unnecessary at this point, though.
VladLazar pushed a commit that referenced this pull request Jul 8, 2024
part of #7418

# Motivation

(reproducing #7418)

When we do an `InMemoryLayer::write_to_disk`, there is a tremendous
amount of random read I/O, as deltas from the ephemeral file (written in
LSN order) are written out to the delta layer in key order.

In benchmarks (#7409) we can
see that this delta layer writing phase is substantially more expensive
than the initial ingest of data, and that within the delta layer write a
significant amount of the CPU time is spent traversing the page cache.

# High-Level Changes

Add a new mode for L0 flush that works as follows:

* Read the full ephemeral file into memory -- layers are much smaller
than total memory, so this is afforable
* Do all the random reads directly from this in memory buffer instead of
using blob IO/page cache/disk reads.
* Add a semaphore to limit how many timelines may concurrently do this
(limit peak memory).
* Make the semaphore configurable via PS config.

# Implementation Details

The new `BlobReaderRef::Slice` is a temporary hack until we can ditch
`blob_io` for `InMemoryLayer` => Plan for this is laid out in
#8183

# Correctness

The correctness of this change is quite obvious to me: we do what we did
before (`blob_io`) but read from memory instead of going to disk.

The highest bug potential is in doing owned-buffers IO. I refactored the
API a bit in preliminary PR
#8186 to make it less
error-prone, but still, careful review is requested.

# Performance

I manually measured single-client ingest performance from `pgbench -i
...`.

Full report:
https://neondatabase.notion.site/2024-06-28-benchmarking-l0-flush-performance-e98cff3807f94cb38f2054d8c818fe84?pvs=4

tl;dr:

* no speed improvements during ingest,  but
* significantly lower pressure on PS PageCache (eviction rate drops to
1/3)
  * (that's why I'm working on this)
* noticable but modestly lower CPU time

This is good enough for merging this PR because the changes require
opt-in.

We'll do more testing in staging & pre-prod.

# Stability / Monitoring

**memory consumption**: there's no _hard_ limit on max `InMemoryLayer`
size (aka "checkpoint distance") , hence there's no hard limit on the
memory allocation we do for flushing. In practice, we a) [log a
warning](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L5741-L5743)
when we flush oversized layers, so we'd know which tenant is to blame
and b) if we were to put a hard limit in place, we would have to decide
what to do if there is an InMemoryLayer that exceeds the limit.
It seems like a better option to guarantee a max size for frozen layer,
dependent on `checkpoint_distance`. Then limit concurrency based on
that.

**metrics**: we do have the
[flush_time_histo](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L3725-L3726),
but that includes the wait time for the semaphore. We could add a
separate metric for the time spent after acquiring the semaphore, so one
can infer the wait time. Seems unnecessary at this point, though.
VladLazar pushed a commit that referenced this pull request Jul 8, 2024
part of #7418

# Motivation

(reproducing #7418)

When we do an `InMemoryLayer::write_to_disk`, there is a tremendous
amount of random read I/O, as deltas from the ephemeral file (written in
LSN order) are written out to the delta layer in key order.

In benchmarks (#7409) we can
see that this delta layer writing phase is substantially more expensive
than the initial ingest of data, and that within the delta layer write a
significant amount of the CPU time is spent traversing the page cache.

# High-Level Changes

Add a new mode for L0 flush that works as follows:

* Read the full ephemeral file into memory -- layers are much smaller
than total memory, so this is afforable
* Do all the random reads directly from this in memory buffer instead of
using blob IO/page cache/disk reads.
* Add a semaphore to limit how many timelines may concurrently do this
(limit peak memory).
* Make the semaphore configurable via PS config.

# Implementation Details

The new `BlobReaderRef::Slice` is a temporary hack until we can ditch
`blob_io` for `InMemoryLayer` => Plan for this is laid out in
#8183

# Correctness

The correctness of this change is quite obvious to me: we do what we did
before (`blob_io`) but read from memory instead of going to disk.

The highest bug potential is in doing owned-buffers IO. I refactored the
API a bit in preliminary PR
#8186 to make it less
error-prone, but still, careful review is requested.

# Performance

I manually measured single-client ingest performance from `pgbench -i
...`.

Full report:
https://neondatabase.notion.site/2024-06-28-benchmarking-l0-flush-performance-e98cff3807f94cb38f2054d8c818fe84?pvs=4

tl;dr:

* no speed improvements during ingest,  but
* significantly lower pressure on PS PageCache (eviction rate drops to
1/3)
  * (that's why I'm working on this)
* noticable but modestly lower CPU time

This is good enough for merging this PR because the changes require
opt-in.

We'll do more testing in staging & pre-prod.

# Stability / Monitoring

**memory consumption**: there's no _hard_ limit on max `InMemoryLayer`
size (aka "checkpoint distance") , hence there's no hard limit on the
memory allocation we do for flushing. In practice, we a) [log a
warning](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L5741-L5743)
when we flush oversized layers, so we'd know which tenant is to blame
and b) if we were to put a hard limit in place, we would have to decide
what to do if there is an InMemoryLayer that exceeds the limit.
It seems like a better option to guarantee a max size for frozen layer,
dependent on `checkpoint_distance`. Then limit concurrency based on
that.

**metrics**: we do have the
[flush_time_histo](https://github.com/neondatabase/neon/blob/23827c6b0d400cbb9a972d4d05d49834816c40d1/pageserver/src/tenant/timeline.rs#L3725-L3726),
but that includes the wait time for the semaphore. We could add a
separate metric for the time spent after acquiring the semaphore, so one
can infer the wait time. Seems unnecessary at this point, though.
@jcsp jcsp changed the title Jcsp/ingest bench pageserver: add bench_ingest Aug 1, 2024
@jcsp jcsp added c/storage/pageserver Component: storage: pageserver a/tech_debt Area: related to tech debt labels Aug 1, 2024
@jcsp jcsp requested a review from arpad-m August 1, 2024 15:45
@jcsp jcsp marked this pull request as ready for review August 1, 2024 15:45
@jcsp jcsp requested a review from a team as a code owner August 1, 2024 15:45
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Looks good - just some questions

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@jcsp jcsp enabled auto-merge (squash) August 5, 2024 12:38
@jcsp jcsp merged commit ca5390a into main Aug 6, 2024
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jcsp added a commit that referenced this pull request Aug 12, 2024
## Problem

We lack a rust bench for the inmemory layer and delta layer write paths:
it is useful to benchmark these components independent of postgres & WAL
decoding.

Related: #8452

## Summary of changes

- Refactor DeltaLayerWriter to avoid carrying a Timeline, so that it can
be cleanly tested + benched without a Tenant/Timeline test harness. It
only needed the Timeline for building `Layer`, so this can be done in a
separate step.
- Add `bench_ingest`, which exercises a variety of workload "shapes"
(big values, small values, sequential keys, random keys)
- Include a small uncontroversial optimization: in `freeze`, only
exhaustively walk values to assert ordering relative to end_lsn in debug
mode.

These benches are limited by drive performance on a lot of machines, but
still useful as a local tool for iterating on CPU/memory improvements
around this code path.

Anecdotal measurements on Hetzner AX102 (Ryzen 7950xd):

```

ingest-small-values/ingest 128MB/100b seq
                        time:   [1.1160 s 1.1230 s 1.1289 s]
                        thrpt:  [113.38 MiB/s 113.98 MiB/s 114.70 MiB/s]
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) low mild
Benchmarking ingest-small-values/ingest 128MB/100b rand: Warming up for 3.0000 s
Warning: Unable to complete 10 samples in 10.0s. You may wish to increase target time to 18.9s.
ingest-small-values/ingest 128MB/100b rand
                        time:   [1.9001 s 1.9056 s 1.9110 s]
                        thrpt:  [66.982 MiB/s 67.171 MiB/s 67.365 MiB/s]
Benchmarking ingest-small-values/ingest 128MB/100b rand-1024keys: Warming up for 3.0000 s
Warning: Unable to complete 10 samples in 10.0s. You may wish to increase target time to 11.0s.
ingest-small-values/ingest 128MB/100b rand-1024keys
                        time:   [1.0715 s 1.0828 s 1.0937 s]
                        thrpt:  [117.04 MiB/s 118.21 MiB/s 119.46 MiB/s]
ingest-small-values/ingest 128MB/100b seq, no delta
                        time:   [425.49 ms 429.07 ms 432.04 ms]
                        thrpt:  [296.27 MiB/s 298.32 MiB/s 300.83 MiB/s]
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) low mild

ingest-big-values/ingest 128MB/8k seq
                        time:   [373.03 ms 375.84 ms 379.17 ms]
                        thrpt:  [337.58 MiB/s 340.57 MiB/s 343.13 MiB/s]
Found 1 outliers among 10 measurements (10.00%)
  1 (10.00%) high mild
ingest-big-values/ingest 128MB/8k seq, no delta
                        time:   [81.534 ms 82.811 ms 83.364 ms]
                        thrpt:  [1.4994 GiB/s 1.5095 GiB/s 1.5331 GiB/s]
Found 1 outliers among 10 measurements (10.00%)


```
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