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Add availability-recovery from systematic chunks (paritytech#1644)
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**Don't look at the commit history, it's confusing, as this branch is
based on another branch that was merged**

Fixes paritytech#598 
Also implements [RFC
paritytech#47](polkadot-fellows/RFCs#47)

## Description

- Availability-recovery now first attempts to request the systematic
chunks for large POVs (which are the first ~n/3 chunks, which can
recover the full data without doing the costly reed-solomon decoding
process). This has a fallback of recovering from all chunks, if for some
reason the process fails. Additionally, backers are also used as a
backup for requesting the systematic chunks if the assigned validator is
not offering the chunk (each backer is only used for one systematic
chunk, to not overload them).
- Quite obviously, recovering from systematic chunks is much faster than
recovering from regular chunks (4000% faster as measured on my apple M2
Pro).
- Introduces a `ValidatorIndex` -> `ChunkIndex` mapping which is
different for every core, in order to avoid only querying the first n/3
validators over and over again in the same session. The mapping is the
one described in RFC 47.
- The mapping is feature-gated by the [NodeFeatures runtime
API](paritytech#2177) so that it
can only be enabled via a governance call once a sufficient majority of
validators have upgraded their client. If the feature is not enabled,
the mapping will be the identity mapping and backwards-compatibility
will be preserved.
- Adds a new chunk request protocol version (v2), which adds the
ChunkIndex to the response. This may or may not be checked against the
expected chunk index. For av-distribution and systematic recovery, this
will be checked, but for regular recovery, no. This is backwards
compatible. First, a v2 request is attempted. If that fails during
protocol negotiation, v1 is used.
- Systematic recovery is only attempted during approval-voting, where we
have easy access to the core_index. For disputes and collator
pov_recovery, regular chunk requests are used, just as before.

## Performance results

Some results from subsystem-bench:

with regular chunk recovery: CPU usage per block 39.82s
with recovery from backers: CPU usage per block 16.03s
with systematic recovery: CPU usage per block 19.07s

End-to-end results here:
paritytech#598 (comment)

#### TODO:

- [x] [RFC paritytech#47](polkadot-fellows/RFCs#47)
- [x] merge paritytech#2177 and
rebase on top of those changes
- [x] merge paritytech#2771 and
rebase
- [x] add tests
- [x] preliminary performance measure on Versi: see
paritytech#598 (comment)
- [x] Rewrite the implementer's guide documentation
- [x] paritytech#3065 
- [x] paritytech/zombienet#1705 and fix
zombienet tests
- [x] security audit
- [x] final versi test and performance measure

---------

Signed-off-by: alindima <alin@parity.io>
Co-authored-by: Javier Viola <javier@parity.io>
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2 people authored and TarekkMA committed Aug 2, 2024
1 parent 645b0f3 commit 6f51f6f
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2 changes: 1 addition & 1 deletion .gitlab/pipeline/zombienet.yml
Original file line number Diff line number Diff line change
@@ -1,7 +1,7 @@
.zombienet-refs:
extends: .build-refs
variables:
ZOMBIENET_IMAGE: "docker.io/paritytech/zombienet:v1.3.104"
ZOMBIENET_IMAGE: "docker.io/paritytech/zombienet:v1.3.105"
PUSHGATEWAY_URL: "http://zombienet-prometheus-pushgateway.managed-monitoring:9091/metrics/job/zombie-metrics"
DEBUG: "zombie,zombie::network-node,zombie::kube::client::logs"

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16 changes: 16 additions & 0 deletions .gitlab/pipeline/zombienet/polkadot.yml
Original file line number Diff line number Diff line change
Expand Up @@ -183,6 +183,22 @@ zombienet-polkadot-functional-0012-spam-statement-distribution-requests:
--local-dir="${LOCAL_DIR}/functional"
--test="0012-spam-statement-distribution-requests.zndsl"

zombienet-polkadot-functional-0013-systematic-chunk-recovery:
extends:
- .zombienet-polkadot-common
script:
- /home/nonroot/zombie-net/scripts/ci/run-test-local-env-manager.sh
--local-dir="${LOCAL_DIR}/functional"
--test="0013-systematic-chunk-recovery.zndsl"

zombienet-polkadot-functional-0014-chunk-fetching-network-compatibility:
extends:
- .zombienet-polkadot-common
script:
- /home/nonroot/zombie-net/scripts/ci/run-test-local-env-manager.sh
--local-dir="${LOCAL_DIR}/functional"
--test="0014-chunk-fetching-network-compatibility.zndsl"

zombienet-polkadot-smoke-0001-parachains-smoke-test:
extends:
- .zombienet-polkadot-common
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8 changes: 7 additions & 1 deletion Cargo.lock

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Original file line number Diff line number Diff line change
Expand Up @@ -56,6 +56,7 @@ impl<Block: BlockT> ActiveCandidateRecovery<Block> {
candidate.receipt.clone(),
candidate.session_index,
None,
None,
tx,
),
"ActiveCandidateRecovery",
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3 changes: 3 additions & 0 deletions cumulus/client/relay-chain-minimal-node/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -285,5 +285,8 @@ fn build_request_response_protocol_receivers<
let cfg =
Protocol::ChunkFetchingV1.get_outbound_only_config::<_, Network>(request_protocol_names);
config.add_request_response_protocol(cfg);
let cfg =
Protocol::ChunkFetchingV2.get_outbound_only_config::<_, Network>(request_protocol_names);
config.add_request_response_protocol(cfg);
(collation_req_v1_receiver, collation_req_v2_receiver, available_data_req_receiver)
}
5 changes: 3 additions & 2 deletions cumulus/test/service/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -152,15 +152,16 @@ impl RecoveryHandle for FailingRecoveryHandle {
message: AvailabilityRecoveryMessage,
origin: &'static str,
) {
let AvailabilityRecoveryMessage::RecoverAvailableData(ref receipt, _, _, _) = message;
let AvailabilityRecoveryMessage::RecoverAvailableData(ref receipt, _, _, _, _) = message;
let candidate_hash = receipt.hash();

// For every 3rd block we immediately signal unavailability to trigger
// a retry. The same candidate is never failed multiple times to ensure progress.
if self.counter % 3 == 0 && self.failed_hashes.insert(candidate_hash) {
tracing::info!(target: LOG_TARGET, ?candidate_hash, "Failing pov recovery.");

let AvailabilityRecoveryMessage::RecoverAvailableData(_, _, _, back_sender) = message;
let AvailabilityRecoveryMessage::RecoverAvailableData(_, _, _, _, back_sender) =
message;
back_sender
.send(Err(RecoveryError::Unavailable))
.expect("Return channel should work here.");
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1 change: 1 addition & 0 deletions polkadot/erasure-coding/Cargo.toml
Original file line number Diff line number Diff line change
Expand Up @@ -19,6 +19,7 @@ sp-trie = { path = "../../substrate/primitives/trie" }
thiserror = { workspace = true }

[dev-dependencies]
quickcheck = { version = "1.0.3", default-features = false }
criterion = { version = "0.5.1", default-features = false, features = ["cargo_bench_support"] }

[[bench]]
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6 changes: 5 additions & 1 deletion polkadot/erasure-coding/benches/README.md
Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,8 @@ cargo bench
## `scaling_with_validators`

This benchmark evaluates the performance of constructing the chunks and the erasure root from PoV and
reconstructing the PoV from chunks. You can see the results of running this bench on 5950x below.
reconstructing the PoV from chunks (either from systematic chunks or regular chunks).
You can see the results of running this bench on 5950x below (only including recovery from regular chunks).
Interestingly, with `10_000` chunks (validators) its slower than with `50_000` for both construction
and reconstruction.
```
Expand Down Expand Up @@ -37,3 +38,6 @@ reconstruct/10000 time: [496.35 ms 505.17 ms 515.42 ms]
reconstruct/50000 time: [276.56 ms 277.53 ms 278.58 ms]
thrpt: [17.948 MiB/s 18.016 MiB/s 18.079 MiB/s]
```

Results from running on an Apple M2 Pro, systematic recovery is generally 40 times faster than
regular recovery, achieving 1 Gib/s.
36 changes: 32 additions & 4 deletions polkadot/erasure-coding/benches/scaling_with_validators.rs
Original file line number Diff line number Diff line change
Expand Up @@ -53,12 +53,16 @@ fn construct_and_reconstruct_5mb_pov(c: &mut Criterion) {
}
group.finish();

let mut group = c.benchmark_group("reconstruct");
let mut group = c.benchmark_group("reconstruct_regular");
for n_validators in N_VALIDATORS {
let all_chunks = chunks(n_validators, &pov);

let mut c: Vec<_> = all_chunks.iter().enumerate().map(|(i, c)| (&c[..], i)).collect();
let last_chunks = c.split_off((c.len() - 1) * 2 / 3);
let chunks: Vec<_> = all_chunks
.iter()
.enumerate()
.take(polkadot_erasure_coding::recovery_threshold(n_validators).unwrap())
.map(|(i, c)| (&c[..], i))
.collect();

group.throughput(Throughput::Bytes(pov.len() as u64));
group.bench_with_input(
Expand All @@ -67,7 +71,31 @@ fn construct_and_reconstruct_5mb_pov(c: &mut Criterion) {
|b, &n| {
b.iter(|| {
let _pov: Vec<u8> =
polkadot_erasure_coding::reconstruct(n, last_chunks.clone()).unwrap();
polkadot_erasure_coding::reconstruct(n, chunks.clone()).unwrap();
});
},
);
}
group.finish();

let mut group = c.benchmark_group("reconstruct_systematic");
for n_validators in N_VALIDATORS {
let all_chunks = chunks(n_validators, &pov);

let chunks = all_chunks
.into_iter()
.take(polkadot_erasure_coding::systematic_recovery_threshold(n_validators).unwrap())
.collect::<Vec<_>>();

group.throughput(Throughput::Bytes(pov.len() as u64));
group.bench_with_input(
BenchmarkId::from_parameter(n_validators),
&n_validators,
|b, &n| {
b.iter(|| {
let _pov: Vec<u8> =
polkadot_erasure_coding::reconstruct_from_systematic(n, chunks.clone())
.unwrap();
});
},
);
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93 changes: 93 additions & 0 deletions polkadot/erasure-coding/src/lib.rs
Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,9 @@ pub enum Error {
/// Bad payload in reconstructed bytes.
#[error("Reconstructed payload invalid")]
BadPayload,
/// Unable to decode reconstructed bytes.
#[error("Unable to decode reconstructed payload: {0}")]
Decode(#[source] parity_scale_codec::Error),
/// Invalid branch proof.
#[error("Invalid branch proof")]
InvalidBranchProof,
Expand Down Expand Up @@ -110,6 +113,14 @@ pub const fn recovery_threshold(n_validators: usize) -> Result<usize, Error> {
Ok(needed + 1)
}

/// Obtain the threshold of systematic chunks that should be enough to recover the data.
///
/// If the regular `recovery_threshold` is a power of two, then it returns the same value.
/// Otherwise, it returns the next lower power of two.
pub fn systematic_recovery_threshold(n_validators: usize) -> Result<usize, Error> {
code_params(n_validators).map(|params| params.k())
}

fn code_params(n_validators: usize) -> Result<CodeParams, Error> {
// we need to be able to reconstruct from 1/3 - eps

Expand All @@ -127,6 +138,41 @@ fn code_params(n_validators: usize) -> Result<CodeParams, Error> {
})
}

/// Reconstruct the v1 available data from the set of systematic chunks.
///
/// Provide a vector containing chunk data. If too few chunks are provided, recovery is not
/// possible.
pub fn reconstruct_from_systematic_v1(
n_validators: usize,
chunks: Vec<Vec<u8>>,
) -> Result<AvailableData, Error> {
reconstruct_from_systematic(n_validators, chunks)
}

/// Reconstruct the available data from the set of systematic chunks.
///
/// Provide a vector containing the first k chunks in order. If too few chunks are provided,
/// recovery is not possible.
pub fn reconstruct_from_systematic<T: Decode>(
n_validators: usize,
chunks: Vec<Vec<u8>>,
) -> Result<T, Error> {
let code_params = code_params(n_validators)?;
let k = code_params.k();

for chunk_data in chunks.iter().take(k) {
if chunk_data.len() % 2 != 0 {
return Err(Error::UnevenLength)
}
}

let bytes = code_params.make_encoder().reconstruct_from_systematic(
chunks.into_iter().take(k).map(|data| WrappedShard::new(data)).collect(),
)?;

Decode::decode(&mut &bytes[..]).map_err(|err| Error::Decode(err))
}

/// Obtain erasure-coded chunks for v1 `AvailableData`, one for each validator.
///
/// Works only up to 65536 validators, and `n_validators` must be non-zero.
Expand Down Expand Up @@ -285,13 +331,41 @@ pub fn branch_hash(root: &H256, branch_nodes: &Proof, index: usize) -> Result<H2

#[cfg(test)]
mod tests {
use std::sync::Arc;

use super::*;
use polkadot_node_primitives::{AvailableData, BlockData, PoV};
use polkadot_primitives::{HeadData, PersistedValidationData};
use quickcheck::{Arbitrary, Gen, QuickCheck};

// In order to adequately compute the number of entries in the Merkle
// trie, we must account for the fixed 16-ary trie structure.
const KEY_INDEX_NIBBLE_SIZE: usize = 4;

#[derive(Clone, Debug)]
struct ArbitraryAvailableData(AvailableData);

impl Arbitrary for ArbitraryAvailableData {
fn arbitrary(g: &mut Gen) -> Self {
// Limit the POV len to 1 mib, otherwise the test will take forever
let pov_len = (u32::arbitrary(g) % (1024 * 1024)).max(2);

let pov = (0..pov_len).map(|_| u8::arbitrary(g)).collect();

let pvd = PersistedValidationData {
parent_head: HeadData((0..u16::arbitrary(g)).map(|_| u8::arbitrary(g)).collect()),
relay_parent_number: u32::arbitrary(g),
relay_parent_storage_root: [u8::arbitrary(g); 32].into(),
max_pov_size: u32::arbitrary(g),
};

ArbitraryAvailableData(AvailableData {
pov: Arc::new(PoV { block_data: BlockData(pov) }),
validation_data: pvd,
})
}
}

#[test]
fn field_order_is_right_size() {
assert_eq!(MAX_VALIDATORS, 65536);
Expand All @@ -318,6 +392,25 @@ mod tests {
assert_eq!(reconstructed, available_data);
}

#[test]
fn round_trip_systematic_works() {
fn property(available_data: ArbitraryAvailableData, n_validators: u16) {
let n_validators = n_validators.max(2);
let kpow2 = systematic_recovery_threshold(n_validators as usize).unwrap();
let chunks = obtain_chunks(n_validators as usize, &available_data.0).unwrap();
assert_eq!(
reconstruct_from_systematic_v1(
n_validators as usize,
chunks.into_iter().take(kpow2).collect()
)
.unwrap(),
available_data.0
);
}

QuickCheck::new().quickcheck(property as fn(ArbitraryAvailableData, u16))
}

#[test]
fn reconstruct_does_not_panic_on_low_validator_count() {
let reconstructed = reconstruct_v1(1, [].iter().cloned());
Expand Down
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