Polkadot's JAM

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8 Jun 2024

Polkadot Virtual Machine (PVM)

The PVM is based on the RISC-V Instruction Set Architecture (ISA), valued for its simplicity and flexibility. RISC-V offers several benefits:

Easily transpiled into common hardware formats such as x86, x64, and ARM.
Strong support from tools like LLVM.

The PVM emphasizes simplicity and security, being sandboxable and providing various execution guarantees. It is deterministic, consensus-sensitive, and friendly to metering, avoiding the complexity found in other VMs.

Advantages over WASM:

WASM, while optimized for web use, struggles with stack management and continuations.
RISC-V places the stack in memory, handling continuations naturally without extra complexity.
PVM achieves excellent execution speeds on conventional hardware (x64 and ARM), offering advantages like free metering compared to WASM.

RISC-V-enabled Continuations:

Facilitates scalable coding on multi-core platforms like JAM.
Asynchronous, parallelized architectures are crucial for scalability in hardware, software, blockchain, and consensus algorithms.

SAFROLE

SAFROLE is a simplified block production algorithm derived from SASSAFRAS. It:

Is minimally opinionated to maximize potential future use cases.
Aims to follow the Ethereum yellow paper model for wide implementation and expertise dissemination.

SAFROLE Features:

SNARK-based, offering constant-time block production and near fork-free operation.
Anonymity ensures the security of block production, preventing spam.

Networking

JAM uses the QUIC protocol, enabling direct connections between validators. This avoids gossip protocols as JAM doesn't handle transactions. Grid-diffusal is used for broader distribution, arranging validators in a grid for efficient package delivery.

Efficient Block Processing

JAM adopts a unique approach by placing the prior state root in the block header, allowing:

Lightweight computations (5% of the block's workload) to be executed immediately.
The remaining 95% of computations to be completed afterward.
More efficient use of block time, improving computation time significantly over traditional setups.

Architectural Differences: JAM vs. Relay Chain

JAM fixes more functionality elements compared to the relay chain, making upgrades challenging but optimizing performance. Key differences include:

Fixed parameters enable network topology and timing optimizations.
Flexibility in application-level functionalities like coretime sales, staking, and governance managed within services.
A non-upgradable chain reduces complexity.

JAM Toaster

A comprehensive test environment designed for large-scale trials and performance assessments of JAM, addressing challenges in understanding network dynamics at scale.

JAM and Substrate

Benchmarks vs. Metering:

JAM's metered system can often replace frequent benchmarking.
Benchmarking remains relevant for performance enhancements and tasks requiring extended execution times.

Cross-Chain Message Passing (XCMP)

JAM mandates full XCMP support for efficient data transmission between parachains, overcoming limitations of HRMP.

Accords

Accords are multi-instance smart contracts enabling direct, trustless interactions between parachains, enhancing token transfers and message integrity.