MIP 4: Batch yield collection to reduce SAVE gas costs

Author	Alex Scott
Discussions-To	https://forum.mstable.org/t/mip-4-batch-yield-collection-to-reduce-save-gas-costs/230
Status	Implemented
Created	2020-10-30

Simple Summary

Currently the yield produced from lending markets Compound/Aave is collected upon each deposit into SAVE. This is an expensive operation and is performed more frequently than is necessary. This proposal provides a more passive method of collecting the yield, reducing SAVE deposit gas costs by >= 50% while delivering a smoother result.

This upgrade is something that could be bundled nicely to mip-3

Abstract

There are 2 primary sources of income from mAsset - SWAP fees, and Lending market income. This MIP will make 2 fundamental changes to the way system income is collected and distributed. The first of which is how the SWAP fees are tracked over time, and the second of which is the frequency of lending market interest is collected. The lending market interest will be collected intermittently and streamed second by second to SAVE.

Motivation

On deposit, currently there are a number of interactions involved in collecting system interest before it is credited to SAVE to the benefit of the existing savers. The flow goes something like SavingsContract > SavingsManager > Masset > BasketManager > eachBasset[PlatformIntegration].

Example of a deposit transaction: tenderly.co/tx/main/0xf73…

Current gas cost: ~390,739

The part of the transaction, from BasketManager onwards, is proposed to be omitted. In this function, for each basset a IPlatformIntegration(integrations[i]).checkBalance(b.addr); call is made - this is expensive due to the way Aave and Compound check balances for underlying assets.

Potential savings: ~210k or ~54%

If we were to continue without this change, the gas costs would increase by ~30k for each bAsset added in to the system.

Specification

Overview

This MIP will solve the problem by making the following changes:

1 - Change the way SWAP fees are recorded in Masset.sol

Currently the SWAP fees are implicitly recorded as deltas in each bAssets vaultBalance property by subtracting the full amount (including fee) during a SWAP. When the interest is collected in BasketManager, the gain is calculated as delta = balance.sub(oldVaultBalance);. Instead of recording fees implicitly like this, we will instead track fees natively in the Masset.sol at the cost of an extra SSTORE to each SWAP/REDEEM.

2 - Change the default behaviour of the collectInterest function in Masset.sol

Simply remove the call basketManager.collectInterest(); and mint mAsset to the amount of fees that have been collected.

3 - Change the method of lending market interest collection & distribution (SavingsManager.sol && Masset.sol)

As opposed to collecting lending market interest each deposit, we will add a new function to Masset.sol that specifically collects the interest from the lending markets and deposits it to the SavingsManager. Following the mechanism in mip-2 in which a publicly accessible function is called intermittently, we will then stream the collected yield over the course of a DAY to SAVE.

Rationale

The ideal solution:

a) Lowers gas costs for SAVE deposit
b) Does not present unfair opportunities for participants to time SAVE deposits and “steal” yield
c) Does not increase gas costs substantially (1 SSTORE is fine) for mint/swap/redeem

The above properties were taken into account and used as benchmarks on the following decisions:

1 - SWAP fee collection: immediate vs streamed

The SWAP fee collection could have been bundled in with the lending market collections and made once per day. While this would offer significant gas advantages, it would give market participants the opportunity to time their SAVE deposits with high SWAP volumes, and thus steal yield from loyal participants (b).

2 - Lending market fee collection: immediate vs streamed

The lending market yield could simply be immediately distributed to SAVE (during the next SWAP fee collection cycle) but this would provide unfair advantages for participants who could time their deposits correctly (b).

3 - SWAP fee tracking

Given the active proposal in MIP-3 which sees gas costs vastly reduced across the board, spending an extra 5k to update a global param is not much of a problem. Instead of setting the variable to 0, we will set it to 1, thus causing 5k for both swap/redeem, as well as 5k for SAVE. This is a compromise for both user groups.

Technical Specification

Contract changes

Masset.sol (UPGRADE)

New property: surplus Tracks the units of fees that have been accrued since last collection (in mAsset terms)

Modify function: collectInterest -> collectFees Turns accrued SWAP fees into mAsset and sends to SavingsManager

New function: collectInterest Collects the lending market interest and sends to the SavingsManager to be streamed over the course of a day

SavingsManager.sol (UPGRADE)

New function: streamInterest Deposits the lending market yield and streams over the course of a day, bundling with existing stream.

Configurable Values (Via MCCP)

N/A

MIP 4: Batch yield collection to reduce SAVE gas costs Source