Remove artificial REX lower limit

[jafri]

Removes REX lower unlent limit from 20% to 0%

[DenisCarriere]

👍 looks good, this change does remove the artificial REX lower limit of 20%

This unlocks the full REX liquidity available to borrowers

REX rental prices are still computed using the Bancor formula, however now has access to the full 100% liquidity of REX

Before

// variables
total_lent = 818945770886 // 81.8mm
total_unlent = 163789519271 // 16.3mm

// calculations
unlent_lower_bound = 2 * total_lent / 10
//=> 163789154177
available_unlent = total_unlent - unlent_lower_bound
//=> 365094

After

// variables
total_lent = 818945770886 // 81.8mm
total_unlent = 163789519271 // 16.3mm

// calculations
unlent_lower_bound = 0;
available_unlent = total_unlent - unlent_lower_bound
//=> 163789519271

[UMU618]

This is a fairer measure.

[sergioyuhjtman]

Have you considered what would happen if total_unlent becomes small? Apparently the rental fee might fall drastically for subsequent loans.
In the case where total_unlent equals 0 the rental fee becomes 0 after the expiration of a loan.
See https://medium.com/eosio/analysis-of-bancor-equations-supporting-rex-74ef4aaeca1c (sections "Loan Expiration" and "Unlent Balance Lower Bound").

[jafri]

@sergioyuhjtman When a loan expires, the rented resources are moved from total_lent to total_unlent, so it could not be 0. Even if the user has auto_renew enabled, remove_loan_from_rex_pool would move total_lent->total_unlent first.

The amount of CPU one gets per EOS is determined by total_unlent / (total_rent + 1). As total_unlent goes down, you get less and less CPU for every EOS.

e.g. if total_unlent was 10,000 EOS and total_rent was 99,990,000 EOS, you would get 0.0001 EOS of CPU/EOS

[andybets]

@jafri it does seem that if there are loans outstanding and rex holders pull out all the unlent rex, total_unlent can go to zero, and thus total_rent is set to 0 (at loan expiration). When that happens, REX is unstable and a future rental paying 1 EOS would get the full unlent amount.

So there is a reason for the buffer. We need to set it above zero.

[jafri]

Fair enough @andybets

That edge case would only apply to the first loan after it hits 0 and requires a full exodus of all tokens.

The minimum unlent lower bound value can be set to 1 instead of 0