Reported here: https://github.com/bitcoin/bitcoin/pull/10244#issuecomment-378035305

Travis log here: https://travis-ci.org/bitcoin/bitcoin/jobs/361243719#L2810

Note the JSON exception:

```
test_framework.authproxy.JSONRPCException: The mempool was not loaded yet (-1) 
```

That exception was introduced here: https://github.com/bitcoin/bitcoin/commit/cb1e319fe9e198c9c5cf5236fe9af5a3d748b9e8#diff-a0c8f511d90e83aa9b5857e819ced344R1617

cc @jtimon 
{
public:
    bool operator()(const CTxMemPoolEntry& a, const CTxMemPoolEntry& b) const
    {
        double a_mod_fee, a_size, b_mod_fee, b_size;
        GetModFeeAndSize(a, a_mod_fee, a_size);
        GetModFeeAndSize(b, b_mod_fee, b_size);
        
        double f1 = a_mod_fee * b_size;
        double f2 = a_size * b_mod_fee;
        if (f1 == f2) {
            return a.GetTime() >= b.GetTime();
        }
        return f1 < f2;
    }
    
    void GetModFeeAndSize(const CTxMemPoolEntry &a, double &mod_fee, double &size) const