Incorrect fg_write_max_bytes_per_sec when set max_bytes_per_sec small than total weight

[Lloyd-Pottiger]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

fg_write_max_bytes_per_sec 1 
bg_write_max_bytes_per_sec 1 
fg_read_max_bytes_per_sec 1 
bg_read_max_bytes_per_sec 1 

3. What did you see instead (Required)

fg_write_max_bytes_per_sec 0
bg_write_max_bytes_per_sec 0 
fg_read_max_bytes_per_sec 0
bg_read_max_bytes_per_sec 0

4. What is your TiFlash version? (Required)

master

5. Why does this bug occur？

in dbms/src/Server/StorageConfigParser.cpp

UInt64 StorageIORateLimitConfig::getFgWriteMaxBytesPerSec() const
{
    if (totalWeight() <= 0 || writeWeight() <= 0)
    {
        return 0;
    }
    return use_max_bytes_per_sec ? max_bytes_per_sec / totalWeight() * fg_write_weight
                                 : max_write_bytes_per_sec / writeWeight() * fg_write_weight;
}

UInt64 StorageIORateLimitConfig::getBgWriteMaxBytesPerSec() const
{
    if (totalWeight() <= 0 || writeWeight() <= 0)
    {
        return 0;
    }
    return use_max_bytes_per_sec ? max_bytes_per_sec / totalWeight() * bg_write_weight
                                 : max_write_bytes_per_sec / writeWeight() * bg_write_weight;
}

UInt64 StorageIORateLimitConfig::getFgReadMaxBytesPerSec() const
{
    if (totalWeight() <= 0 || readWeight() <= 0)
    {
        return 0;
    }
    return use_max_bytes_per_sec ? max_bytes_per_sec / totalWeight() * fg_read_weight
                                 : max_read_bytes_per_sec / readWeight() * fg_read_weight;
}

UInt64 StorageIORateLimitConfig::getBgReadMaxBytesPerSec() const
{
    if (totalWeight() <= 0 || readWeight() <= 0)
    {
        return 0;
    }
    return use_max_bytes_per_sec ? max_bytes_per_sec / totalWeight() * bg_read_weight
                                 : max_read_bytes_per_sec / readWeight() * bg_read_weight;
}

where max_bytes_per_sec and totalWeight() are UInt64 variables. and totalWeight() = 100, so max_bytes_per_sec / totalWeight() = 0.