Eos中，通過抵押token的方式，提供cpu運算時間以供使用者完成交易。
net資源計算方式與cpu類似。
本文旨在理清變數間關係，故略過大部分中間變數，僅記錄關聯度較大的部分。

在eos測試中，用賬號進行高頻的transfer（轉賬）操作，出現了以下報錯。
所以決定做一份關於cpu限制的筆記。

3080004 tx_cpu_usage_exceeded: Transaction exceeded the current CPU usage limit imposed on the transaction
billed CPU time (916 us) is greater than the maximum billable CPU time for the transaction (372 us)
{"billed":916,"billable":372}
thread-0 transaction_context.cpp:361 validate_cpu_usage_to_bill

交易CPU驗證三步曲

主要由transaction_context承擔驗證工作。

• transaction_context::init
  objective_duration_limit = 等於以下四者最小值。
  _deadline = start + objective_duration_limit。

  • config::max_transaction_cpu_usage：config.hpp
  • trx.max_cpu_usage_ms：nodeos --max-cpu-usage-ms
  • resource_limits_manager::get_block_cpu_limit()
    ：動態變化_db.get<resource_limits_config_object>().cpu_limit_parameters.max - _db.get<resource_limits_state_object>().pending_cpu_usage
  • resource_limits_manager::get_account_cpu_limit + leeway：動態變化，使用者視窗期可用總值-已使用值。

• transaction_context::exec
  非延遲類交易立即執行，延遲類交易計算掛起費用（網路、記憶體），其中記憶體費用由交易發起者支付。

• transaction_context::finalize
  驗證交易執行結果，計算費用。

  cpu賬單需滿足以下斷言：
  billed_us >= cfg.min_transaction_cpu_usage：config.hpp
  billed_us <= objective_duration_limit.count()：上面的objective_duration_limit

由此看來，節點間硬體配置差距將導致結果不唯一。

使用者CPU資源使用計算

上面我們大概瞭解了什麼因素會影響cpu_limit的判定，現在要開始弄明白使用者cpu的計算方式。

使用cleos get account我們可以看到cpu相關引數如下：

cpu bandwidth:
staked: 4448951.0162 SYS (total stake delegated from account to self)
delegated: 0.0000 SYS (total staked delegated to account from others)
used: 47.11 ms
available: 85.42 hr
limit: 85.42 hr

// 列印程式碼如下：
std::cout << indent << std::left << std::setw(11) << "used:"      << std::right << std::setw(18) << to_pretty_time( res.cpu_limit.used ) << "\n";
std::cout << indent << std::left << std::setw(11) << "available:" << std::right << std::setw(18) << to_pretty_time( res.cpu_limit.available ) << "\n";
std::cout << indent << std::left << std::setw(11) << "limit:"     << std::right << std::setw(18) << to_pretty_time( res.cpu_limit.max ) << "\n";

其中res.cpu_limit的值源自get_account_cpu_limit_ex，最終與_db中resource_limits*欄位相關聯。
那麼我們需要弄懂相關欄位的含義。

• account_limit

  • _db.get<resource_limits_object, by_owner>(boost::make_tuple(bool pending, account_name account).cpu_weight：
    使用者抵押貨幣後理論cpu時間最大值。
    • pending：false時，記錄上個塊結束時的舊值。
    • pending：true時，記錄當前塊發生交易後的新值。
      當用戶產生交易後，在controller_impl::finalize_block()中更新：
      _db<resource_limits_object, by_owner>(false, account) = _db<resource_limits_object, by_owner>(true, account)

• account_usage

  • _db.get<resource_usage_object, by_owner>：
    視窗期內使用者使用的資源，在add_transaction_usage內增加。

• block_limit

  • _db.get<resource_limits_state_object>：整個塊中使用者計算資源消耗累計，累計方式同account_usage，根據下文config_object的值決定virtual_net_limit，動態調節限制。
    可以參考virtual_net_limit程式碼註釋：

    /**
    * The virtual number of bytes that would be consumed over blocksize_average_window_ms
    * if all blocks were at their maximum virtual size. This is virtual because the
    * real maximum block is less, this virtual number is only used for rate limiting users.
    *
    * It's lowest possible value is max_block_size * blocksize_average_window_ms / block_interval
    * It's highest possible value is 1000 times its lowest possible value
    *
    * This means that the most an account can consume during idle periods is 1000x the bandwidth
    * it is gauranteed under congestion.
    *
    * Increases when average_block_size < target_block_size, decreases when
    * average_block_size > target_block_size, with a cap at 1000x max_block_size
    * and a floor at max_block_size;
    **/

• _db.get<resource_limits_config_object>：記錄著整個硬體資源限制的配置，目前來看似乎是定值。

class resource_limits_config_object : public chainbase::object<resource_limits_config_object_type, resource_limits_config_object> {
    OBJECT_CTOR(resource_limits_config_object);
    id_type id;

    struct elastic_limit_parameters {
    uint64_t target;           // the desired usage
    uint64_t max;              // the maximum usage
    uint32_t periods;          // the number of aggregation periods that contribute to the average usage

    uint32_t max_multiplier;   // the multiplier by which virtual space can oversell usage when uncongested
    ratio    contract_rate;    // the rate at which a congested resource contracts its limit
    ratio    expand_rate;       // the rate at which an uncongested resource expands its limits

    void validate()const; // throws if the parameters do not satisfy basic sanity checks
    };

    elastic_limit_parameters cpu_limit_parameters = {EOS_PERCENT(config::default_max_block_cpu_usage, config::default_target_block_cpu_usage_pct), config::default_max_block_cpu_usage, config::block_cpu_usage_average_window_ms / config::block_interval_ms, 1000, {99, 100}, {1000, 999}};
    elastic_limit_parameters net_limit_parameters = {EOS_PERCENT(config::default_max_block_net_usage, config::default_target_block_net_usage_pct), config::default_max_block_net_usage, config::block_size_average_window_ms / config::block_interval_ms, 1000, {99, 100}, {1000, 999}};

    // window_size = account_cpu_usage_average_window
    uint32_t account_cpu_usage_average_window = config::account_cpu_usage_average_window_ms / config::block_interval_ms;
    uint32_t account_net_usage_average_window = config::account_net_usage_average_window_ms / config::block_interval_ms;
}

// 在controller_impl::finalize_block中執行更新
uint64_t CPU_TARGET = EOS_PERCENT(chain_config.max_block_cpu_usage, chain_config.target_block_cpu_usage_pct);
resource_limits.set_block_parameters(
    { CPU_TARGET, chain_config.max_block_cpu_usage, config::block_cpu_usage_average_window_ms / config::block_interval_ms, max_virtual_mult, {99, 100}, {1000, 999}},
    {EOS_PERCENT(chain_config.max_block_net_usage, chain_config.target_block_net_usage_pct), chain_config.max_block_net_usage, config::block_size_average_window_ms / config::block_interval_ms, max_virtual_mult, {99, 100}, {1000, 999}}
);

• 視窗期（window）

  • exponential_moving_average_accumulator：EMA(指數滑動平均線)計數器
  • 這篇部落格有提到EMA
  • window_size：一個視窗期佔用的block數

轉自：https://www.jianshu.com/p/e3ea61e7ccd9