一，前言
码率分配器，当 gcc 评估出一个可用带宽，由 BitrateAllocator 分配其所包含的 observer，它的 observer 有 AudioSendStream 和 VideoSendStreamImpl 两个，关系图如下：
AudioSendStream 需要满足一下条件才能加入 observer 列表中
二，辅助工具函数
介绍 3 种策略之前先介绍下两个函数 DistributeBitrateRelatively 和 DistributeBitrateEvenly（它们都是对剩余带宽进行分配）。
1），DistributeBitrateRelatively（按需分配）
capacity_bps： observer 所需码率最大码率与最小码率的差值，我的理解为需求量。
bitrate_priority： observer 权重，默认为 1.
1），对所有的 observer 根据 capacity_bps/bitrate_priority 进行从小到大排序
2），优先对需求量小的进行码率分配
3），如果还有剩余，继续分配，bitrate_priority 越大分的的带宽越多
2），DistributeBitrateEvenly（均等分配）
原理比较简单，按 observer 数量均分，见函数
void DistributeBitrateEvenly( const std::vector<AllocatableTrack>& allocatable_tracks, uint32_t bitrate, bool include_zero_allocations, int max_multiplier, std::map<BitrateAllocatorObserver*, int>* allocation) { // 1，先过滤掉非活动的 observer std::multimap<uint32_t, const AllocatableTrack*> list_max_bitrates; for (const auto& observer_config : allocatable_tracks) { if (include_zero_allocations || allocation->at(observer_config.observer) != 0) { list_max_bitrates.insert( {observer_config.config.max_bitrate_bps, &observer_config}); } } auto it = list_max_bitrates.begin(); while (it != list_max_bitrates.end()) { RTC_DCHECK_GT(bitrate, 0); //2， 按数量进行均分 uint32_t extra_allocation = bitrate / static_cast<uint32_t>(list_max_bitrates.size()); // 3，直接追加 uint32_t total_allocation = extra_allocation + allocation->at(it->second->observer); bitrate -= extra_allocation; // 4，当带宽毕竟充足，满足MaxRateAllocation，max_multiplier 为1.5 // 当LowRateAllocation max_multiplier 为1 if (total_allocation > max_multiplier * it->first) { // There is more than we can fit for this observer, carry over to the // remaining observers. bitrate += total_allocation - max_multiplier * it->first; total_allocation = max_multiplier * it->first; } // 5， 为每个observer更新最新的值 allocation->at(it->second->observer) = total_allocation; it = list_max_bitrates.erase(it); }}
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三，3 种分配策略
1），当 bitrate 不能满足 sum_min_bitrates,执 LowRateAllocation
std::map<BitrateAllocatorObserver*, int> LowRateAllocation( const std::vector<AllocatableTrack>& allocatable_tracks, uint32_t bitrate) { std::map<BitrateAllocatorObserver*, int> allocation;
// 1，为每个observer 赋一个初始值，enforce_min_bitrate 默认为0. int64_t remaining_bitrate = bitrate; for (const auto& observer_config : allocatable_tracks) { int32_t allocated_bitrate = 0; if (observer_config.config.enforce_min_bitrate) allocated_bitrate = observer_config.config.min_bitrate_bps;
allocation[observer_config.observer] = allocated_bitrate; remaining_bitrate -= allocated_bitrate; }
// 2，有些observer 可能为非活动的，剔除掉 ？ .......
// 3， 如何带宽还有剩余则执行均等分配 if (remaining_bitrate > 0) DistributeBitrateEvenly(allocatable_tracks, remaining_bitrate, false, 1, &allocation);
RTC_DCHECK_EQ(allocation.size(), allocatable_tracks.size()); return allocation;}
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2），bitrate 介于 sum_min_bitrates 和 sum_max_bitrates 之间，执行 NormalRateAllocation
std::map<BitrateAllocatorObserver*, int> NormalRateAllocation( const std::vector<AllocatableTrack>& allocatable_tracks, uint32_t bitrate, uint32_t sum_min_bitrates) { std::map<BitrateAllocatorObserver*, int> allocation; std::map<BitrateAllocatorObserver*, int> observers_capacities; // 1， 为每个observer 根据配置赋 for (const auto& observer_config : allocatable_tracks) { allocation[observer_config.observer] = observer_config.config.min_bitrate_bps; observers_capacities[observer_config.observer] = observer_config.config.max_bitrate_bps - observer_config.config.min_bitrate_bps; }
bitrate -= sum_min_bitrates; // 2， 当 priority_margin 大于0 时，observer 优先级较高，需要先满足其预设值priority_bitrate_bps， // priority_bitrate_bps 默认为0， for (const auto& observer_config : allocatable_tracks) { int64_t priority_margin = observer_config.config.priority_bitrate_bps - allocation[observer_config.observer]; if (priority_margin > 0 && bitrate > 0) { int64_t extra_bitrate = std::min<int64_t>(priority_margin, bitrate); allocation[observer_config.observer] += rtc::dchecked_cast<int>(extra_bitrate); observers_capacities[observer_config.observer] -= extra_bitrate; bitrate -= extra_bitrate; } } // 3，执行按需分配 if (bitrate > 0) DistributeBitrateRelatively(allocatable_tracks, bitrate, observers_capacities, &allocation);
return allocation;}
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3），大于 sum_max_bitrates ，执行 MaxRateAllocation，这个函数比较简单
std::map<BitrateAllocatorObserver*, int> MaxRateAllocation( const std::vector<AllocatableTrack>& allocatable_tracks, uint32_t bitrate, uint32_t sum_max_bitrates) { std::map<BitrateAllocatorObserver*, int> allocation; // 1， 先满足每个observer 对带宽的最大需求量 for (const auto& observer_config : allocatable_tracks) { allocation[observer_config.observer] = observer_config.config.max_bitrate_bps; bitrate -= observer_config.config.max_bitrate_bps; } // 2，对剩余执行均等分配，kTransmissionMaxBitrateMultiplier 为1.5 DistributeBitrateEvenly(allocatable_tracks, bitrate, true, kTransmissionMaxBitrateMultiplier, &allocation); return allocation;}
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