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聊聊无损网络PFC配置限制和设置指导

流量控制
2022-10-02
2406
更新：2022-10-09 22:11:41

内容提要

> RoCE的流量需要保证运行在无损队列中，无损队列使用了PFC技术，能针对某一队列发送Pause帧，迫使上游停流。设备在进行报文转发时，根据报文的优先级进入对应映射关系的队列中进行调度转发。当某一优先级报文发送速率超过接收速率，导致接收方可用数据缓冲空间不足时，设备通过PFC PAUSE帧反馈给上一跳设备，上一跳设备收到PAUSE帧报文后停止发送本优先级报文，直到再收到PFC XON帧或经过一定的老化时间后才能恢复流量发送。本文讲解具体的PFC配置限制和设置指导。

交换芯片都有固定的Pipeline（转发流水线）， Buffer管理处于入芯片流程和出芯片流程的中间位置。报文处于在这个位置上时，已经知道了该报文的入口和出口信息，因此逻辑上就可以分成入方向和出方向分别对缓存进行管理。

PFC水线是基于入方向缓存管理进行触发的。芯片在入口方向提供了8个队列，我们可以将不同优先级的业务报文映射到不同的队列上，从而实现对不同优先级的报文提供不同的Buffer分配方案。

# 缓存类型

![image.png](https://ngdcn.com/zb_users/upload/2022/10/202210021664716767123970.png)

具体到每个队列，其Buffer分配根据使用场景设计为3部分：保证缓存，共享缓存，Headroom。

# PFC门限设置类型

通过配置PFC缓存门限可以有效解决因缓冲空间不足和入流量队列数量过大，导致发送数据缓冲区尾丢弃等问题。

![image.png](https://ngdcn.com/zb_users/upload/2022/10/202210021664716812639643.png)
<center>PFC缓存门限示意图</center>

PFC目前提供以下门限设置类型：
l Headroom缓存门限（如上图中的headroom）：Headroom存储空间中某802.1p优先级报文的最大使用cell资源。当达到使用的cell资源后，该接口会丢弃收到的报文。

l 反压帧触发门限（如上图中的XOFF）：Shared存储空间中某802.1p优先级报文在该存储空间使用cell资源上限。达到上限后，会触发PFC功能发送PAUSE帧。反压帧触发门限又分为动态反压帧触发门限和静态反压帧触发门限：

l 反压帧停止门限与触发门限间的偏移量（如上图中的ingress-threshold-offset）：当某802.1p优先级报文使用的cell资源减小了一个固定值时，停止发送PFC PAUSE帧，使对端设备恢复流量发送——从Xoff水线回落多少就可以发送Xon的偏移量\[需要保证报文到达之前不欠吞吐——buffer不会过早为0\]。

l PFC预留门限（如上图中的reserved-buffer）：Guaranteed存储空间中为某802.1p优先级报文预留的cell资源。Guaranteed缓存是一个静态水线（固定的、独享的）。

# Headroom缓存门限设置方法

如果想要进行更精确地控制，可以根据如下方法计算Headroom缓存门限值：计算从接收端发送PFC Pause帧到发送端收到PFC Pause帧并真正停止流量发送期间，在途流量大小。公式为：在途流量（bytes）=MTUR+MTUs+Response+2\*link\_delay。其中，

l **MTUR**：队列触发反压帧门限到发送Pause帧之间，如果芯片有大包要发，这个大包的长度。由于这个大包可能来自其它任何队列，所以这个大包的长度应该等于设备允许的最大帧长度9216字节。

l **MTUs**：如果上游设备在准备停止流量发送前，刚好开始发送上一个包，这个包的长度。由于只有PFC指定优先级的报文影响接收端的门限，所以MTUs的长度应该为PFC指定优先级报文的长度。

l **Response**：上游设备收到PFC Pause帧到停止发送的反应时间产生的数据长度，该值固定为3840字节。

# 反压帧触发门限设置方法

对于共享缓存的设置，需要采用更为灵活的动态水线。动态水线能根据当前空闲的Buffer资源，以及当前队列已使用的Buffer资源数量来决定能否继续申请到资源。由于系统中空闲共享Buffer资源与已使用的Buffer资源都是时刻变化的，因此阈值也处于不断变动中。相对于静态水线，动态水线能更灵活、有效的利用Buffer及避免造成不必要的浪费。

PFC动态反压帧触发门限如下：

<figure cid="n328" mdtype="table" style="margin: 1.2em 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; color: rgb(34, 34, 34); letter-spacing: 0.544px; white-space: normal; background-color: rgb(255, 255, 255); overflow-x: auto; cursor: default; font-family: "Open Sans", "Clear Sans", "Helvetica Neue", Helvetica, Arial, "Segoe UI Emoji", sans-serif; font-size: 16px; overflow-wrap: break-word !important; text-align: center;"><table width="676"><thead style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; background-color: rgb(248, 248, 248);"><tr cid="n329" class="firstRow" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229);"><th style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-bottom: 0px; border-top-color: rgb(223, 226, 229); border-right-color: rgb(223, 226, 229); border-left-color: rgb(223, 226, 229); background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">百分比（动态反压帧触发门限值）</th><th style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-bottom: 0px; border-top-color: rgb(223, 226, 229); border-right-color: rgb(223, 226, 229); border-left-color: rgb(223, 226, 229); background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">Alpha值（芯片寄存器配置值，表示队列中cell个数和可用cell个数比值）</th><th style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-bottom: 0px; border-top-color: rgb(223, 226, 229); border-right-color: rgb(223, 226, 229); border-left-color: rgb(223, 226, 229); background-image: initial; background-position: initial; background-size: initial; background-repeat: initial; background-attachment: initial; background-origin: initial; background-clip: initial; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">占可用总缓存的实际比例（alpha/(1+alpha)\*100）%</th></tr></thead><tbody style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important;"><tr cid="n333" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">0</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1/128</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">0.77</td></tr><tr cid="n337" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229); background-color: rgb(248, 248, 248);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1/64</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1.53</td></tr><tr cid="n341" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">2~3</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1/32</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">3.03</td></tr><tr cid="n345" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229); background-color: rgb(248, 248, 248);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">4~5</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1/16</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">5.88</td></tr><tr cid="n349" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">6~11</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1/8</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">11.11</td></tr><tr cid="n353" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229); background-color: rgb(248, 248, 248);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">12~20</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1/4</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">20.00</td></tr><tr cid="n357" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">21~33</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1/2</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">33.33</td></tr><tr cid="n361" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229); background-color: rgb(248, 248, 248);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">34~50</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">1</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">50.00</td></tr><tr cid="n365" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">51~66</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">2</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">66.66</td></tr><tr cid="n369" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229); background-color: rgb(248, 248, 248);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">67~80</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">4</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">80.00</td></tr><tr cid="n373" mdtype="table_row" style="margin: 0px; padding: 0px; outline: 0px; max-width: 100%; box-sizing: border-box; overflow-wrap: break-word !important; break-inside: avoid; break-after: auto; border-width: 1px; border-style: solid; border-color: rgb(223, 226, 229);"><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 66px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">81~100</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 120px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">8</td><td style="margin: 0px; padding: 6px 13px; outline: 0px; word-break: break-all; border-color: rgb(223, 226, 229); max-width: 100%; box-sizing: border-box; min-width: 32px; overflow-wrap: break-word !important;"><span style="box-sizing: border-box; display: inline-block; min-width: 1ch; width: 208px; min-height: 10px; margin: 0px; padding: 0px; outline: 0px; max-width: 100%; font-size: 18px; overflow-wrap: break-word !important;">88.88</td></tr></tbody></table>

动态水线alpha值\=队列可申请缓存量/剩余共享缓存量。队列的α值越大，其在共享缓存中可使用的百分数占比也就越高。队列的α值设置越小，其最大可申请的共享缓存占比就越小。当端口拥塞时就会越早触发PFC流控，PFC流控生效后队列降速，可以很好地确保网络不丢包。

但从性能的角度看，过早触发PFC流控，会导致RDMA网络吞吐下降。因此我们在MMU水线设置时需要选取一个平衡值。

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PFC