Dropbox 從 SPDY 切換到 HTTP/2 發現的現象

Dropbox 將本來的 SPDY 切換到 HTTP/2 後整理了不少資料:「Enabling HTTP/2 for Dropbox web services: experiences and observations」。

大多數都是效能的改善,但「Increased latency for POST requests.」這段頗有趣的,找出了 nginx 的 bug:

POST 的 latency 大約增加了 50%,而實際追蹤問題發現是 nginx 中 SETTINGS_INITIAL_WINDOW_SIZE 預設值的問題,然後提出 patch 改善:「[nginx] HTTP/2: rewritten handling of request body.」:

There is a small issue with setting `SETTINGS_INITIAL_WINDOW_SIZE` to 0: now when client tries to POST data it needs to wait for an additional RTT(between `send HEADERS` and `recv WINDOW_UPDATE`) to start sending data.

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