[Performance] Raise default HTTP/2 initial window size to 16 MiB#2213
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Motivation:
AHC defaulted the HTTP/2 initial window to 65535 (the RFC minimum), so any response larger than ~64 KiB stalls mid-transfer waiting for a WINDOW_UPDATE round-trip. The JDK HttpClient and OkHttp both default this per-stream window to 16 MiB, leaving AHC slower on HTTP/2 out of the box for no reason other than a conservative default.
Modification:
Raise the default http2InitialWindowSize to 16 MiB (16777216) in ahc-default.properties and the AsyncHttpClientConfig interface default, matching the JDK HttpClient and OkHttp. Still overridable via setHttp2InitialWindowSize.
Result:
liminates the per-response WINDOW_UPDATE stall on larger HTTP/2 bodies. In the 64 KiB-response benchmark this lifts HTTP/2 throughput about 64% (6.4k to 10.5k ops/s) and cuts median latency from 256 to 225 us, bringing AHC to parity with the JDK HttpClient and OkHttp. The value stays configurable, with tests covering both the new default and an override.