Fetch COG tiles concurrently in HTTP path to mask RTT#1487
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brendancol merged 2 commits intomainfrom May 5, 2026
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_read_cog_http walked tiles one at a time, blocking on each range request before sending the next. Over a 50 ms RTT link, a 100-tile COG paid 5 s in round trips before any data flowed. Add _HTTPSource.read_ranges, which takes a list of (offset, length) pairs and fetches them through a ThreadPoolExecutor (default 8 workers, override via XRSPATIAL_COG_HTTP_WORKERS). _read_cog_http collects every tile range up front, fetches them in parallel, then decodes and places each tile in input order. read_range is unchanged so other call sites are unaffected. Local file reads do not go through this path. Tests: order preservation, empty-list handling, single-request fast path, concurrency speedup against an artificial-latency fake source, and a round-trip correctness test against a local http.server.
The previous test_read_ranges_concurrency_masks_latency required 20 fake-latency requests at 50ms each to finish under 500ms. On a busy macOS CI runner the run took 519ms — 4% over the budget — and tripped fail-fast across the matrix. Track max-in-flight count in _FakeHTTPSource and assert >= 2. That proves the threadpool dispatches concurrently without depending on wall-clock timing.
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Closes #1480.
Summary
_read_cog_httpwalked tiles one at a time, blocking on each range request before sending the next. Over a 50 ms RTT link, a 100-tile COG paid 5 s in round trips before any data flowed.This PR adds
_HTTPSource.read_ranges, which takes a list of(offset, length)pairs and fetches them through aThreadPoolExecutor. Pool size defaults to 8 and can be overridden withXRSPATIAL_COG_HTTP_WORKERS._read_cog_httpnow collects every tile range up front, fetches them in parallel, then decodes and places each tile in input order._HTTPSource.read_rangeis unchanged, so no other call sites are affected. Local file reads do not go through this path.Measured on the test fake source (20 requests, 50 ms each, 8 workers): sequential bound is 1.00 s, concurrent fetch took 0.15 s, a 6.6x speedup.
Test plan
pytest xrspatial/geotiff/tests/test_cog_http_concurrent.py-- 6 new tests cover ordering, empty list, single-range fast path, concurrency speedup vs an artificial-latency fake, and round-trip correctness against a localhttp.server.pytest xrspatial/geotiff/tests/test_cog.py xrspatial/geotiff/tests/test_reader.py-- existing 32 tests still pass.