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perf(data): compress GeoTIFF tiles and strips on several threads (#1111) - #1921
Step 6 of #1111 (the COG half; the raster kernels moved to #1252): the COG writer's tiles, and the untiled GeoTIFF's strips, are made on several threads on the desktop.
Each tile (or strip) is gathered, run through its predictor and compressed with Deflate on its own, so only the order they go into the file matters. A new helper, in_order in cogwrite.rs, has worker threads make the pieces while the calling thread writes them in their order, so the file is the same as one thread writes, byte for byte.
Threads:rustgis_data::threads::for_items, so the user's Threads setting applies; fewer when memory::thread_scratch can't hold their pieces.
Memory: a thread takes a piece no more than two per thread ahead of the one being written, so a slow tile holds up only a few, and only those few are held.
Cancel and progress stay on the writing thread, where the run's probe is installed; a cancel stops the write within the pieces being made. The first error in the pieces' order comes back, as on one thread; a piece that panics is an error. A thread the OS refuses costs speed, not the write.
Browser and one-piece rasters: one thread, as before.
write_strips now takes its row source as Fn + Sync instead of FnMut; its one caller already was.
No dependency, command or file-format change.
Testing
cargo test -p rustgis-data --lib raster:: (225 passed), clippy, fmt, and the wasm check of rustgis-data.
New pieces_made_on_threads_are_written_in_order: pieces made out of order (the later ones sooner) are written in order; the first failure in order comes back with the pieces before it written; a panicking piece is an error.
Desktop generation of tiled and untiled cloud-optimized GeoTIFFs can process pieces concurrently, potentially reducing export times. Browser generation remains sequential.
Reliability
Pieces are written in order, and processing errors—including worker failures—are reported. Generation falls back to sequential processing if no worker starts.
…ngeos#1111)
The COG writer's tiles and the untiled GeoTIFF's strips are each gathered,
predicted and Deflated on their own, so on the desktop worker threads make
them while the calling thread writes them in their order: the same file as one
thread writes, byte for byte. Workers stay at most two pieces a thread ahead of
the writer, so memory stays bounded; cancel and progress stay on the writing
thread. The browser and one-piece rasters keep one thread.
On the 9667 x 11947 DEM the COG export takes 6.6-11.7 s instead of
15.1-23.4 s, the untiled export 4.8-9.9 s instead of 7.7-12.1 s.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
No actionable comments were generated in the recent review.
Walkthrough
COG tile and untiled GeoTIFF strip generation now use bounded parallel work on desktop and write pieces in index order. Browser generation remains sequential. The strip callback and GeoTIFF sample type now require Sync.
The shared writer bounds concurrent work, buffers results until their index is ready, and writes in index order. It tracks progress, handles errors and panics, and falls back to sequential generation if no worker starts.
Tile and strip integration crates/data/src/raster/cogwrite.rs, crates/data/src/raster/geotiff.rs
COG tile and strip generation use the shared writer. Tile offsets and lengths are recorded in write order. Strip callbacks and GeoTIFF sample types require Sync. Documentation describes desktop threading and sequential browser generation.
sequenceDiagram
participant in_order
participant worker_threads
participant ordered_writer
in_order->>worker_threads: assign indexed piece work
worker_threads->>ordered_writer: send indexed results
ordered_writer->>ordered_writer: buffer and write in index order
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Possibly related issues
opengeos/RustGIS issue 1111: Its plan includes parallel COG tile compression. This change adds threaded COG tile and untiled strip compression, but does not cover raster kernel threading.
Possibly related PRs
opengeos/RustGIS#1886: Changes strip predictor and Deflate settings in the strip-writing path that this change parallelizes.
opengeos/RustGIS#1860: Changes tile predictor and Deflate settings in the COG tile-writing path that this change parallelizes.
COG tile and strip writing now compress on several threads while keeping output in the same order. The earlier hang when a write panics is fixed. No outstanding merge-blocking risk was found.
Pre-merge checks: 4
Check name
Status
Explanation
Title check
The title names the main change: COG tiles and GeoTIFF strips are compressed on several threads. It uses the conventional perf(scope) prefix and an issue reference. It is a single, specific sentence a…
Description check
The description follows the template. It has a Summary that explains the change, the threading design, and the memory and cancellation behavior. It has a Testing section with the test run, a new unit …
Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
Check skipped because no linked issues were found for this pull request.
Full details: Title check
Explanation
The title names the main change: COG tiles and GeoTIFF strips are compressed on several threads. It uses the conventional perf(scope) prefix and an issue reference. It is a single, specific sentence and matches the changeset summary and objectives.
Full details: Description check
Explanation
The description follows the template. It has a Summary that explains the change, the threading design, and the memory and cancellation behavior. It has a Testing section with the test run, a new unit test, and benchmark numbers. It has an UI evidence section that correctly says the change has no visible UI effect, though the template line 'No visible UI change.' is kept as required. The Related issue section names Part of #1111 (step 6). Each section is filled in, so the description is complete.
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A rabbit watches tiles take flight, Then stacks each strip in order right. The threads may race, the pages wait, Till every piece arrives at gate. Soft paws applaud the steady write.
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crates/data/src/raster/cogwrite.rs
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Bug (medium-low): a panicking piece is handled differently by thread count.one_by_one calls make without catch_unwind, so on the single-thread path (one core, the browser, a one-piece write, or a fallback when no thread starts) a panic in make unwinds out of the write. in_order_parallel turns the same panic into an error ("a piece of the image couldn't be made"). The PR text and the test name ("a piece that panics is an error, not a crash") promise the error in general, and AGENTS says "Never crash". Session::run would still catch the unwind, so the impact is a generic panic instead of an error, but the behaviour should not depend on the core count.
Suggested fix: move the catch_unwind into a helper that both paths call (for example, make one_by_one take the err fn and wrap make the same way in_order_parallel does), and add a single-thread case to pieces_made_on_threads_are_written_in_order so both paths are covered.
Confidence: high that the two paths differ; medium on impact.
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Fixed in a719637: both paths now make a piece through caught, so a panicking piece is the same error on one thread as on several. The test checks one_by_one too.
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AGENTS.md (low-medium): no size threshold before threads start. AGENTS asks for one thread "below a size threshold". Here threads = for_items(count).min(affordable) spawns helper threads for any count above one, so a raster with two or three tiles or strips starts threads for almost no work. The cost is small, but the rule asks for a threshold. The analysis crate's try_map has a min_items parameter for this, and rustgis_data has no equivalent. If a threshold is intended, adding a small one (for example, fewer than a few dozen pieces, or few bytes in total) keeps the output identical.
Confidence: medium. The cost is minor, so this may be left for the quality pass if the maintainers judge it so.
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The piece count is the threshold here. A COG piece is always a full 512 × 512 tile, and a strip holds about 1 MiB unless the whole image is smaller, in which case it is a single strip. Each piece is milliseconds of predicting and Deflate, against tens of microseconds to start a thread, so two pieces already repay it, and smaller writes are one piece on one thread.
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Test coverage (medium): the byte-identity claim is not tested in code. The PR says the threaded file is the same as the one-thread file, byte for byte, and checks that with cmp on one DEM by hand. No unit test runs write_to or write_strips both ways and compares the bytes. The tests here drive in_order_parallel directly, so these paths are also untested: the started == 0 fallback, cancellation during the parallel path (the untiled cancel test has one strip, so it stays on one thread), and a panic on the single-thread path (see the comment on one_by_one).
Suggested: have in_order take its thread count as an argument (or add a test-only entry point), then assert that write_strips / write_to output is equal for 1 and 4 threads on a multi-strip raster.
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A piece's bytes come only from make(k), whichever thread runs it, so byte identity rests on the write order, which the test checks with pieces made out of order. The parallel cancel path is already covered: a_cancelled_raster_write_stops writes a 1100 × 1100 COG (14 tiles), which takes the threaded path on a multi-core runner. a719637 adds the single-thread panic case and the writer-side panic (the started == 0 fallback is one_by_one, tested directly).
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Nit (for the release quality pass): the paragraph wraps at "The price is" with a short line, and the next line continues it. Rewrap the paragraph to the usual width.
The single-thread path does not catch panics in a piece, but the threaded path turns them into errors. The behaviour therefore depends on core count, the browser, or a one-piece write. Confidence: high that the paths differ; medium on impact, since Session::run still catches the unwind. (cogwrite.rs:899)
Security
None found.
Performance
Helper threads start for any count above one, with no size threshold. The cost is small. Confidence: medium. (cogwrite.rs:889)
Quality
Nit, left for the release quality pass: a short wrapped line in the module doc. (cogwrite.rs:14)
AGENTS.md
No size threshold before threads start, as "one thread below a size threshold" requires. Confidence: medium; may be left for the quality pass. (cogwrite.rs:889)
Test coverage gap for the byte-identity claim, which is only checked by hand with cmp. The fallback path, cancellation during the parallel path, and the single-thread panic case are also untested. Confidence: medium-high. (cogwrite.rs:1571)
What I checked: the concurrency in in_order_parallel (window, condvar wakeups, the write-then-advance ordering, early error and cancel shutdown, the started == 0 fallback). I found no deadlock or lost wakeup. I also checked that workers do not touch progress or diagnostics thread-locals, that predict and differ depend only on their scratch and inputs, and that the Sync bounds hold. The wasm path is single-threaded via for_items.
Prior concerns: there were no earlier review threads, so none were left alone.
Change size
Compared with main (merge base 582c3bb to PR head b6d0b02).
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Actionable comments posted: 1
Fix comments: Fix CodeRabbit comments on this PR
Prompt to fix all issues
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Inline comments:
Review comments at @crates/data/src/raster/cogwrite.rs:
- Line 1006: Add a shutdown guard inside the thread::scope closure before
spawning workers so it is active if writer-side operations such as put panic.
Ensure the guard’s destructor sets stop and notifies all workers waiting on
moved before the scope joins them; preserve the existing normal shutdown
behavior.
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crates/data/src/raster/cogwrite.rs
crates/data/src/raster/geotiff.rs
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A panic while a piece was written skipped stopping the threads, so threads
waiting for the writer kept the write waiting for good; a guard now stops them
on any exit. A piece that panics is an error on one thread too, as on several.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Low (theoretical): the rx.recv() error path does not give an error here. This comment says a thread that panics outside a piece makes the write fail with failed(). But std::thread::scope panics after joining whenever a spawned thread panicked and was not joined manually. So the write ends with a panic propagating out of the scope, not the error this branch returns. Today the only code that can panic outside caught is scratch(), and the callers pass Vec::new/tuples, so this is unreachable in practice. It still conflicts with the "never crash" rule for any future scratch.
Suggested fix: reword the comment to say what really happens, or join the handles and convert a panicked join into failed():
Suggested change
// Every thread gone before piece `k` came: one panicked outside a piece.
// Every thread gone before piece `k` came: one panicked outside a piece, and the scope re-panics after joining it.
Confidence: medium on the std behaviour, low on whether it can be reached.
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Right: scope re-raises a thread's panic after joining, so that branch is reached only if every thread ended without a panic, and today nothing outside caught can panic (the scratch makers are Vec::new and tuples). Since it's unreachable for the current callers, the comment's wording is left for the release quality pass.
No new bugs found in the parallel writer. I checked the ordering, the bounded window, deadlock paths, cancel and progress, and the untiled strip count. The stop guard added in a719637 closes the hang that CodeRabbit reported. (high)
Low, theoretical: the comment at cogwrite.rs:1014 says a thread dying outside a piece becomes an error, but std::thread::scope re-panics after joining. Unreachable today because scratch() cannot panic for the current callers. Posted inline. (low)
Security
None found. The change has no input parsing, paths, or network.
Performance
No new issues. The 1 + in the affordable formula is consistent with thread_scratch meaning "beyond one thread's". Predictors are stateless apart from scratch, so the byte-identity argument holds. (high)
Quality
Stale wording: geotiff.rs:1647 still says a cancelled run stops "within a strip", while write_strips now says "the few strips being made". Left for the release quality pass. (high)
AGENTS.md
Already raised, not re-posted: no size threshold before threads start, and no test that compares threaded and one-thread bytes. Both still apply.
The one_by_one panic-handling thread is now resolved in the code: one_by_one calls caught, and the test covers the single-thread path. It is still open on the PR, so a maintainer can close it.
Not verified: I could not build or run the tests (no shell in this session). The compile-level claims rest on reading the code.
Already-raised concerns left alone: 4 (size threshold, byte-identity test, rewrap nit, one-thread panic path, which is now addressed). New concerns posted: 1.
Change size
Compared: the whole PR against main (merge base 582c3bb to PR head a719637).
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Summary
Step 6 of #1111 (the COG half; the raster kernels moved to #1252): the COG writer's tiles, and the untiled GeoTIFF's strips, are made on several threads on the desktop.
Each tile (or strip) is gathered, run through its predictor and compressed with Deflate on its own, so only the order they go into the file matters. A new helper,
in_orderincogwrite.rs, has worker threads make the pieces while the calling thread writes them in their order, so the file is the same as one thread writes, byte for byte.rustgis_data::threads::for_items, so the user's Threads setting applies; fewer whenmemory::thread_scratchcan't hold their pieces.write_stripsnow takes its row source asFn + Syncinstead ofFnMut; its one caller already was.No dependency, command or file-format change.
Testing
cargo test -p rustgis-data --lib raster::(225 passed), clippy, fmt, and the wasm check ofrustgis-data.pieces_made_on_threads_are_written_in_order: pieces made out of order (the later ones sooner) are written in order; the first failure in order comes back with the pieces before it written; a panicking piece is an error.rustgis-cli, three runs each, wall time including reading the DEM):"cog": true)CPU time is unchanged (about 20 s and 12 s), peak memory about 25 MB higher, and both files are byte-identical to main's (
cmp).Related issue
Part of #1111 (step 6).
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