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apply_template ran synchronously on the tokio worker threads. Under a burst of long conversations that starves the I/O tasks sharing the runtime, so request bodies stay unread in kernel buffers until clients time out. Hand the worker to the blocking pool for the render, as tokenization already does. Signed-off-by: Pei Li <peili@nvidia.com>
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Priority: ➖ Normal Estimated code review effort: 1 (Trivial) | ~5 minutes Merge Risk: 🔵 Low · up to Applications that invoke this preprocessor from Tokio local tasks can panic while rendering prompts. Use a LocalSet-safe fallback before merge, or explicitly accept this bounded embedding limitation. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
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In `@lib/llm/src/preprocessor.rs`:
- Line 2798: Update OpenAIPreprocessor::preprocess_request around apply_template
to avoid off_runtime and its block_in_place path when invoked from spawn_local;
use a LocalSet-safe synchronous fallback, or refactor through an async helper
that owns Send + 'static inputs before spawn_blocking. Preserve the existing
template-rendering behavior and request/self borrowing semantics.
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| let formatted_prompt = { | ||
| let _nvtx = dynamo_nvtx_range!("preprocess.template"); | ||
| self.apply_template(request) | ||
| off_runtime(|| self.apply_template(request)) |
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🩺 Stability & Availability | 🟡 Minor | ⚡ Quick win
🔎 Supported by static analysis
🏁 Script executed:
sed -n '1360,1410p' lib/llm/src/preprocessor.rs
sed -n '2760,2820p' lib/llm/src/preprocessor.rs
rg -n 'pub (async )?fn preprocess_request|struct OpenAIPreprocessor|fn apply_template|off_runtime|spawn_local|LocalSet' lib Cargo.toml
rg -n 'tokio\s*=|tokio =' Cargo.toml lib/*/Cargo.tomlRepository: ai-dynamo/dynamo
Length of output: 6672
Use a LocalSet-safe blocking path.
OpenAIPreprocessor::preprocess_request can run from spawn_local on a multi-thread Tokio runtime. In that context, off_runtime selects tokio::task::block_in_place, which Tokio does not support for local tasks and can panic.
Because off_runtime is synchronous and its closure borrows self and request, replace this path with a synchronous fallback that does not call block_in_place, or move the rendering operation into an async helper that owns Send + 'static inputs before using spawn_blocking. A direct spawn_blocking replacement is not implementable at the current call site.
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In `@lib/llm/src/preprocessor.rs` at line 2798, Update
OpenAIPreprocessor::preprocess_request around apply_template to avoid
off_runtime and its block_in_place path when invoked from spawn_local; use a
LocalSet-safe synchronous fallback, or refactor through an async helper that
owns Send + 'static inputs before spawn_blocking. Preserve the existing
template-rendering behavior and request/self borrowing semantics.
After applying the fix, consider running `coderabbit review --agent` for local
review. Visit https://docs.coderabbit.ai/cli?utm_source=ghpr
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| /// `block_in_place` is not available. | ||
| fn off_runtime<T>(f: impl FnOnce() -> T) -> T { | ||
| match tokio::runtime::Handle::try_current().map(|h| h.runtime_flavor()) { | ||
| Ok(tokio::runtime::RuntimeFlavor::MultiThread) => tokio::task::block_in_place(f), |
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RuntimeFlavor::MultiThread is also reported while a future is running in a Tokio LocalSet, where block_in_place is explicitly disallowed and panics. Since preprocess_request is public and previously rendered inline, callers using a multithread runtime plus LocalSet now crash whenever a chat template is applied.
🤖 AI Fix
Replace this block_in_place path with an async offload that is valid in local tasks, such as owning the render inputs and awaiting spawn_blocking, and add a LocalSet regression test.
|
Closing this. After the review questions I measured the stages of The mechanism description was also imprecise: If a frontend profile ever shows a runtime worker pinned in template rendering, this can be revisited with that evidence attached. |
`OpenAIPreprocessor::apply_template` renders the chat template synchronously on the tokio worker thread running the request. Tokenization was already moved to the blocking pool for exactly this reason (see the comment on `encode_with_timing`: "a synchronous encode here stalls the frontend tokio runtime for seconds, starving the I/O tasks that share the runtime"), but the render was not. With a burst of a few thousand long conversations the renders saturate the runtime's worker threads and hyper's I/O tasks stop being scheduled: request bodies stay unread in kernel receive buffers, receive windows close, and clients with a TCP user timeout abort the upload before the frontend reads it. The accept loop starves for the same reason, so connections also queue or fail before reaching the application. `off_runtime()` wraps the render in `tokio::task::block_in_place` on the multi-thread runtime, which hands the current worker to the blocking pool and lets tokio start a replacement for the async tasks. On the current-thread runtime (unit tests) it runs inline, since `block_in_place` is unavailable there. `block_in_place` rather than `spawn_blocking` because `apply_template` borrows the request generically (`&R`), so there is no `'static` value to move. This restores the change from the closed PR #15092. It was closed for lack of measurable effect, but that assessment compared throughput on runs that already passed. Measured against the failure rate it was meant to address, on a 2,934-concurrency agentic workload: 3 passes / 9 without it, 5 passes / 6 with it (Fisher one-tailed p = 0.084). Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Signed-off-by: Tzu-Ling <tzulingk@nvidia.com>
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Overview:
OpenAIPreprocessor::apply_templaterenders the chat template synchronously on the tokio worker thread that runs the request. Tokenization was moved to the blocking pool for this reason (see the comment onencode_with_timing), but the render was not. In internal testing with a burst of a few thousand long conversations, the renders saturate the runtime's worker threads and hyper's I/O tasks stop being scheduled: request bodies stay unread in kernel receive buffers (we measured up to ~74 MB across ~600 sockets), the receive windows close, and clients with a TCP user timeout abort the upload before the frontend ever reads it.Details:
off_runtime()helper: on the multi-thread runtime it wraps the closure intokio::task::block_in_place, which hands the current worker to the blocking pool and lets tokio start a replacement worker for the async tasks; on the current-thread runtime (unit tests) it runs inline, sinceblock_in_placeis not available there.preprocess_request_with_optionsrenders the template through it. Nothing else changes;TEMPLATE_SECONDSstill measures the render.block_in_placeis used rather thanspawn_blockingbecauseapply_templateborrows the request generically (&R), so there is no'staticvalue to move into a spawned closure.Where should the reviewer start?
lib/llm/src/preprocessor.rs:off_runtimeand its single call site inpreprocess_request_with_options.Validation
cargo test -p dynamo-llm --lib -- preprocessor: 211 passed.cargo clippy -p dynamo-llm --no-deps --lib -- -D warningsandcargo fmt --check: clean.Related Issues
Complements #15080 (listen backlog); both were found in the same internal test.
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