(backend) implement the post_processor background thread

backends/trtllm/src/lib.rs

@@ -6,9 +6,9 @@ mod utils;
 
 #[cxx::bridge(namespace = "huggingface::tgi::backends")]
 mod ffi {
-
     /// Struct used as shared type between rust and C++ to represent the result
     /// of a single decoding iteration
+    #[derive(Debug, Clone)]
     pub struct GenerationStep {
         request_id: u64,
         token_id: u32,

backends/trtllm/src/looper.rs

@@ -6,19 +6,22 @@ use std::sync::OnceLock;
 use async_trait::async_trait;
 use cxx::UniquePtr;
 use hashbrown::HashMap;
+use log::warn;
 use tokenizers::{Encoding, Tokenizer};
 use tokio::sync::mpsc::{unbounded_channel, UnboundedReceiver, UnboundedSender};
-use tokio::task::JoinHandle;
+use tokio::task::{JoinHandle, spawn_blocking};
+use tokio::time::Instant;
 use tokio_stream::wrappers::UnboundedReceiverStream;
 use tracing::{error, info, Level, span};
 
-use text_generation_router::infer::{Backend, InferError, InferStreamResponse};
+use text_generation_router::{FinishReason, Token};
+use text_generation_router::infer::{Backend, GeneratedText, InferError, InferStreamResponse};
 use text_generation_router::infer::InferError::GenerationError;
 use text_generation_router::validation::{Chunk, ValidationError, ValidGenerateRequest};
 use text_generation_router::validation::ValidationError::UnsupportedModality;
 
 use crate::errors::TensorRtLlmBackendError;
-use crate::ffi::{create_tensorrt_llm_backend, TensorRtLlmBackendImpl};
+use crate::ffi::{create_tensorrt_llm_backend, GenerationStep, TensorRtLlmBackendImpl};
 use crate::utils::first_line;
 
 // Value used to poll the state of the generation stream
@@ -34,15 +37,21 @@ struct ValidGenerateRequestWithTokens {
     inner: ValidGenerateRequest,
 }
 
+struct DecodedTokenContext {
+    tokens: Vec<GenerationStep>,
+    ctx: UnboundedSender<InferResult<InferStreamResponse>>,
+}
+
 fn executor_status_poller(
     mut backend: UniquePtr<TensorRtLlmBackendImpl>,
     mut waiting_requests: UnboundedReceiver<GenerationContext>,
+    mut post_processor_sender: UnboundedSender<DecodedTokenContext>,
 ) {
     // Track the tuple (request_id, stream) for each request
     let mut in_flights = HashMap::<u64, GenerationContext>::with_capacity(128);
 
     // TODO: Does it need a spin-loop?
-    loop {
+    'executor: loop {
         span!(Level::DEBUG, "[in-flight][submit]").in_scope(|| {
             // Is there any request pending to be scheduled?
             let awaiting_requests = waiting_requests.len();
@@ -84,18 +93,40 @@ fn executor_status_poller(
             }
         });
 
-        span!(Level::DEBUG, "[in-flight][poll]").in_scope(|| {
+        if let Err(e) = span!(Level::DEBUG, "[in-flight][poll]").in_scope(|| {
             if backend.num_responses_ready() > 0 {
                 match backend.pin_mut().pull_tokens() {
                     Ok(responses) => {
+                        // worse case scenario is one token for each response: with_capacity(responses.len())
+                        // grouper will group decoded tokens per request to decode multiple tokens
+                        let mut grouper: HashMap<u64, DecodedTokenContext> =
+                            HashMap::with_capacity(responses.len());
+
+                        // Iterate through all the decoded token
                         for step in responses.deref() {
                             let request_id = step.request_id;
+
                             match in_flights.get(&request_id) {
                                 Some(ctx) => {
                                     info!("New token for {} -> {}", request_id, step.token_id);
 
-                                    if step.is_final {
-                                        let _ = in_flights.remove(&step.request_id);
+                                    if !step.has_error {
+                                        let req_group = grouper.entry_ref(&request_id).or_insert(
+                                            DecodedTokenContext {
+                                                tokens: vec![],
+                                                ctx: ctx.streamer.clone(), // Arc::clone() = cheap
+                                            },
+                                        );
+                                        req_group.tokens.push(step.clone()); // Should be ultra cheap
+
+                                        if step.is_final {
+                                            let _ = in_flights.remove(&step.request_id);
+                                        }
+                                    } else {
+                                        warn!(
+                                            "Error for request: {} -> {}",
+                                            request_id, &step.error_msg
+                                        );
                                     }
                                 }
                                 None => {
@@ -103,19 +134,87 @@ fn executor_status_poller(
                                 }
                             }
                         }
+
+                        grouper
+                            .into_values()
+                            .map(|ctx| post_processor_sender.send(ctx))
+                            .collect()?;
                     }
                     Err(err) => {
                         error!("Failed to retrieve tokens from the executor: {}", err);
                     }
                 }
             }
-        });
+
+            Ok(())
+        }) {
+            error!(
+                "Caught an fatal error in the executor's loop, about to exit. {}",
+                e
+            );
+            break 'executor;
+        }
 
         // Hint the CPU we are spin-locking
         hint::spin_loop();
     }
 }
 
+fn post_processor_looper(
+    tokenizer: Tokenizer,
+    mut decoded_tokens: UnboundedReceiver<DecodedTokenContext>,
+) {
+    'post_processor: loop {
+        if decoded_tokens.is_closed() {
+            warn!("Post processor IPC is closed, loop will exit now.");
+            break 'post_processor;
+        }
+
+        if let Some(ctx) = decoded_tokens.blocking_recv() {
+            ctx.tokens.iter().for_each(|step| {
+                let out = match tokenizer.decode(&[step.token_id], true) {
+                    Ok(text) => {
+                        let is_special = tokenizer.get_added_vocabulary().is_special_token(&text);
+                        let token = Token {
+                            id: step.token_id,
+                            text,
+                            logprob: step.log_prob,
+                            special: is_special,
+                        };
+
+                        let response = if !step.is_final {
+                            InferStreamResponse::Intermediate {
+                                token,
+                                top_tokens: vec![],
+                            }
+                        } else {
+                            InferStreamResponse::End {
+                                token,
+                                top_tokens: vec![],
+                                generated_text: GeneratedText {
+                                    text: String::from(""),
+                                    generated_tokens: 0,
+                                    finish_reason: FinishReason::Length,
+                                    seed: None,
+                                },
+                                start: Instant::now(),  // Handle start time
+                                queued: Instant::now(), // Handle queued time
+                            }
+                        };
+
+                        Ok(response)
+                    }
+                    Err(e) => Err(GenerationError(e.to_string())),
+                };
+
+                if let Err(e) = ctx.ctx.send(out) {
+                    warn!("Failed to send back the decoded tokens: {}", e);
+                };
+            });
+        }
+    }
+}
+
 struct GenerationContext {
     request: ValidGenerateRequestWithTokens,
     streamer: UnboundedSender<InferResult<InferStreamResponse>>,
@@ -123,8 +222,9 @@ struct GenerationContext {
 
 pub struct TensorRtLlmBackendV2 {
     tokenizer: Tokenizer,
-    looper: JoinHandle<()>,
-    queue: UnboundedSender<GenerationContext>,
+    executor_looper: JoinHandle<()>,
+    post_processor_looper: JoinHandle<()>,
+    executor: UnboundedSender<GenerationContext>,
 }
 
 impl TensorRtLlmBackendV2 {
@@ -150,20 +250,28 @@ impl TensorRtLlmBackendV2 {
         );
 
         // Allocate the IPC layer to communicate with the backend
-        let (requests_sender, requests_receiver) = unbounded_channel::<GenerationContext>();
+        let (executor_sender, executor_receiver) = unbounded_channel();
+        let (post_processor_sender, post_processor_receiver) = unbounded_channel();
 
         // Create the FFI backend
         let backend = create_tensorrt_llm_backend(&engine_folder, &executor_worker_path)
             .map_err(|e| TensorRtLlmBackendError::Runtime(first_line(e.what(), "Unknown error")))?;
 
-        // Looper is responsible for scheduling and pulling requests state at regular interval
-        let looper =
-            tokio::task::spawn_blocking(move || executor_status_poller(backend, requests_receiver));
+        // Executor looper is responsible for scheduling and pulling requests state at regular interval
+        let executor_looper = spawn_blocking(move || {
+            executor_status_poller(backend, executor_receiver, post_processor_sender)
+        });
+
+        // Post processor looper is responsible from receiving a bunch of tokens, decoding them and sending them back to the user
+        let tokenizer_ = tokenizer.clone();
+        let post_processor_looper =
+            spawn_blocking(move || post_processor_looper(tokenizer_, post_processor_receiver));
 
         Ok(TensorRtLlmBackendV2 {
             tokenizer,
-            looper,
-            queue: requests_sender,
+            executor_looper,
+            post_processor_looper,
+            executor: executor_sender,
         })
     }
 
@@ -212,7 +320,7 @@ impl Backend for TensorRtLlmBackendV2 {
         let (streamer, receiver) = unbounded_channel::<InferResult<InferStreamResponse>>();
 
         // Send the context to the executor for scheduling
-        match self.queue.send(GenerationContext { request, streamer }) {
+        match self.executor.send(GenerationContext { request, streamer }) {
             Ok(_) => Ok(UnboundedReceiverStream::new(receiver)),
             Err(_) => Err(GenerationError(
                 "Failed to submit request to the backend".into(),
@@ -221,6 +329,8 @@ impl Backend for TensorRtLlmBackendV2 {
     }
 
     async fn health(&self, current_health: bool) -> bool {
-        current_health & !self.looper.is_finished()
+        current_health
+            & !self.executor_looper.is_finished()
+            & !self.post_processor_looper.is_finished()
     }
 }