m3serve 0.1.6

High-throughput BGE-M3 inference engine with dense + sparse embeddings

m3serve

Lightweight async inference engine for BAAI/bge-m3 that returns dense and sparse embeddings in a single call — enabling hybrid retrieval without the overhead of a full LLM framework.

Install

pip install m3serve

Usage

from m3serve import Engine

engine = Engine(model_name="BAAI/bge-m3", use_fp16=True)
await engine.start()

result = await engine.embed(["hello world"], return_sparse=True)
# result.dense            -> list[list[float]]  (1024-dim)
# result.sparse_indices   -> list[list[int]]    (token ids with non-zero weight)
# result.sparse_weights   -> list[list[float]]  (corresponding weights)

await engine.stop()

How it works

Three background threads run in a pipeline so the GPU is never idle waiting for tokenisation or post-processing:

Thread 1  encode_pre   tokenise on CPU        ──┐
Thread 2  encode_core  GPU forward pass    ◄──┘  └──►
Thread 3  encode_post  convert to Python lists       └──► resolved Future

Incoming requests are queued and batched by token length (shorter sequences first) to minimise padding waste. Each embed() call is a coroutine that returns as soon as its batch is processed — no polling, no callbacks.

Options

Parameter	Default	Description
model_name	"BAAI/bge-m3"	Any bge-m3 compatible model
device	auto-detected	"cuda:0", "mps", "cpu"
use_fp16	True	Half-precision inference (ignored on CPU)
torch_compile	False	torch.compile the backbone (CUDA only, adds warmup)
max_batch_size	256	Maximum sequences per GPU batch
batch_delay	0.005	Coalescing window in seconds — sleep after first item arrives to let concurrent requests accumulate. Set to ~½ × GPU inference time for your batch size.
tokenizer_threads	4	Number of threads dedicated to tokenization (token_lengths). Each thread holds its own tokenizer copy; all are pre-warmed at start() so no cold deepcopy happens during serving.
max_length	8192	Maximum token length per sequence. Longer inputs are truncated. Lower values reduce memory usage and improve throughput for short-text workloads.

Tuning batch_delay

When the queue goes from empty to non-empty, the preprocess thread sleeps for batch_delay seconds before consuming it. Any requests that arrive during that window get merged into the same GPU batch.

• Low concurrency / latency-sensitive: use Engine(batch_delay=0). At c=1 the window is wasted because there is no one else to wait for.
• High concurrency / throughput-focused: keep the default (0.005). Concurrent requests coalesce into larger batches, amortising the GPU's fixed per-forward-pass cost.

A good starting value is roughly half your typical GPU inference time. This heuristic is also used by Infinity-emb and mirrors Triton's max_queue_delay_microseconds.

Limitations

Single model, single GPU. m3serve runs one bge-m3 instance on one GPU. There is no replica support or multi-GPU sharding.

Coalescing window adds latency at low concurrency. The default batch_delay=0.005 sleeps 5 ms after the first request arrives to let concurrent requests accumulate into a larger batch. At c=1 this sleep is always wasted, adding ~5 ms to every request. Use Engine(batch_delay=0) for single-client or latency-sensitive workloads.

p99 latency can be spiky at medium concurrency. A request that just misses a coalescing window must wait for the next cycle. In practice this means p99 can be 5-10x higher than p50 at moderate concurrency levels (e.g. c=4 to c=8). If your workload has strict p99 SLAs, benchmark under your expected traffic pattern before deploying.

bge-m3 only. The engine is purpose-built for BAAI/bge-m3 and models with the same three-stage encode interface. It is not a general-purpose inference server.