[Doc] Improve OOM troubleshooting (vllm-project#16704)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>
Signed-off-by: Yang Wang <elainewy@meta.com>

Author: DarkLight1337

docs/source/getting_started/troubleshooting.md

@@ -24,7 +24,7 @@ To isolate the model downloading and loading issue, you can use the `--load-form
 
 ## Out of memory
 
-If the model is too large to fit in a single GPU, you will get an out-of-memory (OOM) error. Consider [using tensor parallelism](#distributed-serving) to split the model across multiple GPUs. In that case, every process will read the whole model and split it into chunks, which makes the disk reading time even longer (proportional to the size of tensor parallelism). You can convert the model checkpoint to a sharded checkpoint using <gh-file:examples/offline_inference/save_sharded_state.py>. The conversion process might take some time, but later you can load the sharded checkpoint much faster. The model loading time should remain constant regardless of the size of tensor parallelism.
+If the model is too large to fit in a single GPU, you will get an out-of-memory (OOM) error. Consider adopting [these options](#reducing-memory-usage) to reduce the memory consumption.
 
 ## Generation quality changed
 

docs/source/serving/offline_inference.md

@@ -59,6 +59,8 @@ model = LLM(
 
 Our [list of supported models](#supported-models) shows the model architectures that are recognized by vLLM.
 
+(reducing-memory-usage)=
+
 ### Reducing memory usage
 
 Large models might cause your machine to run out of memory (OOM). Here are some options that help alleviate this problem.
@@ -81,6 +83,12 @@ before initializing vLLM. Otherwise, you may run into an error like `RuntimeErro
 To control which devices are used, please instead set the `CUDA_VISIBLE_DEVICES` environment variable.
 :::
 
+:::{note}
+With tensor parallelism enabled, each process will read the whole model and split it into chunks, which makes the disk reading time even longer (proportional to the size of tensor parallelism).
+
+You can convert the model checkpoint to a sharded checkpoint using <gh-file:examples/offline_inference/save_sharded_state.py>. The conversion process might take some time, but later you can load the sharded checkpoint much faster. The model loading time should remain constant regardless of the size of tensor parallelism.
+:::
+
 #### Quantization
 
 Quantized models take less memory at the cost of lower precision.
@@ -103,23 +111,65 @@ llm = LLM(model="adept/fuyu-8b",
           max_num_seqs=2)
 ```
 
+#### Reduce CUDA Graphs
+
+By default, we optimize model inference using CUDA graphs which take up extra memory in the GPU.
+
+:::{important}
+CUDA graph capture takes up more memory in V1 than in V0.
+:::
+
+You can adjust `compilation_config` to achieve a better balance between inference speed and memory usage:
+
+```python
+from vllm import LLM
+from vllm.config import CompilationConfig, CompilationLevel
+
+llm = LLM(
+    model="meta-llama/Llama-3.1-8B-Instruct",
+    compilation_config=CompilationConfig(
+        level=CompilationLevel.PIECEWISE,
+        # By default, it goes up to max_num_seqs
+        cudagraph_capture_sizes=[1, 2, 4, 8, 16],
+    ),
+)
+```
+
+You can disable graph capturing completely via the `enforce_eager` flag:
+
+```python
+from vllm import LLM
+
+llm = LLM(model="meta-llama/Llama-3.1-8B-Instruct",
+          enforce_eager=True)
+```
+
 #### Adjust cache size
 
 If you run out of CPU RAM, try the following options:
 
 - (Multi-modal models only) you can set the size of multi-modal input cache using `VLLM_MM_INPUT_CACHE_GIB` environment variable (default 4 GiB).
 - (CPU backend only) you can set the size of KV cache using `VLLM_CPU_KVCACHE_SPACE` environment variable (default 4 GiB).
 
-#### Disable unused modalities
+#### Multi-modal input limits
 
-You can disable unused modalities (except for text) by setting its limit to zero.
+You can allow a smaller number of multi-modal items per prompt to reduce the memory footprint of the model:
+
+```python
+from vllm import LLM
+
+# Accept up to 3 images and 1 video per prompt
+llm = LLM(model="Qwen/Qwen2.5-VL-3B-Instruct",
+          limit_mm_per_prompt={"image": 3, "video": 1})
+```
 
+You can go a step further and disable unused modalities completely by setting its limit to zero.
 For example, if your application only accepts image input, there is no need to allocate any memory for videos.
 
 ```python
 from vllm import LLM
 
-# Accept images but not videos
+# Accept any number of images but no videos
 llm = LLM(model="Qwen/Qwen2.5-VL-3B-Instruct",
           limit_mm_per_prompt={"video": 0})
 ```