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docs: update disagg communication guide with AWS EFA benchmark results (#7764) 


Signed-off-by: default avatarRobert Wipfel <rwipfel@nvidia.com>
Co-authored-by: default avatarClaude Opus 4.5 <noreply@anthropic.com>
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docs/kubernetes/disagg-communication-guide.md
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......@@ -12,9 +12,9 @@ This guide explains how prefill and decode workers communicate in Dynamo's disag
## Summary
- **NVLink cannot be used between Kubernetes pods** due to process isolation and GPU partitioning
- **RDMA (InfiniBand/RoCE) is required** for production disaggregated deployments
- **RDMA (InfiniBand, RoCE, or AWS EFA) is required** for production disaggregated deployments
- **Without RDMA, expect 200-500x performance degradation** in Time To First Token (TTFT) — observed ~98s TTFT with TCP vs ~200-500ms with RDMA
- **UCX is the communication layer** that NIXL uses to transfer KV cache between workers
- **UCX or libfabric** are the communication layers that NIXL uses to transfer KV cache between workers
---
......@@ -23,7 +23,7 @@ This guide explains how prefill and decode workers communicate in Dynamo's disag
### Communication Stack
<Frame>
<img src="../assets/img/disagg-comm-stack.svg" alt="Disaggregated inference communication stack showing NIXL, UCX, and transport layers" />
<img src="../assets/img/disagg-comm-stack.svg" alt="Disaggregated inference communication stack showing NIXL, UCX/libfabric, and transport layers" />
</Frame>
### Component Responsibilities
......@@ -31,7 +31,7 @@ This guide explains how prefill and decode workers communicate in Dynamo's disag
| Component | Role | Location |
|-----------|------|----------|
| **NIXL** | High-level KV cache transfer API | Dynamo runtime library |
| **UCX** | Low-level communication framework | System library |
| **UCX or libfabric** | Low-level communication framework | System library |
| **Transports** | Physical data movement | Hardware/kernel drivers |
---
......@@ -115,9 +115,9 @@ When prefill and decode workers are on **different nodes**:
</Frame>
**Requirements for optimal cross-node performance:**
- InfiniBand or RoCE network fabric
- RDMA network fabric (InfiniBand, RoCE, or AWS EFA)
- GPUDirect RDMA enabled (GPU memory registered with NIC)
- Proper UCX configuration
- Proper UCX or libfabric configuration
---
......@@ -224,40 +224,114 @@ env:
### AWS EFA Configuration
> **⚠️ Critical: Zero-Copy RDMA causes crashes on AWS Kernel 6.8+**
>
> On AWS Ubuntu 24.04 with Kernel ≥6.8, using `UCX_RNDV_SCHEME=get_zcopy` triggers a fatal `NIXL_ERR_BACKEND` crash. The EFA provider cannot register CUDA memory due to incomplete DMA-BUF support in `efa_nv_peermem`.
>
> **You MUST use the configuration below** — do not copy the standard InfiniBand settings.
NIXL supports **libfabric** as the backend for AWS EFA deployments. This is the **recommended approach** for disaggregated inference on AWS, achieving ~9.6 GB/s KV transfer bandwidth. See the [AWS EFA with NIXL documentation](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/efa-start-nixl.html) for complete setup instructions.
> **Note: NIXL is migrating from UCX to libfabric for AWS**
> The Dynamo team is transitioning NIXL to use **libfabric** instead of UCX for AWS EFA deployments. This change is driven by:
> - **Better topology awareness**: libfabric provides hierarchical topology awareness similar to NCCL
> - **Native EFA support**: libfabric is the recommended communication layer for AWS EFA
>
> **Current status**: UCX over EFA works but is not recommended for production. Published AWS examples are functional but not performant. Check with the Dynamo team for libfabric availability timeline.
**Requirements:**
- EFA installer version **1.47.0** or later
- Libfabric (installed via EFA installer at `/opt/amazon/efa`)
- GDRCopy for GPU Direct RDMA operations (GPU Operator v26.x installs this automatically)
- EFA-enabled container image (e.g., `nvcr.io/nvidia/ai-dynamo/vllm-runtime:1.0.1-efa-amd64`)
**Required AWS EFA Configuration** (Ubuntu 24.04 + Kernel ≥6.8):
**Kernel Compatibility:**
GDRCopy v2.5.1 has a build failure on kernel 6.15+ due to a `vm_flags_set` redefinition. Pin your Ubuntu EKS AMI to kernel 6.14 or earlier until GDRCopy v2.5.2 is available in GPU Operator.
| Kernel Version | GDRCopy v2.5.1 | GDRCopy v2.5.2 |
|----------------|----------------|----------------|
| 6.14 and below | ✅ Works | ✅ Works |
| 6.15+ | ❌ Build fails | ✅ Works |
**Pod Anti-Affinity (Required):**
EFA is designed for **cross-node** communication. Prefill and decode workers must be scheduled on **different nodes** to avoid EAGAIN errors during KV transfer.
```yaml
VllmDecodeWorker:
extraPodSpec:
affinity:
podAntiAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
- labelSelector:
matchExpressions:
- key: nvidia.com/dynamo-component
operator: In
values:
- VllmPrefillWorker
topologyKey: kubernetes.io/hostname
```
> **Note**: Anti-affinity only needs to be configured on one side (here, the decode worker). The Kubernetes scheduler enforces the constraint symmetrically—if decode cannot be placed with prefill, they will end up on different nodes regardless of which pod has the rule.
**EFA Resource Requests:**
Request EFA interfaces in your pod spec. The p5.48xlarge instance has **32 EFA interfaces** (32 network cards × 1 interface each) with 3200 Gbps total bandwidth. The number of interfaces to allocate per worker depends on your deployment:
| Deployment | EFA per Worker | Rationale |
|------------|----------------|-----------|
| 1P + 1D per node pair | 4 | Achieved ~9.6 GB/s; leaves 24 interfaces for other pods |
| Multi-worker per node | 2-4 | Balance between workers sharing the node |
| Maximum bandwidth | 8-16 | For very large KV cache transfers or TP>1 |
Example with 4 EFA interfaces (validated configuration):
```yaml
extraPodSpec:
mainContainer:
securityContext:
capabilities:
add: ["IPC_LOCK"]
resources:
limits:
vpc.amazonaws.com/efa: "4"
requests:
vpc.amazonaws.com/efa: "4"
```
> **Note**: NIXL/libfabric automatically stripes traffic across all allocated EFA interfaces. The 4-interface configuration achieved ~9.6 GB/s in testing, which is sufficient for Llama-3.1-8B KV cache transfers at ISL=8000. Increase the count if your workload requires higher bandwidth (e.g., larger models or higher TP).
**Environment Variables:**
```yaml
env:
- name: UCX_TLS
value: "srd,cuda_copy,tcp" # SRD is EFA's RDMA transport
- name: UCX_RNDV_SCHEME
value: "auto" # DO NOT use get_zcopy - causes crashes
- name: UCX_RNDV_THRESH
value: "8192" # Avoid CUDA zero-copy for large transfers
- name: NIXL_LOG_LEVEL
value: "INFO"
- name: LD_LIBRARY_PATH
value: "/usr/local/nixl/lib/x86_64-linux-gnu:/opt/amazon/efa/lib64:$(LD_LIBRARY_PATH)"
```
**vLLM Configuration:**
```bash
vllm serve <your-model> \
--kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both","kv_buffer_device":"cuda","kv_connector_extra_config":{"backends":["LIBFABRIC"]}}'
```
| Parameter | Value | Purpose |
|-----------|-------|---------|
| `kv_connector` | `NixlConnector` | Enables NIXL for KV-cache transfer |
| `kv_role` | `kv_both` | Symmetric functionality (producer and consumer) |
| `kv_buffer_device` | `cuda` | Uses GPU memory for KV-cache buffer |
| `backends` | `["LIBFABRIC"]` | Routes NIXL traffic over EFA |
**Verification:**
```bash
# Confirm EFA/libfabric installation
fi_info -p efa -t FI_EP_RDM
# Verify GDRCopy device
ls -la /dev/gdrdrv
# Check NIXL initialization in pod logs (should show 32 EFA devices on p5.48xlarge)
kubectl logs <worker-pod> | grep -i "NIXL\|libfabric\|efa"
```
**Why these settings are mandatory**:
- `UCX_RNDV_SCHEME=auto` prevents UCX from forcing zero-copy RDMA on CUDA buffers
- `UCX_RNDV_THRESH=8192` ensures large KV cache transfers use host-staging instead of GPU-direct (which fails)
- Using `get_zcopy` or threshold `0` will cause `remote invalid RD request` errors and worker crashes
**Expected Log Output:**
**Known Limitations**:
- GPU Direct RDMA is non-functional on AWS EFA with Ubuntu 24.04 + kernel ≥6.8
- Expect 3x performance degradation compared to InfiniBand (host-staged transfers)
- For optimal disaggregated performance, consider clusters with InfiniBand/RoCE, or wait for libfabric support on AWS
```text
NIXL INFO Loaded backend plugin: LIBFABRIC
NIXL INFO Found 32 fabric devices
```
---
......@@ -296,17 +370,22 @@ spec:
### Infrastructure Prerequisites
1. **RDMA Device Plugin**: Exposes `rdma/ib` resources to Kubernetes
1. **RDMA Device Plugin**: Exposes `rdma/ib` or `vpc.amazonaws.com/efa` resources to Kubernetes
```bash
# InfiniBand/RoCE
kubectl get nodes -o jsonpath='{.items[*].status.allocatable.rdma/ib}'
# AWS EFA
kubectl get nodes -o jsonpath='{.items[*].status.allocatable.vpc\.amazonaws\.com/efa}'
```
2. **InfiniBand/RoCE Network**: Physical RDMA fabric connecting nodes
2. **RDMA Network**: One of:
- InfiniBand or RoCE fabric
- AWS EFA (Elastic Fabric Adapter)
3. **GPUDirect RDMA** (optional but recommended):
- NVIDIA driver with GPUDirect enabled
- `nvidia-peermem` kernel module loaded
- NIC firmware supporting GPUDirect
- `nvidia-peermem` kernel module loaded (InfiniBand/RoCE)
- GDRCopy installed (AWS EFA with libfabric)
---
......@@ -426,46 +505,62 @@ kubectl exec <prefill-pod> -- ping -c 3 <decode-pod-ip>
### KV Cache Transfer Overhead
| Configuration | TTFT Overhead | Source |
|---------------|---------------|--------|
| Aggregated (baseline) | 0 | No KV transfer needed |
| Disagg + InfiniBand RDMA with GPUDirect | +200-500ms | *Expected* based on hardware specs |
| Disagg + RoCE RDMA with GPUDirect | +300-800ms | *Expected* based on hardware specs |
| Disagg + Host-staged (no GPUDirect) | +1-3s | *Expected* - CPU bottleneck |
| Disagg + AWS EFA (without GPUDirect) | ~3x slower than aggregated | *Measured* on AWS p5.48xlarge |
| Disagg + TCP fallback | **+90-100s** | *Measured* ~98s TTFT on AWS p5.48xlarge |
| Configuration | TTFT Overhead (avg) | KV Transfer BW | Source |
|---------------|---------------------|----------------|--------|
| Aggregated (baseline) | 0 | N/A | No KV transfer needed |
| Disagg + InfiniBand RDMA with GPUDirect | +200-500ms | 20-50 GB/s | *Expected* based on hardware specs |
| Disagg + RoCE RDMA with GPUDirect | +300-800ms | 10-25 GB/s | *Expected* based on hardware specs |
| Disagg + AWS EFA with libfabric + GDRCopy | **+37ms** | **~9.6 GB/s** | *Measured* on AWS p5.48xlarge (Llama-3.1-8B, ISL=8000, OSL=50) |
| Disagg + Host-staged (no GPUDirect) | +1-3s | 1-3 GB/s | *Expected* - CPU bottleneck |
| Disagg + AWS EFA with UCX (without GPUDirect) | ~3x slower than aggregated | ~1 GB/s | *Measured* on AWS p5.48xlarge |
| Disagg + TCP fallback | **+90-100s** | ~100 MB/s | *Measured* ~98s TTFT on AWS p5.48xlarge |
> **Note**: InfiniBand/RoCE numbers with GPUDirect are expected values based on hardware specifications and have not been validated. AWS measurements reflect EFA without functional GPUDirect RDMA (see [AWS EFA Configuration](#aws-efa-configuration) for details).
> **Note**: For AWS EFA deployments, use libfabric with GDRCopy to enable GPUDirect RDMA. UCX on AWS EFA does not support GPUDirect on kernel ≥6.8 and results in severely degraded performance. See [AWS EFA Configuration](#aws-efa-configuration) for setup instructions.
### When Disaggregated Makes Sense
**Use disaggregated architecture when:**
- Output sequence length (OSL) > 1000 tokens (overhead amortized)
- Input sequence length (ISL) ≥ 4000 tokens (14-22% throughput gain)
- You need independent scaling of prefill vs decode capacity
- Prefill and decode have different hardware requirements
**Use aggregated architecture when:**
- Low-latency TTFT is critical
- Short outputs (OSL under 500 tokens)
- Input sequences under 2000 tokens (minimal disagg benefit)
- RDMA is not available
### Break-Even Analysis
The KV transfer overhead is amortized across output tokens. Example data from **Llama-3.1-8B-Instruct** on AWS p5.48xlarge:
The KV transfer overhead is amortized across output tokens. **Measured data from Llama-3.1-8B-Instruct** on AWS p5.48xlarge with NIXL+libfabric:
```text
Total Latency = TTFT + (OSL × ITL)
KV Transfer Overhead (TTFT min, unqueued):
- Aggregated: ~173ms
- Disaggregated: ~210ms
- KV transfer cost: ~37ms
Performance at ISL=8000, OSL=50, concurrency=10:
- ITL improvement: 41% faster per-token generation
- Throughput gain: 22% higher output throughput
```
Example (Llama-3.1-8B, ISL=4000):
- Aggregated: 218ms + (OSL × 8.0ms)
- Disaggregated: 2400ms + (OSL × 7.8ms)
**Key Insight**: The KV transfer overhead via libfabric+EFA is only **~37ms**. Combined with 41% faster decode (ITL), disaggregated inference delivers **22% higher throughput** for prefill-bound workloads.
Break-even: 2400 - 218 = 2182ms overhead
2182ms / (8.0 - 7.8)ms per token = 10,910 tokens
| Metric | Aggregated | Disaggregated | Difference |
|--------|------------|---------------|------------|
| TTFT (min, unqueued) | 173 ms | 210 ms | +37ms |
| TTFT (p95) | 2097 ms | 1752 ms | **-16%** |
| ITL (avg) | 28.5 ms | 16.9 ms | **-41%** |
| Output throughput (ISL=8000, OSL=50) | 204 tok/s | 248 tok/s | **+22%** |
At OSL=2000: Disagg is 1.1x slower (acceptable)
At OSL=100: Disagg is 3.1x slower (not recommended)
```
**Disagg advantage scales with input length (ISL)** (all at OSL=50, concurrency=10):
| ISL | Throughput Δ | ITL Δ | Recommendation |
|-----|--------------|-------|----------------|
| 1000 | ~0% | -7% | Use aggregated |
| 2000 | +3% | -11% | Either works |
| 4000 | +14% | -18% | Disagg preferred |
| 8000 | **+22%** | **-41%** | **Disagg strongly preferred** |
---
......@@ -503,12 +598,53 @@ kubectl logs <worker-pod> | grep -i "transport\|UCX\|TCP"
**Symptoms**: 3x performance degradation on AWS despite EFA configured
**Root Cause**: GPU Direct RDMA not functional on kernel ≥6.8 with EFA
**Root Cause**: GPU Direct RDMA not functional on kernel ≥6.8 with EFA when using UCX
**Solution**: Use libfabric instead of UCX for AWS EFA deployments. Libfabric with GDRCopy provides efficient GPU Direct RDMA operations on AWS. See the [AWS EFA Configuration](#aws-efa-configuration) section for setup instructions.
**Current Status**: This is a known limitation. Options:
**Alternative options** (if libfabric is not available):
1. Use kernel before 6.8 (Ubuntu 22.04 with kernel 5.15)
2. Accept host-staging performance penalty
3. Wait for AWS to update EFA DMA-BUF support
### Problem: EFA EAGAIN errors (fi_read still retrying)
**Symptoms**: Decode worker logs show repeated EAGAIN errors:
```text
fi_read still retrying EAGAIN on rail 0
fi_read still retrying EAGAIN on rail 1
...
```
**Root Cause**: Prefill and decode workers are scheduled on the **same node**. AWS EFA is designed for cross-node communication and does not function correctly for intra-node transfers.
**Diagnosis**:
```bash
# Check if workers are on the same node
kubectl get pods -o wide | grep vllm
```
If both prefill and decode workers show the same NODE, this is the problem.
**Solution**: Add pod anti-affinity rules to ensure workers are scheduled on different nodes:
```yaml
VllmDecodeWorker:
extraPodSpec:
affinity:
podAntiAffinity:
requiredDuringSchedulingIgnoredDuringExecution:
- labelSelector:
matchExpressions:
- key: nvidia.com/dynamo-component
operator: In
values:
- VllmPrefillWorker
topologyKey: kubernetes.io/hostname
```
> **Note**: Use `nvidia.com/dynamo-component` as the label key, not `app.kubernetes.io/component`. The Dynamo operator uses this label to identify component types.
### Problem: Intermittent transfer failures
......@@ -555,10 +691,16 @@ resources:
### Diagnostic Checklist
- [ ] `rdma/ib` resources visible: `kubectl get nodes -o jsonpath='{..allocatable.rdma/ib}'`
- [ ] NIXL initialized: `kubectl logs <pod> | grep "Backend"`
- [ ] Transfer bandwidth > 1 GB/s (check Grafana metrics)
**For UCX deployments:**
- [ ] UCX sees RDMA devices: `ucx_info -d | grep "Transport: rc"`
- [ ] UCX sees GPU memory: `ucx_info -d | grep "memory types.*cuda"`
- [ ] NIXL initialized with UCX: `kubectl logs <pod> | grep "Backend UCX"`
- [ ] Transfer bandwidth > 1 GB/s (check Grafana metrics)
**For libfabric deployments (AWS EFA):**
- [ ] EFA devices available: `fi_info -p efa`
- [ ] GDRCopy installed: `ls /dev/gdrdrv`
---
......
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