RTX4090 GPUDirect P2P Unlocked!

Fri, 07 Feb 2025 11:18:00 +0800

If you want see the test results directly, please jump to the Test the unlocked performance section.

What is P2P?

From NVIDIA: GPUDirect Peer to Peer enables GPU-to-GPU copies as well as loads and stores directly over the memory fabric (PCIe, NVLink). GPUDirect Peer to Peer is supported natively by the CUDA Driver.

As you can see, GPUDirect P2P allows for direct memory access between GPUs, bypassing the CPU and system memory. This can lead to significant performance improvements in multi-GPU systems. For example, in a multi-GPU LLM inference system, the data can be directly transferred between GPUs without involving the CPU, reducing latency and improving throughput.

Why is P2P disabled on RTX4090?

Easy, NVIDIA wants to make money. They want you to buy the more expensive Tesla GPUs if you want P2P.

Test machine configuration

CPU: 2 x AMD EPYC 7542 (32C 64T, 225W TDP, 2.9GHz base, 3.4GHz boost)
MEM: 2 x 8 x 32GB 3200MT/s DDR4 ECC
GPU: 8 x NVIDIA GeForce RTX 4090 (24GB GDDR6X, 450W TDP)

Topology (ideally, the GPUs should be behind a PCIe switch, but our test machine doesn’t have one):

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 GPU0 GPU1 GPU2 GPU3 GPU4 GPU5 GPU6 GPU7 NIC0 NIC1 NIC2 NIC3 CPU Affinity NUMA Affinity GPU NUMA ID
GPU0 X SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS 24-31,88-95 3 N/A
GPU1 SYS X SYS SYS SYS SYS SYS SYS SYS SYS SYS SYS 16-23,80-87 2 N/A
GPU2 SYS SYS X SYS SYS SYS SYS SYS SYS SYS SYS SYS 8-15,72-79 1 N/A
GPU3 SYS SYS SYS X SYS SYS SYS SYS SYS SYS SYS SYS 0-7,64-71 0 N/A
GPU4 SYS SYS SYS SYS X SYS SYS SYS SYS SYS SYS SYS 56-63,120-127 7 N/A
GPU5 SYS SYS SYS SYS SYS X SYS SYS SYS SYS SYS SYS 48-55,112-119 6 N/A
GPU6 SYS SYS SYS SYS SYS SYS X SYS PHB PHB PHB PHB 40-47,104-111 5 N/A
GPU7 SYS SYS SYS SYS SYS SYS SYS X SYS SYS SYS SYS 32-39,96-103 4 N/A
NIC0 SYS SYS SYS SYS SYS SYS PHB SYS X PIX PHB PHB
NIC1 SYS SYS SYS SYS SYS SYS PHB SYS PIX X PHB PHB
NIC2 SYS SYS SYS SYS SYS SYS PHB SYS PHB PHB X PIX
NIC3 SYS SYS SYS SYS SYS SYS PHB SYS PHB PHB PIX X

Legend:

 X = Self
 SYS = Connection traversing PCIe as well as the SMP interconnect between NUMA nodes (e.g., QPI/UPI)
 NODE = Connection traversing PCIe as well as the interconnect between PCIe Host Bridges within a NUMA node
 PHB = Connection traversing PCIe as well as a PCIe Host Bridge (typically the CPU)
 PXB = Connection traversing multiple PCIe bridges (without traversing the PCIe Host Bridge)
 PIX = Connection traversing at most a single PCIe bridge
 NV# = Connection traversing a bonded set of # NVLinks

NIC Legend:

 NIC0: mlx5_0
 NIC1: mlx5_1
 NIC2: mlx5_2
 NIC3: mlx5_3

Test the original performance

PCI BAR

Only 256MB of BAR memory is available. Note that I did enabled Resizable BAR in the BIOS (not my fault).

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# nvidia-smi -q | grep -i bar -A 3
 BAR1 Memory Usage
 Total : 256 MiB
 Used : 1 MiB
 Free : 255 MiB

NCCL Tests

Building NCCL Tests

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git clone --depth=1 https://github.com/NVIDIA/nccl-tests.git
cd nccl-tests
make -j$(nproc)

Runnning:

Around 14.5GB/s of bandwidth.

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# ./build/all_reduce_perf -b 8 -e 128M -f 2 -g 8
# nThread 1 nGpus 8 minBytes 8 maxBytes 134217728 step: 2(factor) warmup iters: 5 iters: 20 agg iters: 1 validation: 1 graph: 0
#
# Using devices
# Rank 0 Group 0 Pid 1756513 on <REDACTED> device 0 [0x01] NVIDIA GeForce RTX 4090
# Rank 1 Group 0 Pid 1756513 on <REDACTED> device 1 [0x25] NVIDIA GeForce RTX 4090
# Rank 2 Group 0 Pid 1756513 on <REDACTED> device 2 [0x41] NVIDIA GeForce RTX 4090
# Rank 3 Group 0 Pid 1756513 on <REDACTED> device 3 [0x61] NVIDIA GeForce RTX 4090
# Rank 4 Group 0 Pid 1756513 on <REDACTED> device 4 [0x81] NVIDIA GeForce RTX 4090
# Rank 5 Group 0 Pid 1756513 on <REDACTED> device 5 [0xa1] NVIDIA GeForce RTX 4090
# Rank 6 Group 0 Pid 1756513 on <REDACTED> device 6 [0xc1] NVIDIA GeForce RTX 4090
# Rank 7 Group 0 Pid 1756513 on <REDACTED> device 7 [0xe1] NVIDIA GeForce RTX 4090
#
# out-of-place in-place 
# size count type redop root time algbw busbw #wrong time algbw busbw #wrong
# (B) (elements) (us) (GB/s) (GB/s) (us) (GB/s) (GB/s)
 8 2 float sum -1 38.83 0.00 0.00 0 40.43 0.00 0.00 0
 16 4 float sum -1 38.73 0.00 0.00 0 38.59 0.00 0.00 0
 32 8 float sum -1 41.85 0.00 0.00 0 42.55 0.00 0.00 0
 64 16 float sum -1 42.55 0.00 0.00 0 42.65 0.00 0.00 0
 128 32 float sum -1 42.42 0.00 0.01 0 42.88 0.00 0.01 0
 256 64 float sum -1 42.44 0.01 0.01 0 42.53 0.01 0.01 0
 512 128 float sum -1 42.58 0.01 0.02 0 42.68 0.01 0.02 0
 1024 256 float sum -1 44.04 0.02 0.04 0 42.39 0.02 0.04 0
 2048 512 float sum -1 42.91 0.05 0.08 0 42.61 0.05 0.08 0
 4096 1024 float sum -1 43.18 0.09 0.17 0 42.69 0.10 0.17 0
 8192 2048 float sum -1 44.02 0.19 0.33 0 42.96 0.19 0.33 0
 16384 4096 float sum -1 43.10 0.38 0.67 0 42.99 0.38 0.67 0
 32768 8192 float sum -1 44.18 0.74 1.30 0 43.63 0.75 1.31 0
 65536 16384 float sum -1 45.66 1.44 2.51 0 45.28 1.45 2.53 0
 131072 32768 float sum -1 67.27 1.95 3.41 0 54.86 2.39 4.18 0
 262144 65536 float sum -1 124.8 2.10 3.68 0 121.1 2.17 3.79 0
 524288 131072 float sum -1 196.9 2.66 4.66 0 200.5 2.61 4.58 0
 1048576 262144 float sum -1 222.8 4.71 8.24 0 226.1 4.64 8.12 0
 2097152 524288 float sum -1 373.4 5.62 9.83 0 358.6 5.85 10.23 0
 4194304 1048576 float sum -1 580.8 7.22 12.64 0 592.0 7.09 12.40 0
 8388608 2097152 float sum -1 1041.8 8.05 14.09 0 1040.2 8.06 14.11 0
 16777216 4194304 float sum -1 1972.8 8.50 14.88 0 1979.5 8.48 14.83 0
 33554432 8388608 float sum -1 3905.6 8.59 15.04 0 3919.0 8.56 14.98 0
 67108864 16777216 float sum -1 7907.1 8.49 14.85 0 7905.3 8.49 14.86 0
 134217728 33554432 float sum -1 16253 8.26 14.45 0 16227 8.27 14.47 0
# Out of bounds values : 0 OK
# Avg bus bandwidth : 4.85265
#

P2P Bandwidth and Latency

Building:

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git clone --depth=1 https://github.com/NVIDIA/cuda-samples.git
cd cuda-samples/Samples/5_Domain_Specific/p2pBandwidthLatencyTest
make -j$(nproc)

Running:

About 21GB/s bidirectional bandwidth
10us+ GPU-GPU latency

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# ./p2pBandwidthLatencyTest
P2P Connectivity Matrix
 D\D 0 1 2 3 4 5 6 7
 0 1 0 0 0 0 0 0 0
 1 0 1 0 0 0 0 0 0
 2 0 0 1 0 0 0 0 0
 3 0 0 0 1 0 0 0 0
 4 0 0 0 0 1 0 0 0
 5 0 0 0 0 0 1 0 0
 6 0 0 0 0 0 0 1 0
 7 0 0 0 0 0 0 0 1
Unidirectional P2P=Disabled Bandwidth Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 909.49 18.77 19.56 20.23 18.72 19.29 19.34 19.83
 1 19.95 919.12 18.93 20.18 18.83 19.23 19.24 19.80
 2 19.87 19.55 919.66 20.27 18.82 19.28 19.29 19.77
 3 19.92 19.48 19.56 918.58 18.82 19.23 19.19 19.80
 4 19.88 20.58 19.74 20.54 920.15 18.38 18.89 18.71
 5 20.04 20.19 19.80 20.37 18.75 921.29 18.88 19.08
 6 20.04 20.11 19.71 20.60 18.77 18.58 920.74 18.62
 7 20.02 20.21 19.73 20.33 18.37 18.74 18.89 921.29
Unidirectional P2P=Enabled Bandwidth (P2P Writes) Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 910.02 18.83 19.55 20.27 18.76 19.36 19.26 19.83
 1 19.95 920.20 18.92 20.19 18.83 19.26 19.21 19.80
 2 19.82 19.61 919.66 20.28 18.86 19.29 19.34 19.75
 3 19.91 19.47 19.60 920.20 18.83 19.26 19.16 19.81
 4 19.78 20.43 19.75 20.56 920.25 18.60 18.85 18.75
 5 20.02 20.24 19.76 20.45 18.80 922.37 18.89 19.15
 6 20.06 20.12 19.75 20.59 18.80 18.58 921.25 18.61
 7 20.02 20.16 19.80 20.38 18.41 18.72 18.87 920.83
Bidirectional P2P=Disabled Bandwidth Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 916.15 21.03 21.53 21.54 20.69 21.02 21.10 21.06
 1 21.39 920.74 21.59 21.52 20.55 21.02 20.50 21.18
 2 21.15 21.48 921.83 21.20 20.58 21.11 20.98 21.09
 3 21.38 21.67 21.42 922.65 20.53 21.05 20.65 21.21
 4 20.86 20.89 20.86 20.95 923.69 19.99 20.11 19.97
 5 20.90 20.97 21.00 21.02 20.22 922.10 20.15 20.30
 6 20.94 20.62 20.89 20.61 19.98 20.07 923.11 19.99
 7 21.07 21.13 21.10 21.22 20.00 20.29 20.11 923.46
Bidirectional P2P=Enabled Bandwidth Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 917.23 20.86 21.49 21.56 20.63 20.99 21.05 21.13
 1 21.30 922.37 21.61 21.56 20.53 21.08 20.49 21.15
 2 21.15 21.47 922.36 21.16 20.50 21.06 20.94 21.10
 3 21.39 21.60 21.39 922.92 20.63 21.09 20.69 21.22
 4 20.89 20.86 20.81 20.89 923.46 20.12 20.05 20.02
 5 20.88 20.98 20.99 20.87 20.20 923.65 20.12 20.39
 6 21.05 20.63 20.89 20.67 20.07 20.05 922.92 20.02
 7 21.01 21.12 21.06 21.19 20.01 20.31 20.13 923.16
P2P=Disabled Latency Matrix (us)
 GPU 0 1 2 3 4 5 6 7
 0 1.54 20.26 11.68 11.60 12.65 15.80 12.19 11.75
 1 11.60 1.48 11.58 11.66 16.03 12.13 12.55 14.59
 2 20.27 11.59 1.43 20.08 13.17 17.14 16.31 11.41
 3 11.50 11.59 11.51 1.47 13.11 16.14 14.04 11.61
 4 12.53 14.27 12.75 12.65 1.51 12.47 12.49 12.41
 5 11.88 11.73 11.65 11.63 12.28 1.42 14.95 15.40
 6 12.27 12.63 12.42 12.68 12.25 12.30 1.47 11.89
 7 11.46 16.67 11.44 11.47 12.30 13.33 13.57 1.41

 CPU 0 1 2 3 4 5 6 7
 0 4.04 12.23 10.01 9.91 10.88 10.78 10.60 11.29
 1 9.79 3.15 9.85 10.06 10.77 10.56 10.50 10.48
 2 10.04 9.63 3.24 9.94 10.86 10.65 10.70 10.63
 3 9.75 9.61 9.85 3.26 10.92 10.82 10.77 10.67
 4 10.50 10.26 10.61 10.56 3.57 11.55 11.48 11.26
 5 10.52 10.07 10.40 10.48 11.42 3.52 11.32 11.24
 6 10.24 10.20 10.44 10.47 11.42 11.29 3.54 11.35
 7 10.26 10.09 10.50 10.41 11.47 11.31 11.36 3.53
P2P=Enabled Latency (P2P Writes) Matrix (us)
 GPU 0 1 2 3 4 5 6 7
 0 1.54 11.67 11.75 20.27 14.49 15.77 16.95 15.91
 1 11.67 1.48 11.44 11.59 15.22 12.17 13.20 13.18
 2 11.58 20.27 1.43 11.50 14.60 13.73 13.21 11.38
 3 11.58 20.06 12.12 1.47 13.16 16.17 14.67 11.67
 4 12.88 12.78 13.92 12.58 1.50 12.46 12.59 12.50
 5 11.48 11.93 11.78 11.88 19.18 1.42 11.59 11.40
 6 11.95 11.70 14.90 14.00 12.42 11.41 1.46 11.63
 7 12.96 11.45 11.60 11.53 20.55 11.32 12.94 1.40

 CPU 0 1 2 3 4 5 6 7
 0 3.21 9.61 9.86 9.79 10.69 10.48 10.55 10.44
 1 9.67 3.12 9.81 9.81 10.64 10.46 10.66 10.43
 2 9.89 9.62 3.20 9.98 10.96 10.67 10.75 10.72
 3 9.80 9.65 9.89 3.20 10.86 10.78 10.78 10.61
 4 10.41 10.24 10.60 10.50 3.52 11.34 11.49 11.28
 5 10.27 10.14 11.93 14.06 14.53 4.55 16.99 19.67
 6 20.68 20.61 20.39 20.63 20.86 14.35 5.21 14.46
 7 9.89 9.66 9.85 9.85 10.81 10.62 10.68 3.41

How to unlock P2P on RTX4090?

I encountered this legendary modified NVIDIA driver by tinygrad that unlocks P2P on RTX4090 GPUs which blows my mind. Let’s give it a try!

Disable IOMMU

If the following command returns an empty list, it means that the IOMMU is disabled, otherwise, you need to disable it. Consult your motherboard manual on how to disable IOMMU.

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ll /sys/class/iommu/

Enable Resizable BAR

Enable Resizable BAR in the BIOS. It’s required for the modified NVIDIA driver to work. Consult your motherboard manual on how to enable Resizable BAR.

Uninstall the official NVIDIA driver

We will install our own modified NVIDIA driver, so we need to uninstall the official NVIDIA driver first.

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nvidia-uninstall
reboot

Install the official NVIDIA driver without kernel modules

Visit the modified NVIDIA driver by tinygrad. Check out the branches with p2p in the name. There are multiple versions. You should choose one that fits. For example, I am using the 550.90.07-p2p branch. Remember the version number 550.90.07, you will need it later.

Download the official driver with the same version number as you chose (550.90.07 in my case) from the NVIDIA website. The driver I downloaded is NVIDIA-Linux-x86_64-550.90.07.run.

Install the official driver without the kernel modules because we will install our own modified kernel modules later.

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./NVIDIA-Linux-x86_64-550.90.07.run --no-kernel-modules

Install the modified kernel modules

Clone the modified NVIDIA driver by tinygrad and install. Remember to replace the branch name with the one you chose.

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git clone --depth=1 -b 550.90.07-p2p https://github.com/tinygrad/open-gpu-kernel-modules.git
cd open-gpu-kernel-modules
# Just in case
rmmod nvidia_drm nvidia_modeset nvidia_uvm nvidia
# Build and install the modified kernel modules
make modules -j$(nproc)
make modules_install
depmod
nvidia-smi -pm 1

Test the unlocked performance

PCI BAR

Now the BAR memory is fully available.

256MB -> 32GB.

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# nvidia-smi -q | grep -i bar -A 3
 BAR1 Memory Usage
 Total : 32768 MiB
 Used : 24211 MiB
 Free : 8557 MiB

NCCL Tests

14.47GB/s -> 20.64GB/s: 42% improvement

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# nThread 1 nGpus 8 minBytes 8 maxBytes 134217728 step: 2(factor) warmup iters: 5 iters: 20 agg iters: 1 validation: 1 graph: 0
#
# Using devices
# Rank 0 Group 0 Pid 33895 on <REDACTED> device 0 [0x01] NVIDIA GeForce RTX 4090
# Rank 1 Group 0 Pid 33895 on <REDACTED> device 1 [0x25] NVIDIA GeForce RTX 4090
# Rank 2 Group 0 Pid 33895 on <REDACTED> device 2 [0x41] NVIDIA GeForce RTX 4090
# Rank 3 Group 0 Pid 33895 on <REDACTED> device 3 [0x61] NVIDIA GeForce RTX 4090
# Rank 4 Group 0 Pid 33895 on <REDACTED> device 4 [0x81] NVIDIA GeForce RTX 4090
# Rank 5 Group 0 Pid 33895 on <REDACTED> device 5 [0xa1] NVIDIA GeForce RTX 4090
# Rank 6 Group 0 Pid 33895 on <REDACTED> device 6 [0xc1] NVIDIA GeForce RTX 4090
# Rank 7 Group 0 Pid 33895 on <REDACTED> device 7 [0xe1] NVIDIA GeForce RTX 4090
#
# out-of-place in-place
# size count type redop root time algbw busbw #wrong time algbw busbw #wrong
# (B) (elements) (us) (GB/s) (GB/s) (us) (GB/s) (GB/s)
 8 2 float sum -1 38.82 0.00 0.00 0 38.16 0.00 0.00 0
 16 4 float sum -1 38.31 0.00 0.00 0 38.12 0.00 0.00 0
 32 8 float sum -1 38.57 0.00 0.00 0 40.87 0.00 0.00 0
 64 16 float sum -1 40.71 0.00 0.00 0 40.65 0.00 0.00 0
 128 32 float sum -1 40.51 0.00 0.01 0 41.66 0.00 0.01 0
 256 64 float sum -1 41.63 0.01 0.01 0 41.38 0.01 0.01 0
 512 128 float sum -1 41.67 0.01 0.02 0 42.45 0.01 0.02 0
 1024 256 float sum -1 41.70 0.02 0.04 0 41.67 0.02 0.04 0
 2048 512 float sum -1 41.74 0.05 0.09 0 41.74 0.05 0.09 0
 4096 1024 float sum -1 42.35 0.10 0.17 0 42.39 0.10 0.17 0
 8192 2048 float sum -1 42.14 0.19 0.34 0 41.88 0.20 0.34 0
 16384 4096 float sum -1 43.19 0.38 0.66 0 42.73 0.38 0.67 0
 32768 8192 float sum -1 42.17 0.78 1.36 0 42.61 0.77 1.35 0
 65536 16384 float sum -1 47.58 1.38 2.41 0 46.95 1.40 2.44 0
 131072 32768 float sum -1 72.03 1.82 3.18 0 71.50 1.83 3.21 0
 262144 65536 float sum -1 119.7 2.19 3.83 0 118.6 2.21 3.87 0
 524288 131072 float sum -1 166.6 3.15 5.51 0 159.8 3.28 5.74 0
 1048576 262144 float sum -1 216.1 4.85 8.49 0 217.2 4.83 8.45 0
 2097152 524288 float sum -1 321.5 6.52 11.42 0 322.6 6.50 11.37 0
 4194304 1048576 float sum -1 478.7 8.76 15.33 0 480.1 8.74 15.29 0
 8388608 2097152 float sum -1 798.6 10.50 18.38 0 818.0 10.26 17.95 0
 16777216 4194304 float sum -1 1472.8 11.39 19.94 0 1472.1 11.40 19.94 0
 33554432 8388608 float sum -1 2870.3 11.69 20.46 0 2866.4 11.71 20.49 0
 67108864 16777216 float sum -1 5705.0 11.76 20.59 0 5695.6 11.78 20.62 0
 134217728 33554432 float sum -1 11382 11.79 20.64 0 11380 11.79 20.64 0
# Out of bounds values : 0 OK
# Avg bus bandwidth : 6.11168
#

P2P Bandwidth and Latency

P2P is now available: see the 1s in the P2P Connectivity Matrix
Bandwidth: 21.39GB/s -> 50.15GB/s: 134% improvement
Latency: 11.67us -> 1.19us: 89% reduction

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P2P Connectivity Matrix
 D\D 0 1 2 3 4 5 6 7
 0 1 1 1 1 1 1 1 1
 1 1 1 1 1 1 1 1 1
 2 1 1 1 1 1 1 1 1
 3 1 1 1 1 1 1 1 1
 4 1 1 1 1 1 1 1 1
 5 1 1 1 1 1 1 1 1
 6 1 1 1 1 1 1 1 1
 7 1 1 1 1 1 1 1 1
Unidirectional P2P=Disabled Bandwidth Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 912.13 18.86 19.89 20.25 18.34 19.64 19.25 19.45
 1 20.56 920.74 19.44 20.27 18.46 19.72 19.19 19.40
 2 20.36 19.72 922.37 20.28 18.52 19.65 19.22 19.40
 3 20.43 19.68 20.04 922.49 18.47 19.59 19.15 19.41
 4 20.20 20.15 20.25 20.26 921.83 19.04 18.84 18.36
 5 20.45 20.09 20.17 20.46 18.29 922.37 18.73 18.60
 6 20.39 20.18 20.24 20.54 18.44 19.09 922.49 18.65
 7 20.53 20.14 20.12 20.51 18.17 19.11 18.74 923.46
Unidirectional P2P=Enabled Bandwidth (P2P Writes) Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 913.74 25.60 25.85 25.85 21.36 22.41 21.74 22.48
 1 25.78 938.91 25.79 25.79 20.70 22.39 22.53 21.63
 2 25.70 25.95 939.00 25.78 21.42 21.59 22.21 22.16
 3 25.60 25.93 25.53 938.42 22.14 22.30 21.70 22.45
 4 22.41 21.61 22.35 22.49 940.70 25.69 25.96 25.90
 5 21.34 22.38 21.72 22.41 25.77 939.57 25.95 25.87
 6 20.57 22.27 22.32 21.60 25.75 26.01 941.73 25.83
 7 21.32 21.52 22.39 22.48 25.51 25.83 25.75 938.44
Bidirectional P2P=Disabled Bandwidth Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 918.85 20.94 21.69 21.67 20.66 21.44 20.98 21.26
 1 21.48 921.29 21.57 21.54 20.20 20.65 20.87 20.70
 2 21.34 21.63 921.56 21.13 20.66 21.28 20.89 21.21
 3 21.40 21.72 21.62 921.83 20.16 20.92 20.89 20.89
 4 20.93 20.33 20.87 20.45 922.90 20.07 20.21 19.96
 5 21.40 20.86 21.26 20.90 20.23 923.69 20.18 20.33
 6 20.93 20.87 20.89 20.90 20.27 20.25 923.67 20.19
 7 21.19 20.66 21.03 20.87 20.02 20.28 20.41 923.41
Bidirectional P2P=Enabled Bandwidth Matrix (GB/s)
 D\D 0 1 2 3 4 5 6 7
 0 916.45 50.10 50.46 50.29 39.90 41.85 40.83 41.68
 1 50.15 919.60 50.19 50.22 38.93 41.79 41.87 40.82
 2 49.91 50.42 920.74 50.53 40.19 40.94 41.86 41.61
 3 50.08 50.48 50.35 919.66 40.22 41.69 40.92 41.78
 4 41.60 40.94 41.86 41.93 919.93 49.80 50.36 50.22
 5 40.25 41.68 39.96 41.64 49.75 921.83 50.20 50.66
 6 39.59 41.58 41.83 40.95 49.89 50.43 921.56 50.59
 7 40.36 40.86 41.76 41.82 49.96 50.43 50.28 921.01
P2P=Disabled Latency Matrix (us)
 GPU 0 1 2 3 4 5 6 7
 0 1.50 11.60 20.17 20.18 13.08 11.27 14.58 19.33
 1 11.68 1.38 11.74 20.17 12.60 11.39 14.35 14.59
 2 11.33 11.34 1.31 11.50 16.96 20.53 16.53 20.53
 3 11.51 11.51 11.51 1.37 17.86 11.37 15.80 20.54
 4 11.47 11.79 11.44 12.57 1.38 12.24 12.93 11.73
 5 11.48 11.79 11.64 11.64 14.40 1.44 13.46 14.35
 6 13.59 15.05 13.35 12.52 12.06 11.67 1.40 12.34
 7 19.32 19.95 11.39 11.44 12.87 12.23 14.05 1.38

 CPU 0 1 2 3 4 5 6 7
 0 3.28 10.21 10.15 9.83 10.92 10.75 10.88 10.71
 1 10.07 3.14 9.81 9.56 10.58 10.41 10.61 10.35
 2 10.13 9.68 3.28 9.62 10.71 10.56 10.73 10.48
 3 9.81 9.46 9.68 3.13 10.60 10.41 10.52 10.35
 4 10.60 10.20 10.40 10.16 3.48 11.19 11.37 11.07
 5 10.37 10.02 10.29 10.04 11.10 3.49 11.24 11.00
 6 10.51 10.20 10.35 10.11 11.34 11.09 3.47 11.16
 7 10.40 10.11 10.30 10.06 11.17 11.07 11.22 3.42
P2P=Enabled Latency (P2P Writes) Matrix (us)
 GPU 0 1 2 3 4 5 6 7
 0 1.48 1.24 1.26 1.25 1.41 1.41 1.36 1.42
 1 1.19 1.38 1.18 1.18 1.29 1.42 1.29 1.26
 2 1.21 1.20 1.31 1.19 1.30 1.34 1.44 1.41
 3 1.21 1.19 1.17 1.38 1.37 1.41 1.30 1.44
 4 1.69 1.69 1.69 1.64 1.38 1.50 1.59 1.56
 5 1.64 1.60 1.60 1.62 1.50 1.43 1.56 1.48
 6 1.59 1.53 1.57 1.54 1.48 1.44 1.39 1.44
 7 1.58 1.58 1.57 1.56 1.48 1.47 1.45 1.37

 CPU 0 1 2 3 4 5 6 7
 0 3.27 2.84 2.76 2.74 2.80 2.76 2.76 2.75
 1 2.80 3.13 2.66 2.74 2.70 2.70 2.68 2.69
 2 2.83 2.80 3.26 2.85 2.85 2.78 2.77 2.76
 3 2.78 2.69 2.70 3.24 2.69 2.70 2.98 2.67
 4 3.18 3.11 3.10 3.09 3.55 3.12 3.09 3.08
 5 3.12 3.04 3.10 3.05 3.06 3.50 3.08 3.04
 6 3.20 3.11 3.18 3.11 3.11 3.16 3.57 3.08
 7 3.12 3.06 3.08 3.12 3.08 3.07 3.04 3.58

How it works? By tinygrad.

I am not an expert in such low-level hacking (but it’s soooo interesting and I want to learn). This part is from the modified NVIDIA driver by tinygrad (Thank you tinygrad). I just put it here for reference.

Normally, P2P on NVIDIA cards uses MAILBOXP2P. This is some hardware interface designed to allow GPUs to transfer memory back in the days of small BAR. It is not present or disabled in hardware on the 4090s, and that’s why P2P doesn’t work. There was a bug in early versions of the driver that reported that it did work, and it was actually sending stuff on the PCIe bus. However, because the mailbox hardware wasn’t present, these copies wouldn’t go to the right place. You could even crash the system by doing something like torch.zeros(10000,10000).cuda().to("cuda:1")

In some 3090s and all 4090s, NVIDIA added large BAR support.

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tiny@tiny14:~$ lspci -s 01:00.0 -v
01:00.0 VGA compatible controller: NVIDIA Corporation AD102 [GeForce RTX 4090] (rev a1) (prog-if 00 [VGA controller])
 Subsystem: Micro-Star International Co., Ltd. [MSI] Device 510b
 Physical Slot: 49
 Flags: bus master, fast devsel, latency 0, IRQ 377
 Memory at b2000000 (32-bit, non-prefetchable) [size=16M]
 Memory at 28800000000 (64-bit, prefetchable) [size=32G]
 Memory at 28400000000 (64-bit, prefetchable) [size=32M]
 I/O ports at 3000 [size=128]
 Expansion ROM at b3000000 [virtual] [disabled] [size=512K]
 Capabilities: <access denied>
 Kernel driver in use: nvidia
 Kernel modules: nvidiafb, nouveau, nvidia_drm, nvidia

Notice how BAR1 is size 32G. In H100, they also added support for a PCIe mode that uses the BAR directly instead of the mailboxes, called BAR1P2P. So, what happens if we try to enable that on a 4090?

We do this by bypassing the HAL and calling a bunch of the GH100 methods directly. Methods like kbusEnableStaticBar1Mapping_GH100, which maps the entire VRAM into BAR1. This mostly just works, but we had to disable the use of that region in the MapAperture function for some reason. Shouldn’t matter.

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[ 3491.654009] NVRM: kbusEnableStaticBar1Mapping_GH100: Static bar1 mapped offset 0x0 size 0x5e9200000
[ 3491.793389] NVRM: kbusEnableStaticBar1Mapping_GH100: Static bar1 mapped offset 0x0 size 0x5e9200000

Perfect, we now have the VRAM mapped. However, it’s not that easy to get P2P. When you run ./simpleP2P from cuda-samples, you get this error.

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[ 3742.840689] NVRM: kbusCreateP2PMappingForBar1P2P_GH100: added PCIe BAR1 P2P mapping between GPU2 and GPU3
[ 3742.840762] NVRM: kbusCreateP2PMappingForBar1P2P_GH100: added PCIe BAR1 P2P mapping between GPU3 and GPU2
[ 3742.841089] NVRM: nvAssertFailed: Assertion failed: (shifted >> pField->shift) == value @ field_desc.h:272
[ 3742.841106] NVRM: nvAssertFailed: Assertion failed: (shifted & pField->maskPos) == shifted @ field_desc.h:273
[ 3742.841281] NVRM: nvAssertFailed: Assertion failed: (shifted >> pField->shift) == value @ field_desc.h:272
[ 3742.841292] NVRM: nvAssertFailed: Assertion failed: (shifted & pField->maskPos) == shifted @ field_desc.h:273
[ 3742.865948] NVRM: GPU at PCI:0000:01:00: GPU-49c7a6c9-e3a8-3b48-f0ba-171520d77dd1
[ 3742.865956] NVRM: Xid (PCI:0000:01:00): 31, pid=21804, name=simpleP2P, Ch 00000013, intr 00000000. MMU Fault: ENGINE CE3 HUBCLIENT_CE1 faulted @ 0x7f97_94000000. Fault is of type FAULT_INFO_TYPE_UNSUPPORTED_KIND ACCESS_TYPE_VIRT_WRITE

Failing with an MMU fault. So you dive into this and find that it’s using GMMU_APERTURE_PEER as the mapping type. That doesn’t seem supported in the 4090. So let’s see what types are supported, GMMU_APERTURE_VIDEO,GMMU_APERTURE_SYS_NONCOH, and GMMU_APERTURE_SYS_COH. We don’t care about being coherent with the CPU’s L2 cache, but it does have to go out the PCIe bus, so we rewrite GMMU_APERTURE_PEER to GMMU_APERTURE_SYS_NONCOH. We also no longer set the peer id that was corrupting the page table.

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cudaMemcpyPeer / cudaMemcpy between GPU0 and GPU1: 24.21GB/s
Preparing host buffer and memcpy to GPU0...
Run kernel on GPU1, taking source data from GPU0 and writing to GPU1...
Run kernel on GPU0, taking source data from GPU1 and writing to GPU0...
Copy data back to host from GPU0 and verify results...
Verification error @ element 1: val = 0.000000, ref = 4.000000
Verification error @ element 2: val = 0.000000, ref = 8.000000

Progress! ./simpleP2P appears to work, however the copy isn’t happening. The address is likely wrong. It turns out they have a separate field for the peer address called fldAddrPeer, we change that to fldAddrSysmem. We also print out the addresses and note that the physical BAR address isn’t being added properly, they provide a field fabricBaseAddress for GMMU_APERTURE_PEER, we reuse it and put the BAR1 base address in there.

P2P on Charlie Chiang's blog

RTX4090 GPUDirect P2P Unlocked!

What is P2P?

Why is P2P disabled on RTX4090?

Test machine configuration

Test the original performance

PCI BAR

NCCL Tests

P2P Bandwidth and Latency

How to unlock P2P on RTX4090?

Disable IOMMU

Enable Resizable BAR

Uninstall the official NVIDIA driver

Install the official NVIDIA driver without kernel modules

Install the modified kernel modules

Test the unlocked performance

PCI BAR

NCCL Tests

P2P Bandwidth and Latency

How it works? By tinygrad.