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doc(tuning_and_security): extend the performance tuning section

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Extend the performance tuning section of documentation
to include explanation on the `--L1s.L1_num_tx_sym_per_thread` option
and on the AMD EPYC core complex behavior
This commit is contained in:
Romain Beurdouche
2026-02-19 09:57:08 +00:00
parent 1fdc6b7515
commit b1786e2277
2 changed files with 104 additions and 0 deletions
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doc/NR_SA_Tutorial_COTS_UE.md
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@@ -191,6 +191,7 @@ sudo ethtool -G enp1s0f0 tx 4096 rx 4096
- Enable Performance Mode `sudo cpupower idle-set -D 0`
- If you get real-time problems on heavy UL traffic, reduce the maximum UL MCS using an additional command-line switch: `--MACRLCs.[0].ul_max_mcs 14`.
- You can also reduce the number of LDPC decoder iterations, which will make the LDPC decoder take less time: `--L1s.[0].max_ldpc_iterations 4`.
- Read more on system tuning in the [dedicated document on performance tuning](./tuning_and_security.md#performance-tuning).
### 6.3 Uplink issues related with noise on the DC carriers

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doc/tuning_and_security.md
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@@ -67,6 +67,109 @@ sudo sysctl -n -e -q -w net.core.wmem_default=134217728
sudo sysctl -n -e -q -w net.core.wmem_max=134217728
```
### System tuning
In order to get an optimal real-time behavior, a few tunings can be performed on the host system:
- The use of isolated cores for the softmodem prevents competitions on the usage of core between the softmodem and other processes.
Core isolation is enabled through the kernel command line.
**Warning: modifying the kernel command line can harm the OS behavior. Proceed with caution.**
Refer to the [OAI 7.2 Fronthaul Interface Tutorial](./ORAN_FHI7.2_Tutorial.md) for examples.
### Softmodem tuning
The way the NR softmodem uses the computing ressource can be configured.
It can have a significant effect on the performance and real-time behavior:
- The L1 TX and L1 RX threads are the two main threads
executing the L1 RX and L1 TX pipelines.
These threads are ideally assigned to two dedicated cores.
To be dedicated, the cores should be isolated in the
kernel parameters and not be assigned elsewhere.
They can be assigned to specified cores with options
`--L1s.[0].L1_tx_thread_core` and `--L1s.[0].L1_rx_thread_core`
followed by a core id.
- The thread pool is a group of processor cores over which
some baseband processing worker cores execute.
It is configured by providing a list of core ids
after option `--thread-pool`.
`-1` can also be passed instead of a core id
in order to use a floating core.
By default, the thread pool is 8 floating cores.
- PDSCH generation (i.e., layer mapping and precoding) is by default executed
in the L1 TX thread but can be multithreaded using the thread pool.
This is enabled by option `--L1s.[0].L1_num_tx_sym_per_thread` followed
by the number of symbols that should be processed in each thread.
This option can also be set in the gNB configuration file in field
`L1_num_tx_sym_per_thread` in the `L1s` section.
### Workarounds
If the real-time performance remains bad after tuning the system and softmodem,
some workarounds allow to lower the computing demand
at the cost of lower network performance:
- If you get real-time problems on heavy UL traffic,
reduce the maximum UL MCS using an additional
command-line switch: `--MACRLCs.[0].ul_max_mcs 14`.
This comes at the cost of a lower spectral efficiency
(i.e., less data for the same radio resource).
- You can also reduce the number of LDPC decoder iterations,
which will make the LDPC decoder take less time:
`--L1s.[0].max_ldpc_iterations 4`.
The default number of LDPC iterations is 8.
Lowering the number of iteration comes at the cost
of more unsuccessful transmissions.
OAI offers multiple implementation of LDPC coding,
including offloading to an accelerator,
the number of LDPC iteration should be chosen accordingly.
### Known hardware behaviors
Here is a **non-exhaustive** list of known behaviors related to hardware architecture:
- On some AMD EPYC series processors with Zen architecture
(at least every Zen4, Zen4c, Zen5 and Zen5c based processors
experience this behavior),
the processor is made of multiple dies holding one or multiple
core complexes which are groups of cores with an L3 cache.
This means that cores from different core complexes
do not share the same L3 cache and communication between
these cores implies inter L3 cache communication
within a die or, even worse, between dies,
which has a cost in term of latency.
Depending on the system configuration,
the NUMA topology may reflect this physical topology,
which can induce even further latency for
inter core complex communication.
The softmodem is sensitive to this latency and its performance
can be harmed if it uses cores across the border of dies or core complexes,
especially when multithreading of PDSCH generation is enabled
(argument of `--L1s.[0].L1_num_tx_sym_per_thread` is superior to 0).
**Solution**: We recommend to use only one core complex
or one die for allocating cores to nr-softmodem process.
- NUMA architecture: Make sure you don't assign cores to nr-softmodem from
different numa nodes. Cores from different numa cores can induce latency.
It is preferred to use the cores from numa node
which is used by the Fronthaul NIC.
**Example**: How to assign cores to `nr-softmodem` process for gNB/DU connected to an o-ran 7.2 O-RU on EPYC 9575F (64 Zen5)
assuming that we have configured `L1s.[0].L1_num_tx_sym_per_thread = 1`
There are 8 cores per core complex and one core complex per die. You can visulize this topology using
```bash
# Ubuntu 25.04
for X in $(seq 0 63); do echo -n "cpu$X -> die "; cat /sys/devices/system/cpu/cpu$X/topology/die_id; done
```
A 100MHz 4x4 FR1 gNB/DU with FHI 7.2 can be executed with full capabilities (4DL and 2UL layers)
on a single core complex (cpus 0-7 in this example) by folowing the instructions of the
[OAI 7.2 Fronthaul Interface Tutorial](./ORAN_FHI7.2_Tutorial.md)
with the following core assignment:
- `L1s.[0].L1_tx_thread_core = 0`
- `L1s.[0].L1_rx_thread_core = 1`
- `RUs.[0].ru_thread_core = 2`
- `fhi_72.system_core = 3`
- `fhi_72.io_core = 4`
- `fhi_72.worker_cores.[0] = 5` (minimum 1 core)
- `thread-pool: 2,3,6,7`
(thread pool can overlap with `RUs.[0].ru_thread_core` and `fhi_72.system_core`)
## Capabilities
Historically, all softmodems are executed as `root`, typically using `sudo`.