IOMMU-based tuning

Tune with IOMMU

IOMMU (Input–Output Memory Management Unit) controls how I/O devices access memory. In many cloud environments, SmartNICs offload IOMMU-related work. On Arm Neoverse bare‑metal systems, you can often improve Tomcat networking performance by disabling strict mode and enabling passthrough (setting iommu.strict=0 and iommu.passthrough=1).

Configure IOMMU settings

Edit the GRUB configuration to set IOMMU to passthrough and disable strict invalidations:

    

        
        
sudo vi /etc/default/grub

    

Add or update the kernel command line:

    

        
        
GRUB_CMDLINE_LINUX="iommu.strict=0 iommu.passthrough=1"

    

Update GRUB and reboot to apply the settings:

    

        
        
sudo update-grub && sudo reboot

    

Verify that IOMMU is in passthrough mode after reboot:

    

        
        
sudo dmesg | grep iommu

    

You will notice that the IOMMU is already in passthrough mode:

    

        
        
[    0.000000] Kernel command line: BOOT_IMAGE=/vmlinuz-6.14.0-1011-aws root=PARTUUID=1c3f3c20-db6b-497c-8727-f6702f73a5b2 ro iommu.strict=0 iommu.passthrough=1 console=tty1 console=ttyS0 nvme_core.io_timeout=4294967295 panic=-1
[    0.855658] iommu: Default domain type: Passthrough (set via kernel command line)

    

Validate performance after IOMMU tuning

Prepare the Arm Neoverse bare‑metal server (ensure your ${net} interface variable is set; if not, set it to your NIC name, for example net=enP11p4s0), align queues, and restart Tomcat:

    

        
        
for no in {96..103}; do sudo bash -c "echo 1 > /sys/devices/system/cpu/cpu${no}/online"; done
for no in {0..95} {104..191}; do sudo bash -c "echo 0 > /sys/devices/system/cpu/cpu${no}/online"; done

# Ensure NIC queue count matches the number of online CPUs (example: 8)
sudo ethtool -L ${net} combined 8

# Restart Tomcat with a higher file‑descriptor limit
~/apache-tomcat-11.0.10/bin/shutdown.sh 2>/dev/null
ulimit -n 65535 && ~/apache-tomcat-11.0.10/bin/startup.sh

    

Run wrk2 on the x86_64 benchmarking client to measure throughput and latency:

    

        
        
ulimit -n 65535 && wrk -c1280 -t128 -R500000 -d60 http://${tomcat_ip}:8080/examples/servlets/servlet/HelloWorldExample

    

Sample results after IOMMU tuning:

    

        
          Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     4.92s     2.49s   10.08s    62.27%
    Req/Sec     3.36k    56.23     3.58k    69.64%
  25703668 requests in 1.00m, 13.33GB read
Requests/sec: 428628.50
Transfer/sec:    227.69MB