DPAS Lab Guide

Prerequisites

You must have already completed the platform-specific QEMU installation and Ubuntu VM setup.

• macOS Setup Guide
• Linux Setup Guide
• Windows Setup Guide

Part 1: DPAS Kernel Installation

Install the DPAS-patched kernel using the provided deb packages.

1-1. Transfer Packages

Transfer the downloaded deb files from the host to the VM.

# Run on the host terminal
scp -P 2222 linux-image-5.18.0-dpas_*.deb <username>@localhost:~/
scp -P 2222 linux-headers-5.18.0-dpas_*.deb <username>@localhost:~/

1-2. Install and Reboot

# Run inside the VM
sudo dpkg -i ~/linux-image-5.18.0-dpas_*.deb ~/linux-headers-5.18.0-dpas_*.deb
sudo reboot

1-3. Verify Installation

uname -r
# 5.18.0-dpas

Part 2: Lab — Performance Comparison by Mode

2-1. Benchmark Scripts

Create the following 4 scripts inside the VM. Usage: ./script.sh <device> <cpu> <numjobs>

fioint.sh — Interrupt Mode

#!/bin/bash
sudo modprobe -r nvme; sudo modprobe nvme poll_queues=0
fio --filename=/dev/$1 --size=100m --direct=1 --bs=4k --ioengine=pvsync2 \
    --iodepth=1 --rw=randread --runtime=10 --numjobs=$3 --time_based \
    --group_reporting --name=test --eta-newline=1 --cpus_allowed=$2 \
    --nice=-10 --prioclass=2 --prio=0

fiocp.sh — Continuous Polling Mode

#!/bin/bash
sudo modprobe -r nvme; sudo modprobe nvme poll_queues=2
fio --filename=/dev/$1 --ramp_time=3 --size=100m --direct=1 --bs=4k \
    --ioengine=pvsync2 --iodepth=1 --rw=randread --runtime=10 \
    --numjobs=$3 --time_based --group_reporting --name=test \
    --eta-newline=1 --cpus_allowed=$2 --nice=-10 --prioclass=2 \
    --prio=0 --hipri

fiopas.sh — PAS (Polling after Sleep) Mode

#!/bin/bash
sudo modprobe -r nvme; sudo modprobe nvme poll_queues=2
echo 0 > /sys/block/$1/queue/io_poll_delay
echo 1 > /sys/block/$1/queue/pas_enabled
echo 1 > /sys/block/$1/queue/pas_adaptive_enabled
fio --filename=/dev/$1 --direct=1 --bs=4k --ioengine=pvsync2 \
    --iodepth=1 --rw=randread --runtime=10 --numjobs=$3 --time_based \
    --group_reporting --name=test --eta-newline=1 --cpus_allowed=$2 \
    --nice=-10 --prioclass=2 --prio=0 --hipri

fiodpas.sh — DPAS (Dynamic PAS) Mode

#!/bin/bash
sudo modprobe -r nvme; sudo modprobe nvme poll_queues=2
echo 0 > /sys/block/$1/queue/io_poll_delay
echo 1 > /sys/block/$1/queue/pas_enabled
echo 1 > /sys/block/$1/queue/pas_adaptive_enabled
echo 1 > /sys/block/$1/queue/switch_enabled
echo 10 > /sys/block/$1/queue/switch_param1
echo 10 > /sys/block/$1/queue/switch_param2
echo 10 > /sys/block/$1/queue/switch_param3
echo 1 > /sys/block/$1/queue/switch_param4
fio --filename=/dev/$1 --ramp_time=3 --size=100m --direct=1 --bs=4k \
    --ioengine=pvsync2 --iodepth=1 --rw=randread --runtime=10 \
    --numjobs=$3 --time_based --group_reporting --name=test \
    --eta-newline=1 --cpus_allowed=$2 --nice=-10 --prioclass=2 \
    --prio=0 --hipri
cat /sys/block/$1/queue/switch_stat
dmesg | tail -10

Grant execute permissions:

chmod +x fioint.sh fiocp.sh fiopas.sh fiodpas.sh

2-2. Running Experiments

Pin to CPU 0 and compare the 4 modes while varying the number of jobs.

# Example: INT mode, 1 job
./fioint.sh nvme0n1 0 1

# Repeat for each mode with jobs = 1, 2, 4, 8
# INT:  ./fioint.sh nvme0n1 0 {1,2,4,8}
# CP:   ./fiocp.sh  nvme0n1 0 {1,2,4,8}
# PAS:  ./fiopas.sh nvme0n1 0 {1,2,4,8}
# DPAS: ./fiodpas.sh nvme0n1 0 {1,2,4,8}

2-3. Recording Results

Record the IOPS and avg latency from the fio output of each run.

2-4. Verifying DPAS Mode Transition

After running DPAS, check the switch_stat and dmesg output:

Expected Behavior

2-5. Analysis Questions

1. What causes the IOPS difference between INT and CP modes when jobs=1?
2. Why does IOPS not increase significantly when adding more jobs in CP mode?
3. Explain why DPAS approaches CP performance at jobs=1 and approaches INT performance at jobs=4+ from the perspective of mode transition.
4. Why is the pas io count always less than the polled io count in switch_stat?

Part 3: DPAS sysfs Interface

The DPAS kernel provides the following parameters under /sys/block/nvme0n1/queue/.

Part 4: Building the Kernel from Source Challenge

Instead of using deb packages, apply the DPAS patch directly to the kernel source and build it yourself.

4-1. Install Build Dependencies

sudo apt install -y build-essential libncurses-dev bison flex libssl-dev \
    libelf-dev bc pahole dwarves zstd

4-2. Download Kernel Source

cd ~
wget https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.18.tar.xz
tar xf linux-5.18.tar.xz
cd linux-5.18

4-3. Apply the DPAS Patch

# After transferring the patch from host to VM
patch -p1 < ~/dpas.patch

4-4. Kernel Configuration (Minimized)

cp /boot/config-$(uname -r) .config
yes '' | make localmodconfig

4-5. Build

make -j$(nproc) bindeb-pkg

Upon successful build, linux-image-*.deb and linux-headers-*.deb files will be generated in the home directory.

4-6. Install

sudo dpkg -i ~/linux-image-5.18.0-dpas_*.deb ~/linux-headers-5.18.0-dpas_*.deb
sudo reboot

Troubleshooting

Reference: Limitations of the QEMU Environment