eBPF DCstat

Plugin: ebpf-go.plugin Module: dcstat

Overview

Monitor directory cache events given an overall vision about files on memory or storage device. Setting dcstat = yes enables collection and the host-wide charts only; the per-application and per-cgroup charts additionally require apps = yes and/or cgroups = yes in the [global] section. reference, slow and miss are independent kernel counters and are reported independently: an interval with no fast-path lookups can still report slow-path lookups or misses. The C implementation of this collector suppressed slow and miss whenever reference did not move in the same interval.

Attach a kprobe to lookup_fast and a kretprobe to d_lookup, using the object flavor selected for the running kernel.

This collector is only supported on the following platforms:

• Linux

This collector supports collecting metrics from multiple instances of this integration, including remote instances.

The plugin needs setuid because it loads data inside kernel. Netdata sets necessary permission during installation time.

eBPF DCstat can be monitored further using the following other integrations:

• Applications
• Containers

Default Behavior

Auto-Detection

The plugin checks kernel compilation flags (CONFIG_KPROBES, CONFIG_BPF, CONFIG_BPF_SYSCALL, CONFIG_BPF_JIT), presence of BTF files, and the configured object flavor to decide which eBPF program will be attached. Because lookup_fast is a static kernel function, the collector resolves its real symbol name from /proc/kallsyms before attaching.

Limits

The default configuration for this integration does not impose any limits on data collection.

Performance Impact

This thread will add overhead every time that an internal kernel function monitored by this thread is called.

Setup

Prerequisites

Compile kernel

Check if your kernel was compiled with necessary options (CONFIG_KPROBES, CONFIG_BPF, CONFIG_BPF_SYSCALL, CONFIG_BPF_JIT) in /proc/config.gz or inside /boot/config file. Some cited names can be different according to the preferences of Linux distributions. When you do not have options set, it is necessary to get the kernel source code from https://kernel.org or a kernel package from your distribution, this last is preferred. The kernel compilation has a well defined pattern, but distributions can deliver their configuration files with different names.

Now follow steps:

1. Copy the configuration file to /usr/src/linux/.config.
2. Select the necessary options: make oldconfig
3. Compile your kernel image: make bzImage
4. Compile your modules: make modules
5. Copy your new kernel image for boot loader directory
6. Install the new modules: make modules_install
7. Generate an initial ramdisk image (initrd) if it is necessary.
8. Update your boot loader

Configuration

Options

The dcstat option in section [ebpf programs] controls whether the module is loaded. The options listed below are [global] overrides.

Config options

Option	Description	Default	Required
update every	Data collection frequency. This configuration value takes precedence over the runtime interval passed by pluginsd.	10	no
ebpf load mode	Legacy compatibility option. entry and return are accepted as no-ops; object flavor selects the eBPF runtime.	entry	no
apps	Enable or disable integration with apps.plugin	no	no
cgroups	Enable or disable integration with cgroup.plugin	no	no
pid table size	Number of elements stored inside hash tables used to monitor calls per PID.	32768	no
ebpf object flavor	Select the dcstat object flavor to load. Available values are arena, buffer ring (also accepted as buffer), and legacy (also accepted as tracing). The default is buffer, and the collector falls back when the requested flavor is not available on the system.	buffer	no
ebpf type format	Legacy key from the C ebpf.plugin, kept so migrated configurations still parse. It maps onto ebpf object flavor: legacy forces the kprobe-based tracing objects, while co-re and auto are accepted no-ops that leave the flavor to auto-detection. Prefer ebpf object flavor in new configurations.	auto	no
ebpf co-re tracing	Select the attach method used by plugin when co-re is defined in previous option. Two options are available: trampoline (Option with lowest overhead), and probe (the same of legacy code).	trampoline	no
maps per core	Define how plugin will load their hash maps. When enabled (yes) plugin will load one hash table per core, instead to have centralized information.	yes	no
btf path	Override the default BTF source directory. The collector checks this path for a vmlinux BTF file used by CO-RE loading.	/sys/kernel/btf	no
collect pid	Controls which PIDs are stored in the BPF hash tables. real parent stores only the real parent PID (lowest overhead); parent stores the immediate parent PID; all stores every PID used by the process (highest overhead).	real parent	no
lifetime	Set default lifetime for thread when enabled by cloud.	300	no

via File

The configuration file name for this integration is ebpf.d.conf.

You can edit the configuration file using the edit-config script from the Netdata config directory.

cd /etc/netdata 2>/dev/null || cd /opt/netdata/etc/netdata
sudo ./edit-config ebpf.d.conf

Examples

There are no configuration examples.

Alerts

There are no alerts configured by default for this integration.

Metrics

Metrics grouped by scope.

The scope defines the instance that the metric belongs to. An instance is uniquely identified by a set of labels.

Per eBPF DCstat instance

These metrics show how often file lookups were resolved by the kernel directory cache, host-wide.

This scope has no labels.

Metrics:

Metric	Description	Dimensions	Unit
filesystem.dc_hit_ratio	Percentage of directory lookups resolved by the cache — (reference - miss) / reference. It measures lookup resolution, not how many files are held in the cache. Computed from cumulative counters since the eBPF program attached, matching the legacy dcstat contract.	ratio	%
filesystem.dc_reference	Variables used to calculate hit ratio. Published as per-interval totals with the absolute algorithm, preserving the legacy dcstat metric contract.	reference, slow, miss	files