feat(tuned): enable tuned and disable THP

[hunleyd]

Implement a specialized tuned profile for PostgreSQL to disable Transparent Huge Pages (THP) and ensure proper service ordering.

Background: Transparent Huge Pages (THP)
In Linux, memory is typically managed in 4KB pages. Huge Pages allow the system to manage memory in much larger chunks (e.g., 2MB or 1GB), which reduces the overhead of the Translation Lookaside Buffer (TLB).

Transparent Huge Pages (THP) is a kernel feature that attempts to automate this by dynamically allocating huge pages for processes. While beneficial for some workloads, it is generally detrimental to database systems like PostgreSQL for several reasons:

1. Memory Bloat & Fragmentation: THP can lead to significant memory waste. If a process only needs a small portion of a 2MB page, the entire 2MB is still allocated. In highly concurrent database environments, this often leads to rapid memory exhaustion.
2. Latency Spikes (khugepaged): The kernel's khugepaged thread periodically scans memory to "collapse" standard pages into huge pages. This process can cause unpredictable latency spikes and CPU contention, often referred to as "stalls," which are unacceptable for high-performance database transactions.
3. Inefficient I/O: PostgreSQL manages its own shared buffers and is optimized for 8KB blocks. The mismatch between the kernel's 2MB THP allocations and PostgreSQL's internal memory management can lead to inefficient paging and increased I/O pressure during page faults.

Summary of Changes

1. PostgreSQL Service Ordering (ansible/tasks/setup-postgres.yml)

• SystemD Overrides: Created a directory /etc/systemd/system/postgresql.service.d to house service customizations.
• Dependency Management: Added a Unit dependency (After=tuned.service) via overrides.conf. This ensures that the tuned profile—and the resulting kernel optimizations—are fully applied before the PostgreSQL daemon initializes.

2. PostgreSQL-Specific Tuned Profile (ansible/tasks/setup-tuned.yml)

• Profile Definition: Created a new tuned profile directory and configuration at /etc/tuned/postgresql/tuned.conf.
• THP Deactivation:
  • Runtime: Sets vm.transparent_hugepages=never to disable THP at the kernel level immediately.
  • Boot-time: Appends transparent_hugepages=never to the bootloader command line to ensure THP remains disabled after system reboots.
• Service Activation: Restarts the tuned service to register the new profile and executes tuned-adm profile postgresql to apply the optimizations.

These changes ensure a stabler and more predictable performance profile for PostgreSQL by preventing kernel-level memory management interference.