What is SQL Server tempdb allocation contention (PFS, GAM, SGAM bottlenecks)?

tempdb contention occurs when multiple sessions compete for allocation pages like PFS, GAM, and SGAM.

tempdb uses special system pages to manage space allocation. PFS tracks page free space, GAM tracks global extent allocation, and SGAM tracks mixed extent allocation. Under high concurrency, these pages become hot spots because many sessions try to allocate temporary objects simultaneously. This leads to latch contention (PAGELATCH_UP). SQL Server mitigates this by recommending multiple tempdb data files with equal sizing and proportional fill behavior.