In December, Metin (a coworker of mine) discussed an inability to scale a simple task (parallel scans of many independent tables) to many cores (it’s here). As a ramp-up task at Citus I was tasked to figure out what the heck was going on here.
I have a pretty extensive writeup here (whose length is more a result of my inexperience with the workings of PostgreSQL than anything else) and was looking for some feedback. In short, my conclusion is that a working set larger than memory results in backends piling up on BufFreelistLock. As much as possible I removed anything that could be blamed for this: Hyper-Threading is disabled zone reclaim mode is disabled numactl was used to ensure interleaved allocation kernel.sched_migration_cost was set to highly disable migration kernel.sched_autogroup_enabled was disabled transparent hugepage support was disabled For a way forward, I was thinking the buffer allocation sections could use some of the atomics Andres added here. Rather than workers grabbing BufFreelistLock to iterate the clock hand until they find a victim, the algorithm could be rewritten in a lock-free style, allowing workers to move the clock hand in tandem. Alternatively, the clock iteration could be moved off to a background process, similar to what Amit Kapila proposed here. Is this assessment accurate? I know 9.4 changes a lot about lock organization, but last I looked I didn’t see anything that could alleviate this contention: are there any plans to address this? —Jason
