Re: [RFC PATCH 34/37] mm: rcu safe vma freeing only for multithreaded user space

[Matthew Wilcox]

On Tue, Apr 06, 2021 at 06:44:59PM -0700, Michel Lespinasse wrote:
> Performance tuning: as single threaded userspace does not use
> speculative page faults, it does not require rcu safe vma freeing.
> Turn this off to avoid the related (small) extra overheads.
> 
> For multi threaded userspace, we often see a performance benefit from
> the rcu safe vma freeing - even in tests that do not have any frequent
> concurrent page faults ! This is because rcu safe vma freeing prevents
> recently released vmas from being immediately reused in a new thread.

Why does that provide a performance benefit?  Recently released
VMAs are cache-hot, and NUMA-local.  I'd expect the RCU delay to be
performance-negative.