On Wed, Apr 07, 2021 at 03:50:06AM +0100, Matthew Wilcox wrote: > 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.
I only have the observation and no full explanation for it. Just try it on wis-mmap and wis-malloc threaded cases. Of course this all washes away when dealing with more realistic macro benchmarks.
