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

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