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

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Date:   Thu, 8 Apr 2021 00:53:44 -0700
From:   Michel Lespinasse <michel@...pinasse.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Michel Lespinasse <michel@...pinasse.org>,
        Linux-MM <linux-mm@...ck.org>,
        Laurent Dufour <ldufour@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Michal Hocko <mhocko@...e.com>,
        Rik van Riel <riel@...riel.com>,
        Paul McKenney <paulmck@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Rom Lemarchand <romlem@...gle.com>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [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.