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From: Andy Lutomirski <luto@kernel.org>
Date: Wed, 21 Jun 2017 22:19:30 -0700
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Subject: Re: [PATCH v3 00/11] PCID and improved laziness
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Borislav Petkov <bp@alien8.de>,
        Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Nadav Amit <nadav.amit@gmail.com>, Rik van Riel <riel@redhat.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Arjan van de Ven <arjan@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>
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On Wed, Jun 21, 2017 at 11:23 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, Jun 20, 2017 at 10:22 PM, Andy Lutomirski <luto@kernel.org> wrote:
>> There are three performance benefits here:
>
> Side note: can you post the actual performance numbers, even if only
> from some silly test program on just one platform? Things like lmbench
> pipe benchmark or something?
>
> Or maybe you did, and I just missed it. But when talking about
> performance, I'd really like to always see some actual numbers.

Here are some timings using KVM:

pingpong between two processes using eventfd:
patched: 883ns
unpatched: 1046ns (with considerably higher variance)

madvise(MADV_DONTNEED); write to the page; switch CPUs:
patched: ~12.5us
unpatched: 19us

The latter test is a somewhat contrived example to show off the
improved laziness.  Current kernels send an IPI on each iteration if
the system is otherwise idle.