linux-kernel - Re: [RFC, PATCHv1 00/28] 5-level paging

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Date:   Fri, 9 Dec 2016 08:49:47 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>,
        Ingo Molnar <mingo@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC, PATCHv1 00/28] 5-level paging

On 12/09/2016 02:37 AM, Kirill A. Shutemov wrote:
> On other hand, large virtual address space would put more pressure on
> cache -- at least one more page table per process, if we make 56-bit VA
> default.

For a process only using a small amount of its address space, the
mid-level paging structure caches will be very effective since the page
walks are all very similar.  You may take a cache miss on the extra
level on the *first* walk, but you only do that once per context switch.
 I bet the CPU is also pretty aggressive about filling those things when
it sees a new CR3 and they've been forcibly emptied.  So, you may never
even _see_ the latency from that extra miss.