linux-kernel - Re: [PATCH] mm: introduce MADV_CLR

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Date:   Thu, 1 Jun 2017 11:35:02 +0300
From:   Mike Rapoport <rppt@...ux.vnet.ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Andrea Arcangeli <aarcange@...hat.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        linux-mm <linux-mm@...ck.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH] mm: introduce MADV_CLR_HUGEPAGE

On Thu, Jun 01, 2017 at 10:09:09AM +0200, Michal Hocko wrote:
> On Thu 01-06-17 09:53:02, Mike Rapoport wrote:
> > On Tue, May 30, 2017 at 04:39:41PM +0200, Michal Hocko wrote:
> > > On Tue 30-05-17 16:04:56, Andrea Arcangeli wrote:
> > > > 
> > > > UFFDIO_COPY while not being a major slowdown for sure, it's likely
> > > > measurable at the microbenchmark level because it would add a
> > > > enter/exit kernel to every 4k memcpy. It's not hard to imagine that as
> > > > measurable. How that impacts the total precopy time I don't know, it
> > > > would need to be benchmarked to be sure.
> > > 
> > > Yes, please!
> > 
> > I've run a simple test (below) that fills 1G of memory either with memcpy
> > of ioctl(UFFDIO_COPY) in 4K chunks.
> > The machine I used has two "Intel(R) Xeon(R) CPU E5-2680 0 @ 2.70GHz" and
> > 128G of RAM.
> > I've averaged elapsed time reported by /usr/bin/time over 100 runs and here
> > what I've got:
> > 
> > memcpy with THP on: 0.3278 sec
> > memcpy with THP off: 0.5295 sec
> > UFFDIO_COPY: 0.44 sec
> 
> I assume that the standard deviation is small?

Yes.
 
> > That said, for the CRIU usecase UFFDIO_COPY seems faster that disabling THP
> > and then doing memcpy.
> 
> That is a bit surprising. I didn't think that the userfault syscall
> (ioctl) can be faster than a regular #PF but considering that
> __mcopy_atomic bypasses the page fault path and it can be optimized for
> the anon case suggests that we can save some cycles for each page and so
> the cumulative savings can be visible.
> 
> -- 
> Michal Hocko
> SUSE Labs
>