linux-kernel - Re: [PATCH -v3 00/10] THP swap: Delay splitting THP during swapping out

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Date:   Thu, 22 Sep 2016 20:38:06 -0400
From:   Rik van Riel <riel@...hat.com>
To:     Shaohua Li <shli@...nel.org>, "Huang, Ying" <ying.huang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, tim.c.chen@...el.com,
        dave.hansen@...el.com, andi.kleen@...el.com, aaron.lu@...el.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Vladimir Davydov <vdavydov@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH -v3 00/10] THP swap: Delay splitting THP during swapping
 out

On Thu, 2016-09-22 at 15:56 -0700, Shaohua Li wrote:
> On Wed, Sep 07, 2016 at 09:45:59AM -0700, Huang, Ying wrote:
> > 
> > - It will help the memory fragmentation, especially when the THP is
> >   heavily used by the applications.  The 2M continuous pages will
> > be
> >   free up after THP swapping out.
> 
> So this is impossible without THP swapin. While 2M swapout makes a
> lot of
> sense, I doubt 2M swapin is really useful. What kind of application
> is
> 'optimized' to do sequential memory access?

I suspect a lot of this will depend on the ratio of storage
speed to CPU & RAM speed.

When swapping to a spinning disk, it makes sense to avoid
extra memory use on swapin, and work in 4kB blocks.

When swapping to NVRAM, it makes sense to use 2MB blocks,
because that storage can handle data faster than we can
manage 4kB pages in the VM.

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