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

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Date:   Thu, 22 Sep 2016 16:53:02 -0700
From:   Andi Kleen <andi@...stfloor.org>
To:     "Chen\, Tim C" <tim.c.chen@...el.com>
Cc:     Shaohua Li <shli@...nel.org>,
        "Huang\, Ying" <ying.huang@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Hansen\, Dave" <dave.hansen@...el.com>,
        "Kleen\, Andi" <andi.kleen@...el.com>,
        "Lu\, Aaron" <aaron.lu@...el.com>,
        "linux-mm\@kvack.org" <linux-mm@...ck.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>,
        Minchan Kim <minchan@...nel.org>,
        Rik van Riel <riel@...hat.com>,
        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

"Chen, Tim C" <tim.c.chen@...el.com> writes:

>>
>>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?

Anything that touches regions larger than 4K and we want to do the
kernel do minimal work to manage the swapping.

>
> We waste a lot of cpu cycles to re-compact 4K pages back to a large page
> under THP.  Swapping it back in as a single large page can avoid
> fragmentation and this overhead.

Also splitting something just to merge it again is wasteful.

A lot of big improvements in the block and VM and network layers
over the years came from avoiding that kind of wasteful work.

-Andi