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

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Date:   Mon, 19 Sep 2016 08:59:22 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Minchan Kim <minchan@...nel.org>,
        "Chen, Tim C" <tim.c.chen@...el.com>
Cc:     "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@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hugh Dickins <hughd@...gle.com>, Shaohua Li <shli@...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

On Mon, 2016-09-19 at 16:11 +0900, Minchan Kim wrote:
> Hi Tim,
> 
> On Tue, Sep 13, 2016 at 11:52:27PM +0000, Chen, Tim C wrote:
> > 
> > > 
> > > > 
> > > > 
> > > > - Avoid CPU time for splitting, collapsing THP across swap out/in.
> > > Yes, if you want, please give us how bad it is.
> > > 
> > It could be pretty bad.  In an experiment with THP turned on and we
> > enter swap, 50% of the cpu are spent in the page compaction path.  
> It's page compaction overhead, especially, pageblock_pfn_to_page.
> Why is it related to overhead THP split for swapout?
> I don't understand.

Today you have to split a large page into 4K pages to swap it out.
Then after you swap in all the 4K pages, you have to re-compact
them back into a large page.

If you can swap the large page out as a contiguous unit, and swap
it back in as a single large page, the splitting and re-compaction
back into a large page can be avoided.

Tim