linux-kernel - Re: [PATCH v4 0/9] mm/swap: Regular page swap optimizations

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Date:   Tue, 3 Jan 2017 13:34:11 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Jan Kara <jack@...e.cz>
CC:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ying Huang <ying.huang@...el.com>, <dave.hansen@...el.com>,
        <ak@...ux.intel.com>, <aaron.lu@...el.com>, <linux-mm@...ck.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.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Hillf Danton <hillf.zj@...baba-inc.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Jonathan Corbet <corbet@....net>,
        Peter Zijlstra <peterz@...radead.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v4 0/9] mm/swap: Regular page swap optimizations

Hi Jan,

On Mon, Jan 02, 2017 at 04:48:41PM +0100, Jan Kara wrote:
> Hi,
> 
> On Tue 27-12-16 16:45:03, Minchan Kim wrote:
> > > Patch 3 splits the swap cache radix tree into 64MB chunks, reducing
> > >         the rate that we have to contende for the radix tree.
> > 
> > To me, it's rather hacky. I think it might be common problem for page cache
> > so can we think another generalized way like range_lock? Ccing Jan.
> 
> I agree on the hackyness of the patch and that page cache would suffer with
> the same contention (although the files are usually smaller than swap so it
> would not be that visible I guess). But I don't see how range lock would
> help here - we need to serialize modifications of the tree structure itself
> and that is difficult to achieve with the range lock. So what you would
> need is either a different data structure for tracking swap cache entries
> or a finer grained locking of the radix tree.

Thanks for the comment, Jan.

I think there are more general options. One is to shrink batching pages like
Mel and Tim had approached.

https://patchwork.kernel.org/patch/9008421/
https://patchwork.kernel.org/patch/9322793/

Or concurrent page cache by peter.

https://www.kernel.org/doc/ols/2007/ols2007v2-pages-311-318.pdf

Ccing Nick who might have an interest on lockless page cache.

Thanks.