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

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Date:   Thu, 5 Jan 2017 15:15:57 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     "Huang, Ying" <ying.huang@...el.com>
CC:     Jan Kara <jack@...e.cz>, Tim Chen <tim.c.chen@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        <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 Huang,

On Tue, Jan 03, 2017 at 01:43:43PM +0800, Huang, Ying wrote:
> Hi, Minchan,
> 
> Minchan Kim <minchan@...nel.org> writes:
> 
> > 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/
> 
> This helps to reduce the lock contention on radix tree of swap cache.
> But splitting swap cache has much better performance.  So we switched
> from that solution to current solution.
> 
> > Or concurrent page cache by peter.
> >
> > https://www.kernel.org/doc/ols/2007/ols2007v2-pages-311-318.pdf
> 
> I think this is good, it helps swap and file cache.  But I don't know
> whether other people want to go this way and how much effort will be
> needed.
> 
> In contrast, splitting swap cache is quite simple, for implementation
> and review.  And the effect is good.

I think general approach is better but I don't want to be a a party pooper
if every people are okay with this. I just wanted to point out we need to
consider more general approach and I did my best.

Decision depends on you guys.

Thanks.