linux-kernel - Re: [PATCH v5 3/9] mm/swap: Split swap cache into 64MB trunks

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Date:   Wed, 11 Jan 2017 15:19:37 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Tim Chen <tim.c.chen@...ux.intel.com>,
        "Huang, Ying" <ying.huang@...el.com>, dave.hansen@...el.com,
        aaron.lu@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Hugh Dickins <hughd@...gle.com>,
        Shaohua Li <shli@...nel.org>, 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.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>
Subject: Re: [PATCH v5 3/9] mm/swap: Split swap cache into 64MB trunks

> Switching from a single radix-tree to an array of radix-trees to reduce
> contention seems a bit hacky.  That we can do this and have everything
> continue to work tells me that we're simply using an inappropriate data
> structure to hold this info.

What would you use instead?

A tree with fine grained locking?

FWIW too fine grained locking (e.g. on every node) is usually a bad idea: 

it slows down the single thread performance and it causes much more overhead
when there is actual contention because too much time is spent bouncing cache
lines around.

So I actually like the "a little bit more fine grained, but not too much"
approach.

Or a hash table? 

Not sure if this would work here.

-Andi