linux-kernel - Re: [RFC PATCH v5 13/16] slub: Enable balancing slabs across nodes

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Message-ID: <20190521014457.GA27676@eros.localdomain>
Date:   Tue, 21 May 2019 11:44:57 +1000
From:   "Tobin C. Harding" <me@...in.cc>
To:     Roman Gushchin <guro@...com>
Cc:     "Tobin C. Harding" <tobin@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Viro <viro@....linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Pekka Enberg <penberg@...helsinki.fi>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Christopher Lameter <cl@...ux.com>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Andreas Dilger <adilger@...ger.ca>,
        Waiman Long <longman@...hat.com>,
        Tycho Andersen <tycho@...ho.ws>, Theodore Ts'o <tytso@....edu>,
        Andi Kleen <ak@...ux.intel.com>,
        David Chinner <david@...morbit.com>,
        Nick Piggin <npiggin@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v5 13/16] slub: Enable balancing slabs across nodes

On Tue, May 21, 2019 at 01:04:10AM +0000, Roman Gushchin wrote:
> On Mon, May 20, 2019 at 03:40:14PM +1000, Tobin C. Harding wrote:
> > We have just implemented Slab Movable Objects (SMO).  On NUMA systems
> > slabs can become unbalanced i.e. many slabs on one node while other
> > nodes have few slabs.  Using SMO we can balance the slabs across all
> > the nodes.
> > 
> > The algorithm used is as follows:
> > 
> >  1. Move all objects to node 0 (this has the effect of defragmenting the
> >     cache).
> 
> This already sounds dangerous (or costly). Can't it be done without
> cross-node data moves?
>
> > 
> >  2. Calculate the desired number of slabs for each node (this is done
> >     using the approximation nr_slabs / nr_nodes).
> 
> So that on this step only (actual data size - desired data size) has
> to be moved?

This is just the most braindead algorithm I could come up with.  Surely
there are a bunch of things that could be improved.  Since I don't know
the exact use case it seemed best not to optimize for any one use case.

I'll review, comment on, and test any algorithm you come up with!

thanks,
Tobin.