linux-kernel - Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v7

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Message-ID: <20161207155750.yfsizliaoodks5k4@techsingularity.net>
Date:   Wed, 7 Dec 2016 15:57:50 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Christoph Lameter <cl@...ux.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux-Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: page_alloc: High-order per-cpu page allocator v7

On Wed, Dec 07, 2016 at 08:52:27AM -0600, Christoph Lameter wrote:
> On Wed, 7 Dec 2016, Mel Gorman wrote:
> 
> > SLUB has been the default small kernel object allocator for quite some time
> > but it is not universally used due to performance concerns and a reliance
> > on high-order pages. The high-order concerns has two major components --
> 
> SLUB does not rely on high order pages. It falls back to lower order if
> the higher orders are not available. Its a performance concern.
> 

Ok -- While SLUB does not rely on high-order pages for functional
correctness, it perfoms better if high-order pages are available.

> This is also an issue for various other kernel subsystems that really
> would like to have larger contiguous memory area. We are often seeing
> performance constraints due to the high number of 4k segments when doing
> large scale block I/O f.e.
> 

Which is related to the fundamentals of fragmentation control in
general. At some point there will have to be a revisit to get back to
the type of reliability that existed in 3.0-era without the massive
overhead it incurred. As stated before, I agree it's important but
outside the scope of this patch.

-- 
Mel Gorman
SUSE Labs