netdev - Re: [PATCH 0/1] mm: Remove the SLAB allocator

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Date:   Wed, 17 Apr 2019 13:27:43 +0000
From:   Christopher Lameter <cl@...ux.com>
To:     Jesper Dangaard Brouer <netdev@...uer.com>
cc:     Pekka Enberg <penberg@....fi>, Michal Hocko <mhocko@...nel.org>,
        "Tobin C. Harding" <me@...in.cc>, Vlastimil Babka <vbabka@...e.cz>,
        "Tobin C. Harding" <tobin@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Tejun Heo <tj@...nel.org>, Qian Cai <cai@....pw>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Mel Gorman <mgorman@...hsingularity.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexander Duyck <alexander.duyck@...il.com>
Subject: Re: [PATCH 0/1] mm: Remove the SLAB allocator

On Wed, 17 Apr 2019, Jesper Dangaard Brouer wrote:

> I do think SLUB have a number of pathological cases where SLAB is
> faster.  If was significantly more difficult to get good bulk-free
> performance for SLUB.  SLUB is only fast as long as objects belong to
> the same page.  To get good bulk-free performance if objects are
> "mixed", I coded this[1] way-too-complex fast-path code to counter
> act this (joined work with Alex Duyck).

Right. SLUB usually compensates for that with superior allocation
performance.

> > It's, of course, worth thinking about other pathological cases too.
> > Workloads that cause large allocations is one. Workloads that cause lots
> > of slab cache shrinking is another.
>
> I also worry about long uptimes when SLUB objects/pages gets too
> fragmented... as I said SLUB is only efficient when objects are
> returned to the same page, while SLAB is not.

??? Why would SLUB pages get more fragmented? SLUB has fragmentation
prevention methods that SLAB does not have.