linux-kernel - Re: [RFC PATCH] mm, slab: Reduce space complexity of alien

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Date:   Tue, 5 Oct 2021 13:18:38 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Christoph Lameter <cl@...two.de>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [RFC PATCH] mm, slab: Reduce space complexity of alien_cache
 using rbtree

On Mon, Oct 04, 2021 at 04:05:23PM +0200, Christoph Lameter wrote:
> On Sun, 3 Oct 2021, Hyeonggon Yoo wrote:
> 
> > As remote node allocation isn't that frequent and better avoided,
> > this patch tries to compromise some execution time for memory usage.
> 
> Remote node allocation is typical for large memory loads and essential to
> balance the memory allocation over multiple nodes. Large NUMA systems
> exist because the memory requirements of the application cannot be
> satisfied by what a single node has to offer and usually code threads from
> multiple nodes may access remote memory.
>

> > Remove init_reap_node, next_reap_node and related variables
> > introduced by commit 8fce4d8e3b9e ("[PATCH] slab: Node rotor for freeing
> > alien caches and remote per cpu pages."), Because it has only one tree
> > for all remote nodes.
> 
> Rotors to spread the memory allocated are essential for the performance
> of large memory loads! Memory must be spread evenly otherwise memory
> accesses will overload a single node.

Sorry for making a noise. It was because I misunderstood NUMA.
Thought it should be not frequent because kernel's default memory allocation policy was
allocating from local node.

It would have been better If I asked why it uses much memory first, not writing code.
I'll be more careful next time. Thank you for reviewing!

Thanks,
Hyeonggon