Message-ID: <cbfaeb40-13c7-15b6-177d-07b3f3c4a0a9@suse.cz>
Date:   Tue, 14 Dec 2021 18:24:58 +0100
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To:     Christoph Lameter <cl@gentwo.org>,
        Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc:     Matthew Wilcox <willy@infradead.org>,
        Christoph Lameter <cl@gentwo.de>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>
References: <20211017042852.GA3050@kvm.asia-northeast3-a.c.our-ratio-313919.internal>
 <20211017133618.GA7989@kvm.asia-northeast3-a.c.our-ratio-313919.internal>
 <20211017135708.GA8442@kvm.asia-northeast3-a.c.our-ratio-313919.internal>
 <YWw1n6y/AGED14HD@casper.infradead.org>
 <CAB=+i9Tor-tmZuB8YjATT_rv68nnF2W_TvMvyGp55AGaSyKynw@mail.gmail.com>
 <alpine.DEB.2.22.394.2110251016260.3145@gentwo.de>
 <20211028100414.GA2928@kvm.asia-northeast3-a.c.our-ratio-313919.internal>
 <YXqSoo+b9RTclW/2@casper.infradead.org> <20211210110835.GA632811@odroid>
 <alpine.DEB.2.22.394.2112101305500.290446@gentwo.de>
From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: Do we really need SLOB nowdays?
In-Reply-To: <alpine.DEB.2.22.394.2112101305500.290446@gentwo.de>
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On 12/10/21 13:06, Christoph Lameter wrote:
> On Fri, 10 Dec 2021, Hyeonggon Yoo wrote:
> 
>> > > (But I still have doubt if we can run linux on machines like that.)
>> >
>> > I sent you a series of articles about making Linux run in 1MB.
>>
>> After some time playing with the size of kernel,
>> I was able to run linux in 6.6MiB of RAM. and the SLOB used
>> around 300KiB of memory.
> 
> What is the minimal size you need for SLUB?
 
Good question. Meanwhile I tried to compare Slab: in /proc/meminfo on a virtme run:
virtme-run --mods=auto --kdir /home/vbabka/wrk/linux/ --memory 2G,slots=2,maxmem=4G --qemu-opts --smp 4

Got ~30800kB with SLOB, 34500kB with SLUB without DEBUG and PERCPU_PARTIAL.
Then did a quick and dirty patch (below) to never load c->slab in
___slab_alloc() and got to 32200kB. Fiddling with
slub_min_order/slub_max_order didn't actually help, probably due to causing
more internal fragmentation.

So that's relatively close, but on a really small system the difference can
be possibly more prominent. Also my test doesn't account for text/data or
percpu usage differences.

diff --git a/mm/slub.c b/mm/slub.c
index 68aa112e469b..fd9c853971d1 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -3054,6 +3054,8 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node,
 		 */
 		goto return_single;
 
+	goto return_single;
+
 retry_load_slab:
 
 	local_lock_irqsave(&s->cpu_slab->lock, flags);