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From: Jan Glauber <>
To: Arnd Bergmann <>
Cc: Catalin Marinas <>,
	Will Deacon <>,
	Linux ARM <>,
	Linux Kernel Mailing List <>
Subject: Re: [PATCH 2/2] arm64: defconfig: Raise NR_CPUS to 256
Date: Tue, 6 Mar 2018 15:02:01 +0100	[thread overview]
Message-ID: <20180306140201.GB7428@hc> (raw)
In-Reply-To: <>

On Tue, Mar 06, 2018 at 02:12:29PM +0100, Arnd Bergmann wrote:
> On Fri, Mar 2, 2018 at 3:37 PM, Jan Glauber <> wrote:
> > ThunderX1 dual socket has 96 CPUs and ThunderX2 has 224 CPUs.
> Are you sure about those numbers? From my counting, I would have expected
> twice that number in both cases: 48 cores, 2 chips and 2x SMT for ThunderX
> vs 52 Cores, 2 chips and 4x SMT for ThunderX2.

That's what I have on those machines. I counted SMT as normal CPUs as it
doesn't make a difference for the config. I've not seen SMT on ThunderX.

The ThunderX2 number of 224 is already with 4x SMT (and 2 chips) but
there may be other versions planned that I'm not aware of.

> > Therefore raise the default number of CPUs from 64 to 256
> > by adding an arm64 specific option to override the generic default.
> Regardless of what the correct numbers for your chips are, I'd like
> to hear some other opinions on how high we should raise that default
> limit, both in arch/arm64/Kconfig and in the defconfig file.
> As I remember it, there is a noticeable cost for taking the limit beyond
> BITS_PER_LONG, both in terms of memory consumption and also
> runtime performance (copying and comparing CPU masks).

OK, that explains the default. My unverified assumption is that
increasing the CPU masks wont be a noticable performance hit.

Also, I don't think that anyone who wants performance will use
defconfig. All server distributions would bump up the NR_CPUS anyway
and really small systems will probably need to tune the config

For me defconfig should produce a usable system, not with every last
driver configured but with all the basics like CPUs, networking, etc.
fully present.

> I'm sure someone will keep coming up with even larger configurations
> in the future, so we should try to decide how far we can take the
> defaults for the moment without impacting users of the smallest
> systems. Alternatively, you could add some measurements that
> show how much memory and CPU time is used up on a typical
> configuration for a small system (4 cores, no SMT, 512 MB RAM).
> If that's low enough, we could just do it anyway.

OK, I'll take a look.


  reply	other threads:[~2018-03-06 14:02 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-03-02 14:37 [PATCH 1/2] arm64: defconfig: enable THUNDER_NIC_VF Jan Glauber
2018-03-02 14:37 ` [PATCH 2/2] arm64: defconfig: Raise NR_CPUS to 256 Jan Glauber
2018-03-06 13:12   ` Arnd Bergmann
2018-03-06 14:02     ` Jan Glauber [this message]
2018-03-06 14:30       ` Arnd Bergmann
2018-03-26  8:52       ` Jan Glauber
2018-03-26  9:28         ` Arnd Bergmann
2018-03-26 10:02           ` Jan Glauber
2018-04-30  9:36 ` [PATCH 1/2] arm64: defconfig: enable THUNDER_NIC_VF Jan Glauber

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