LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Jan Glauber <jan.glauber@caviumnetworks.com>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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: <CAK8P3a2ORBU-_7=JeCHSuM8YtC7zeO1VPxQzny_8BrpyVGCKKw@mail.gmail.com>

On Tue, Mar 06, 2018 at 02:12:29PM +0100, Arnd Bergmann wrote:
> On Fri, Mar 2, 2018 at 3:37 PM, Jan Glauber <jglauber@cavium.com> 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
anyway.

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.

--Jan

  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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20180306140201.GB7428@hc \
    --to=jan.glauber@caviumnetworks.com \
    --cc=arnd@arndb.de \
    --cc=catalin.marinas@arm.com \
    --cc=linux-arm-kernel@lists.infradead.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=will.deacon@arm.com \
    --subject='Re: [PATCH 2/2] arm64: defconfig: Raise NR_CPUS to 256' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).