[Buildroot] [PATCH v2 1/1] erlang: remove non-SMP build option

[Yann E. MORIN <yann.morin.1998@free.fr>]

Frank, All,

On 2017-12-20 08:52 -0500, Frank Hunleth spake thusly:
> On Wed, Dec 20, 2017 at 8:28 AM, Yann E. MORIN <yann.morin.1998@free.fr> wrote:
> > On 2017-12-20 11:26 +0100, Thomas Petazzoni spake thusly:
> >> On Tue, 19 Dec 2017 13:45:44 -0500, Frank Hunleth wrote:
> >> > The non-SMP scheduler was deprecated with the Erlang/OTP 20.0 release and
> >> > slated for removal with the next major Erlang release. Since the non-SMP
> >> > scheduler isn't even built anymore, this option no longer has the
> >> > intended effect of saving space or compile time. The SMP scheduler
> >> > supports both SMP and non-SMP processors, so removing the option will
> >> > not break any platforms.
> >> >
> >> > Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
> >> > ---
> >> > Changes v1 -> v2:
> >> >   - Added text to Config.in.legacy to explain the removal of the option
> >> >     (suggested by Yann - I added to Config.in.legacy after thinking about
> >> >      it, since anyone who previously selected this option may be surprised
> >> >      to see the SMP scheduler in their process list.)
> >> >
> >> >  Config.in.legacy         | 10 ++++++++++
> >> >  package/erlang/Config.in | 10 ----------
> >> >  package/erlang/erlang.mk |  4 ----
> >> >  3 files changed, 10 insertions(+), 14 deletions(-)
> >> >
> >> > diff --git a/Config.in.legacy b/Config.in.legacy
> >> > index decbace..1607df6 100644
> >> > --- a/Config.in.legacy
> >> > +++ b/Config.in.legacy
> >> > @@ -153,6 +153,16 @@ config BR2_PACKAGE_GNUPG2_GPGV2
> >> >       The gpgv2 executable is now named gpgv. The config option
> >> >       has been renamed accordingly.
> >> >
> >> > +config BR2_PACKAGE_ERLANG_SMP
> >> > +   bool "erlang smp option removed"
> >> > +   select BR2_LEGACY
> >> > +   help
> >> > +     This option used to disable Erlang's SMP scheduler to save
> >> > +     space. Since Erlang 20.0 deprecated the non-SMP scheduler and
> >> > +     no longer builds it, it is no longer relevant. See the Erlang
> >> > +     20.0 release notes for more information. Note that the SMP
> >> > +     scheduler runs on both SMP and non-SMP CPUs.
> >>
> >> As Yann explained, we do not want a Config.in.legacy option. Indeed,
> >> the new behavior is identical to having the option enabled, so there is
> >> no point in having a Config.in.legacy entry.
> >
> > Yes, indeed.
> >
> > Frank, what I meant was  that, usually we have to add a legacy entry to
> > tell users what is happenning. However, in this particular case, it is
> > not needed because a build with this option enabled previously will now
> > yield the same result as  build now with this option removed.
> 
> That's not true. Before if you enabled the option, you'd get the
> uniprocessor scheduler. With the change, you get the SMP scheduler.
> 
> In the .mk file, if you were to enable BR2_PACKAGE_ERLANG_SMP, that
> would pass "--disable-smp-support" to the configure script.

That's wrong, because the code was:

    ifeq ($(BR2_PACKAGE_ERLANG_SMP),)
    ERLANG_CONF_OPTS += --disable-smp-support
    endif

If BR2_PACKAGE_ERLANG_SMP was set, it would not fulfill the condition,
and we would *not* pass --disable-smp-support. But if it was unset, then
the condition would be true, and we'd pass --disable-smp-support.

Now, with your change, we will no longer pass --disable-smp-support,
ever, so this is back to option enabled.

(note that we can't account for the case where the option was disabled;
that's a limitation, but we don't really care).

> That
> forces the uniprocessor scheduler. With this patch, you only can get
> the SMP scheduler. I'm not sure how many people would do this, but
> operationally, you'd see a process called "beam.smp" running on your
> system now. The uniprocessor scheduler made a process called "beam".
> That seems like a different result to me?
> 
> Looking over the Erlang release notes, the situation may even be worse
> now since they've added a --enable-plain-scheduler option to build the
> uniprocessor scheduler, so the current option BR2_PACKAGE_ERLANG_SMP
> option may not even work. I don't know, but the uniprocessor scheduler
> is not worth spending time on any more since it's going to be removed
> soon.

Well, depends. Are there erlang packages that require the UP scheduler?

From your commit log, it looks like the SMP scheduler can cope with
non-SMP stuff as well, so there is no risk of breakage.

Thus, I'm fine with not considering a UP option.

> > But this it is usualy to add a legacy entry, for the reviewr's sake, it
> > is important that the commit log explains why a legacy entry is not
> > needed.
> 
> I'm fine either way. I'm stuck in the thinking that this change isn't
> identical to the previous behavior, so I'm not sure how to explain it
> in the commit message. If someone indulges me with text, I'll gladly
> send a v3 or feel free to merge one of my other versions.

What about:

    erlang: remove non-SMP build option

    [your commit log]

    We do not need to add a legacy entry, because the new behaviour is
    the same as with the option previously set (i.e. SMP enabled).

    Signed-off-by: You.

Regards,
Yann E. MORIN.

> Frank
> 
> >
> > Sorry if I was not explicit enough about this...
> >
> > Regards,
> > Yann E. MORIN.
> >
> >> Best regards,
> >>
> >> Thomas
> >> --
> >> Thomas Petazzoni, CTO, Free Electrons
> >> Embedded Linux and Kernel engineering
> >> http://free-electrons.com
> >> _______________________________________________
> >> buildroot mailing list
> >> buildroot at busybox.net
> >> http://lists.busybox.net/mailman/listinfo/buildroot
> >
> > --
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-- 
.-----------------.--------------------.------------------.--------------------.
|  Yann E. MORIN  | Real-Time Embedded | /"\ ASCII RIBBON | Erics' conspiracy: |
| +33 662 376 056 | Software  Designer | \ / CAMPAIGN     |  ___               |
| +33 223 225 172 `------------.-------:  X  AGAINST      |  \e/  There is no  |
| http://ymorin.is-a-geek.org/ | _/*\_ | / \ HTML MAIL    |   v   conspiracy.  |
'------------------------------^-------^------------------^--------------------'