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From: Noah Goldstein <goldstein.w.n@gmail.com>
To: Florian Weimer <fweimer@redhat.com>
Cc: libc-alpha@sourceware.org
Subject: Re: [PATCH 2/2] x86: Add generic CPUID data dumper to ld.so --list-diagnostics
Date: Mon, 11 Sep 2023 11:31:12 -0500	[thread overview]
Message-ID: <CAFUsyfJ-dSeOob-Ac6cMfVngFvTKi4BJK5XZLMpE+EHm9JEGbg@mail.gmail.com> (raw)
In-Reply-To: <CAFUsyfKyS3FA=m-dRY3o3d=oPc-e5=9zPDrCM72ou=4K4uk91Q@mail.gmail.com>

On Mon, Sep 11, 2023 at 11:28 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> On Mon, Sep 11, 2023 at 11:26 AM Florian Weimer <fweimer@redhat.com> wrote:
> >
> > * Noah Goldstein:
> >
> > > On Sun, Sep 10, 2023 at 11:24 PM Florian Weimer <fweimer@redhat.com> wrote:
> > >>
> > >> * Noah Goldstein:
> > >>
> > >> > On Fri, Sep 8, 2023 at 3:10 PM Florian Weimer <fweimer@redhat.com> wrote:
> > >> >>
> > >> >> This is surprisingly difficult to implement if the goal is to produce
> > >> >> reasonably sized output.  With the current approaches to output
> > >> >> compression (suppressing zeros and repeated results between CPUs,
> > >> >> folding ranges of identical subleaves, dealing with the %ecx
> > >> >> reflection issue), the output is less than 600 KiB even for systems
> > >> >> with 256 threads.
> > >> >>
> > >> > Maybe should just output a complete json?
> > >>
> > >> JSON cannot directly represent 64-bit integers, so it would need some
> > >> custom transformation for other parts of the --list-diagnostics output.
> > >>
> > >> > Then users can pretty easily write scripts to extract the exact information
> > >> > they are after. Or or the dumper can be extended in the future to let
> > >> > the user specify fields/values to dump so it can be configured to be more
> > >> > reasonable?
> > >>
> > >> I'm not sure what is unreasonable about the current implementation?  I
> > >> complained about how hard it is getting the data and distilling it into
> > >> something that is not a gigantic data blob.
> > >>
> > >> To be clear, without only trivial zero-values suppression, brute-force
> > >> enumeration (cutting off at 512 subleaves) results in roughly 8 KiB of
> > >> raw data per *CPU*.  It's even larger for recent CPUs which have more of
> > >> the funny ECX behavior (where unsupported subleaves do not come back as
> > >> zero).
> > >
> > > Maybe I misunderstand but the commit message is saying a 256 core system
> > > dumps 600KB?
> >
> > For all CPUs, after hex encoding.  So that's more like 1,000 bytes of
> > data per CPU (and the 8 KiB number was just an estimate, for two ECX
> > runaways, if you have more of those the number grows quickly).
> >
> > > If so, that seems like a lot for a person to just grok hence why I'd
> > > favor a standardized format.
> >
> > Sure, this calls out for automated processing.  We have Python parsing
> > code in the testsuite, which could be repurposed.
> >
> > > If JSON isn't really feasible for technical reasons, however, so be it.
> >
> > There is that 53 bit problem, and we'd still have to use string keys for
> > the objects.
>
> We can't find meaningful names for the information? If not do we even
> want to dump it?
>
> Although I'm probably trivializing the problem, a quick glance at:
> https://github.com/google/cpu_features
> and there json dump skips everything but feature flags.

Thats not right actually, they dump itlb/cache info but don't do per-thread.
> >
> > Thanks,
> > Florian
> >

  reply	other threads:[~2023-09-11 16:31 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-09-08 20:09 [PATCH 1/2] elf: Wire up _dl_diagnostics_cpu_kernel Florian Weimer
2023-09-08 20:10 ` [PATCH 2/2] x86: Add generic CPUID data dumper to ld.so --list-diagnostics Florian Weimer
2023-09-10 19:56   ` Noah Goldstein
2023-09-11  4:24     ` Florian Weimer
2023-09-11 16:16       ` Noah Goldstein
2023-09-11 16:25         ` Florian Weimer
2023-09-11 16:28           ` Noah Goldstein
2023-09-11 16:31             ` Noah Goldstein [this message]
2023-09-11 17:48             ` Florian Weimer
2023-09-11 18:35               ` Noah Goldstein
2023-09-11 16:08   ` Adhemerval Zanella Netto
2023-09-11 16:19     ` Florian Weimer
2023-09-11 16:41       ` Adhemerval Zanella Netto
2024-02-09 19:08 [PATCH 0/2] Enhanced x86 CPU diagnostics Florian Weimer
2024-02-09 19:08 ` [PATCH 2/2] x86: Add generic CPUID data dumper to ld.so --list-diagnostics Florian Weimer
2024-02-09 19:34   ` H.J. Lu
2024-02-10  0:44     ` H.J. Lu

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