From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: X86 ML <x86@kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: Remove hpet vclock support
Date: Tue, 4 Feb 2014 21:29:40 +0100 (CET) [thread overview]
Message-ID: <alpine.DEB.2.02.1402042126090.24986@ionos.tec.linutronix.de> (raw)
In-Reply-To: <CALCETrUwdS9c1RgWLN4Wvimejh56N0tnCmgsSgPKGv5bVCn8ZQ@mail.gmail.com>
On Tue, 4 Feb 2014, Andy Lutomirski wrote:
> On Tue, Feb 4, 2014 at 11:31 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> > On Fri, 31 Jan 2014, Andy Lutomirski wrote:
> >
> >> The HPET is so amazingly slow that this is barely a win. It adds
> >
> > That's nonsense. It's definitely a win to access HPET directly
> > especially on older systems with TSC wreckage. Why do you want to
> > enforce a syscall if we can read out the data straight from user
> > space. The systems which are forced to use HPET have slower syscalls
> > than those which have a proper TSC.
> >
>
> I'm actually curious whether anyone cares about this particular
> performance difference. On all my HPET systems, the actual HPET read
> takes ~500ns, whereas the overhead from a syscall is ~50ns. (This is
> ignoring the CONFIG_AUDIT_SYSCALL wreckage, which I'm trying to fix.)
> I certainly care about 10% performance changes in clock_gettime, but
> that's only because it's already fast enough to call very frequently.
> If it took anywhere near 500ns, I would just stop using it, so the
> 50ns difference wouldn't matter for my application.
>
> It's certainly true that, on older hardware, syscalls are slower, but
> I suspect that the HPET is at least as slow, and old enough hardware
> won't even have a usable HPET.
Well, on one reference system which is forced to use hpet the
systemcall overhead with your patch amounts with a real world
application to a whopping 20% versus the vdso based HPET access.
> On newish things (probably Nehalem and up that have non-buggy BIOS),
> HPET is AFAIK completely pointless.
True, but there is a world outside of the "we have access to the
latest hardware" universe. Linux has served that world very well and I
see no reason why we should not continue to do so.
Thanks,
tglx
prev parent reply other threads:[~2014-02-04 20:29 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-01-31 23:19 [PATCH] x86: Remove hpet vclock support Andy Lutomirski
2014-02-01 15:43 ` Clemens Ladisch
2014-02-01 17:10 ` Andy Lutomirski
2014-02-04 19:31 ` Thomas Gleixner
2014-02-04 19:41 ` Andy Lutomirski
2014-02-04 20:29 ` Thomas Gleixner [this message]
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