linux-kernel - Re: [PATCH] x86: Remove hpet vclock support

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Date:	Tue, 4 Feb 2014 21:29:40 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...capital.net>
cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Remove hpet vclock support

On Tue, 4 Feb 2014, Andy Lutomirski wrote:
> On Tue, Feb 4, 2014 at 11:31 AM, Thomas Gleixner <tglx@...utronix.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
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