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Date: Fri, 30 Mar 2018 14:17:25 +0200
From: Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 00/11] Use global pages with PTI
Message-ID: <20180330121725.zcklh36ulg7crydw@gmail.com>
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To: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, Linus Torvalds <torvalds@linux-foundation.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Andrea Arcangeli <aarcange@redhat.com>, Andrew Lutomirski <luto@kernel.org>, Kees Cook <keescook@google.com>, Hugh Dickins <hughd@google.com>, =?iso-8859-1?Q?J=FCrgen_Gro=DF?= <jgross@suse.com>, the arch/x86 maintainers <x86@kernel.org>, namit@vmware.com


* Ingo Molnar <mingo@kernel.org> wrote:

> > No Global pages (baseline): 186.951 seconds time elapsed  ( +-  0.35% )
> > 28 Global pages (this set): 185.756 seconds time elapsed  ( +-  0.09% )
> >                              -1.195 seconds (-0.64%)
> > 
> > Lower is better here, obviously.
> > 
> > I also re-checked everything using will-it-scale's llseek1 test[2] which
> > is basically a microbenchmark of a halfway reasonable syscall.  Higher
> > here is better.
> > 
> > No Global pages (baseline): 15783951 lseeks/sec
> > 28 Global pages (this set): 16054688 lseeks/sec
> > 			     +270737 lseeks/sec (+1.71%)
> > 
> > So, both the kernel compile and the microbenchmark got measurably faster.
> 
> Ok, cool, this is much better!
> 
> Mind re-sending the patch-set against latest -tip so it can be merged?
> 
> At this point !PCID Intel hardware is not a primary concern, if something bad 
> happens on them with global pages we can quirk global pages off on them in some 
> way, or so.

BTW., the expectation on !PCID Intel hardware would be for global pages to help 
even more than the 0.6% and 1.7% you measured on PCID hardware: PCID already 
_reduces_ the cost of TLB flushes - so if there's not even PCID then global pages 
should help even more.

In theory at least. Would still be nice to measure it.

Thanks,

	Ingo