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SubjectRe: [PATCH] x86: fix /proc/meminfo DirectMap

* Hugh Dickins <hugh@veritas.com> wrote:

> On Fri, 15 Aug 2008, Andi Kleen wrote:
> > On Fri, Aug 15, 2008 at 01:58:32PM +0100, Hugh Dickins wrote:
> > > Do we actually want these DirectMap lines in the x86 /proc/meminfo?
> > > I can see they're interesting to CPA developers and TLB optimizers,
> > > but they don't fit its usual "where has all my memory gone?" usage.
> >
> > It was intended for the "why is my computer going slower" usage.
>
> Yes, that's what I meant by the TLB optimizers. But it's going to be
> a fractional effect, isn't it, when you're trying to get the last 1%
> out of the machine? And in such a case, you might wonder more what
> all the 4k ones are actually being used for (no problem at all if
> they've ended up behind vmalloced module text).

i cannot see any performance difference myself between 2MB and 1GB TLBs.

There are measurements that Andi Kleen did originally in this commit:

commit 8346ea17aa20e9864b0f7dc03d55f3cd5620b8c1
Author: Andi Kleen <andi@firstfloor.org>
Date: Wed Mar 12 03:53:32 2008 +0100

x86: split large page mapping for AMD TSEG

[lower is better]
no split stddev split stddev delta
Elapsed Time 87.146 (0.727516) 84.296 (1.09098) -3.2%
User Time 274.537 (4.05226) 273.692 (3.34344) -0.3%
System Time 34.907 (0.42492) 34.508 (0.26832) -1.1%
Percent CPU 322.5 (38.3007) 326.5 (44.5128) +1.2%

=> About 3.2% improvement in elapsed time for kernbench.
[...]

meanwhile i have Barcelona class hardware myself and i cannot reproduce
these claimed improvements in kernbench performance. gbpages versus
no-gbpages results are dead on the same, within statistical noise.

( i'm sure it could make some difference in synthetic user-space
workloads - but gbpages are not exposed to user-space anyway. )

Ingo


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