Re: [PATCH 10/13] x86: mm: Enable deferred struct page initialisation on x86-64

From: Mel Gorman
Date: Fri Apr 24 2015 - 11:20:17 EST

On Fri, Apr 24, 2015 at 10:35:49AM -0400, Waiman Long wrote:
> On 04/23/2015 05:23 AM, Mel Gorman wrote:
> >On Wed, Apr 22, 2015 at 04:45:00PM -0700, Andrew Morton wrote:
> >>On Wed, 22 Apr 2015 18:07:50 +0100 Mel Gorman<mgorman@xxxxxxx> wrote:
> >>
> >>>--- a/arch/x86/Kconfig
> >>>+++ b/arch/x86/Kconfig
> >>>@@ -32,6 +32,7 @@ config X86
> >>> select ARCH_SUPPORTS_INT128 if X86_64
> >>Put this in the "config X86_64" section and skip the "X86_64&&"?
> >>
> >Done.
> >
> >>Can we omit the whole defer_meminit= thing and permanently enable the
> >>feature? That's simpler, provides better test coverage and is, we
> >>hope, faster.
> >>
> >Yes. The intent was to have a workaround if there were any failures like
> >Waiman's vmalloc failures in an earlier version but they are bugs that
> >should be fixed.
> >
> >>And can this be used on non-NUMA? Presumably that won't speed things
> >>up any if we're bandwidth limited but again it's simpler and provides
> >>better coverage.
> >Nothing prevents it. There is less opportunity for parallelism but
> >improving coverage is desirable.
> >
> Memory access latency can be more than double for local vs. remote
> node memory. Bandwidth can also be much lower depending on what kind
> of interconnect is between the 2 nodes. So it is better to do it in
> a NUMA-aware way.

I do not believe that is what he was asking. He was asking if we could
defer memory initialisation even when there is only one node. It does not
gain much in terms of boot times but it improves testing coverage.

> Within a NUMA node, however, we can split the
> memory initialization to 2 or more local CPUs if the memory size is
> big enough.

I considered it but discarded the idea. It'd be more complex to setup and
the two CPUs could simply end up contending on the same memory bus as
well as contending on zone->lock.

Mel Gorman
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