Re: huge tlb support

On Tuesday, August 21, 2012 05:56:58 PM Robert Haas wrote:
> On Tue, Aug 21, 2012 at 11:31 AM, Andres Freund <andres(at)2ndquadrant(dot)com>
wrote:
> > On Tuesday, August 21, 2012 05:30:28 PM Robert Haas wrote:
> >> On Thu, Aug 16, 2012 at 10:53 PM, David Gould <daveg(at)sonic(dot)net> wrote:
> >> > A warning, on RHEL 6.1 (2.6.32-131.4.1.el6.x86_64 #1 SMP) we have had
> >> > horrible problems caused by transparent_hugepages running postgres on
> >> > largish systems (128GB to 512GB memory, 32 cores). The system
> >> > sometimes goes 99% system time and is very slow and unresponsive to
> >> > the point of not successfully completing new tcp connections. Turning
> >> > off
> >> > transparent_hugepages fixes it.
> >>
> >> Yikes! Any idea WHY that happens?
Afair there were several bugs that could cause that in earlier version of the
hugepage feature. The prominent was something around never really stopping to
search for mergeable pages even though the probability was small or such.

I am not a rhel person, so I cannot directly interpret that kernel version, is
that the latest kernel?

> >> I'm inclined to think this torpedos any idea we might have of enabling
> >> hugepages automatically whenever possible. I think we should just add
> >> a GUC for this and call it good. If the state of the world improves
> >> sufficiently in the future, we can adjust, but I think for right now
> >> we should just do this in the simplest way possible and move on.
> >
> > He is talking about transparent hugepages not hugepages afaics.
>
> Hmm. I guess you're right. But why would it be different?
Because in this case explicit hugepage usage reduces the pain instead of
increasing it. And we cannot do much against transparent hugepages being
enabled by default.
Unless I misremember how things work the problem is/was independent of
anonymous mmap or sysv shmem.

Greetings,

Andres
--
Andres Freund http://www.2ndQuadrant.com/
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