[Discuss] khugepaged + vmware = massive CPU load

[dsr at randomstring.org (Dan Ritter)]

On Thu, Dec 10, 2015 at 09:31:13PM -0500, Daniel Barrett wrote:
> 
> I just spent 3 weeks trying to solve a performance problem that showed
> up when I upgraded to Ubuntu 15.10 (from 14.04). Here are the details
> in case anybody else runs into it.

...
 
> I don't understand much about khugepaged except that it's supposed to
> be (ironically) a performance optimization:
> 
>   http://www.phoronix.com/scan.php?page=article&item=linux_transparent_hugepages
> 
> Can anybody explain what might have been happening here, and whether I
> am losing anything important by disabling transparent hugepages?

Normally your system keeps track of memory in 4KB pages. They're
analogous to filesystem blocks. Performance is improved when you
can satisfy memory allocation requests with less overhead, so
hugepages were created. A hugepage is a 2MB page.

In the 2.x kernel series where they were invented, hugepages had
to be turned on explicitly (there's a sysctl) and applications
had to request them.

Transparent hugepages are an attempt to extend the benefits of
the reduced overhead even to applications that don't ask for
them. Guess what? This doesn't always work.

I have no transparent hugepages in use on any of the systems I
work on, and explicit hugepages are enabled for database servers
and some similar heavy lifters.

You aren't missing anything by turning them off.

-dsr-