kernel-packages team mailing list archive

[Todd Vierling <tv@xxxxxxx>]

Based on what we've been seeing maintaining Oracle Linux, there are many
workloads for which THP results in a ~9-10% performance hit overall on
the system, and this includes kernels up through 3.8.13 (which we're
using as the baseline for a new kernel in that distro). It has to do
with the fact that the memory allocator uses the slowpath almost all the
time, rather than a faster path when memory fragmentation is low. It's
not that THP is _enabled_ so much that it is _compiled into the kernel
at all_; as a result, we're probably going to remove it for the time
being, and re-enable it when the allocator is revamped.

However, there is also good reason for choosing madvise as the default.
Transparent hugepages *cannot be swapped out*. Read that carefully: when
you use them, the allocated memory is always in-core, as if it had been
mlock()ed into place. While systems with plenty of memory shouldn't need
to worry about this (hell, I run my systems without swap at all!), it's
a consideration to keep in mind. Applications for which huge pages may
provide proven benefit, but aren't required, should start to adopt
MADV_HUGEPAGE as an advisory call; this might include databases, Web
browsers, and other data-intensive applications. That way, you don't run
the risk of hitting memory walls thanks to a bunch of smaller system
processes that just happen to use hugepages because it's set to
"always".

-- 
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https://bugs.launchpad.net/bugs/743688

Title:
  Transparent HugePages not enabled in 11.04 kernel

Status in “linux” package in Ubuntu:
  Confirmed

Bug description:
  Transparent hugepages, which is a major performance enhancement in the
  2.6.38 kernel is not enabled in the 11.04 kernel.:

  $ cat /etc/issue 
  Ubuntu natty (development branch) \n \l

  $ grep TRANSPARENT /boot/config-2.6.38-7-server
  # CONFIG_TRANSPARENT_HUGEPAGE is not set
  $ grep TRANSPARENT /boot/config-2.6.38-7-generic
  # CONFIG_TRANSPARENT_HUGEPAGE is not set
  $ 

  If this is the way 11.04 kernels will be built, it's a bummer.

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