Re: [PATCH v2 2/4] mm: make the threshold of enabling THP configurable

From: Cong Wang
Date: Wed Jun 22 2011 - 07:08:14 EST

ä 2011å06æ22æ 14:32, David Rientjes åé:
On Wed, 22 Jun 2011, Cong Wang wrote:

Either way, this patch isn't needed since it has no benefit over doing it
through an init script.

If you were right, CONFIG_TRANSPARENT_HUGEPAGE_ALWAYS is not needed,
you can do it through an init script.

They are really two different things: config options like
for command line options when you want the _default_ behavior to be
specified. They could easily be done on the command line just as they can
be done in the config. They typically have far reaching consequences
depending on whether they are enabled or disabled and warrant the entry in
the config file.

This patch, however, is not making the heuristic any easier to work with;
in fact, if the default were ever changed or the value is changed on your
kernel, then certain kernels will have THP enabled by default and others
will not. That's why I suggested an override command line option like
transparent_hugepage=force to ignore any disabling heursitics either
present or future.

Actually, if we move this out of kernel, to user-space, everything
you worried will be solved by just changing the user-space code.
Just add the following pseudo code into your init script,

if [ $total_memory -lt 512 ]
echo never > /sys/kernel/mm/transparent_hugepage/enabled

BTW, "=force" makes people confused with "=always", like "=never"
actually means "=disabled".

If you were right, the 512M limit is not needed neither, you have
transparent_hugepage=never boot parameter and do the check of
512M later in an init script. (Actually, moving the 512M check to
user-space is really more sane to me.)

It's quite obvious that the default behavior intended by the author is
that it is defaulted off for systems with less than 512M of memory.
Obfuscating that probably isn't a very good idea, but I'm always in favor
of command lines that allow users to override settings when they really do
know better.

The better way to express this is to add one line in Kconfig help said
"Please set CONFIG_THP_NEVER=y when you have less than 512M memory",
rather than enforcing a decision in code.

From either aspect, I don't think the current 512M check code in kernel
is a good thing.

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