Re: [PATCH 1/3] mm: completely disable THP by transparent_hugepage=never

From: Cong Wang
Date: Tue Jun 21 2011 - 00:09:02 EST

ä 2011å06æ21æ 03:21, Andrea Arcangeli åé:
On Mon, Jun 20, 2011 at 02:25:58PM -0400, Vivek Goyal wrote:
So I see some opprotunity there to save memory. But this 10kB
definitely sounds trivial amount to me.

Agree with you and Rik. Also I already avoided the big memory waste
(that for example isn't avoided in the ksmd and could be optimized
away without decreasing flexibility of KSM, and ksmd surely runs on
the kdump kernel too...) that is to make khugepaged exit and release
kernel stack when enabled=never (either done by sysfs or at boot with
transparent_hugepage=never) and all other structs associated with a
(temporarily) useless kernel thread.

I agree to disable ksm in kdump kernel, thanks for pointing this out!
I will look into later, and probably send a patch for this too.

The khugepaged_slab_init and mm_slot_hash_init() maybe could be
deferred to when khugepaged starts, and be released when it shutdown
but it makes it more tricky/racey. If you really want to optimize
that, without preventing to ever enable THP again despite all .text
was compiled in and ready to run. You will likely save more if you
make ksmd exit when run=0 (which btw is a much more common config than
enabled=never with THP). And slots hashes are allocated by ksm too so
you could optimize those too if you want and allocate them only by the
time ksmd starts.

The thing is that we can save ~10K by adding 3 lines of code as this
patch showed, where else in kernel can you save 10K by 3 lines of code?
(except some kfree() cases, of course) So, again, why not have it? ;)

As long as it'd still possible to enable the feature again as it is
possible now without noticing an altered behavior from userland, I'm
not entirely against optimizing for saving ~8k of ram even if it
increases complexity a bit (more kernel code will increase .text a bit
though, hopefully not 8k more of .text ;).

Why do we _force_ the feature to be tunable even when user completely
don't want to disable it? Why not provide a way to let the user to decide
which is better for him?

When programming kernel, providing a mechanism rather than a policy is
what I always keep in mind, I don't know why you violate this rule here,
to be honest. :-/

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