LWN: Comments on "Triggering huge-page collapse from user space"

Triggering huge-page collapse from user space

jezuch — Tue, 22 Mar 2022 12:53:25 +0000

FWIW I immediately understood the intended meaning. Caveat: I'm not a native speaker and I read way too much about astrophysics :)

Triggering huge-page collapse from user space

zokeefe — Fri, 18 Mar 2022 15:27:19 +0000

Thanks for the kind writeup! :)

> This call will attempt to collapse length bytes of memory beginning at begin into huge pages. There does not appear to be any specific alignment requirement for those parameters, even though huge pages do have alignment requirements.

I should call out what happens when parameters are passed that don't align with architecture hugepage size/alignment; thanks for pointing out that I didn't mention this anywhere.

> If begin points to a base page in the middle of the address range that the huge page containing it will cover, then pages before begin will become part of the result. In other words, begin will be aligned backward to the proper beginning address for the containing huge page. The same is true for length, which will be increased if necessary to encompass a full huge page."

A small correction: the opposite actually happens; we forward align the start and backward align the end. Else, we'd have to make a decision on what to do if the new range fell outside the VMA(s). IOW, we clamp the provided range(s) to be hugepage aligned/sized.

Triggering huge-page collapse from user space

zokeefe — Fri, 18 Mar 2022 15:08:40 +0000

I thought about "coalesce", but "collapse" is just what the operation is ubiquitously referred to in the kernel and existing apis (e.g. /sys/kernel/mm/transparent_hugepage/khugepaged/pages_collapsed) and stats (e.g. /proc/vmstat:thp_collapse_alloc[_failed]).

Triggering huge-page collapse from user space

pitb0ss — Fri, 18 Mar 2022 13:34:34 +0000

Perhaps coalesce is another good word

Triggering huge-page collapse from user space

ncm — Fri, 18 Mar 2022 01:17:06 +0000

The most apt English word is "consolidate".

(That did not stop business reporters from preferring "conglomerate", some decades back.)

Triggering huge-page collapse from user space

mtaht — Wed, 16 Mar 2022 00:44:40 +0000

aggregate, rather than "collapse" might be a better word.

Triggering huge-page collapse from user space

Wol — Tue, 15 Mar 2022 08:16:23 +0000

Think of the energy. It takes an release of energy to make things explode, a minimal flow of energy to make things collapse, while an implosion gives of a blast of energy.

So I guess collapse is the right word here. There's a minimal change observable from outside the system, while what was there is still there, just looking a lot smaller because all the empty space has been squeezed out.

Cheers,
Wol

Triggering huge-page collapse from user space

edeloget — Tue, 15 Mar 2022 07:01:50 +0000

Both are true. Some programs are perfectly happy with a lot of small, non-contiguous virtual memory ranges because they never do large allocations.

Triggering huge-page collapse from user space

Nikratio — Tue, 15 Mar 2022 05:49:43 +0000

"Processes do not always need large, virtually contiguous memory ranges," - that should be "physically", not "virtually" I think?

Triggering huge-page collapse from user space

Karellen — Tue, 15 Mar 2022 00:41:43 +0000

I suppose it's the difference between a solid object collapsing into individual pieces, like a wall collapsing into a disconnected pile of bricks; or a sparsely connected group of elements collapsing into a single mass, like a house of cards collapsing into a dense pile of cards.

Do things generally collapse outwards, or inwards?

Triggering huge-page collapse from user space

flussence — Mon, 14 Mar 2022 20:47:30 +0000

I'd just think of it like pulling down a shelving unit to free up space for larger items; you're removing structure after all.

In terms of difficulty I'd say huge pages lies somewhere between rearranging occupied shelves without breaking the contents and safely containing a black hole…

Triggering huge-page collapse from user space

droundy — Mon, 14 Mar 2022 17:23:02 +0000

Me too! It's a little more like a black hole collapse...

Triggering huge-page collapse from user space

Sesse — Mon, 14 Mar 2022 16:57:15 +0000

I originally assumed “collapse” meant that the huge page would collapse and splinter into individual pages, but no, it's the opposite!