linux-kernel - Re: [PATCH 1/1] mm/khugepaged: reduce process visible downtime by pre-zeroing hugepage

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Message-ID: <ef409d5e-5652-4fff-933c-49bda6d75018@redhat.com>
Date: Mon, 11 Mar 2024 17:19:26 +0100
From: David Hildenbrand <david@...hat.com>
To: Lance Yang <ioworker0@...il.com>, akpm@...ux-foundation.org
Cc: mhocko@...e.com, zokeefe@...gle.com, shy828301@...il.com,
 xiehuan09@...il.com, songmuchun@...edance.com, minchan@...nel.org,
 linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] mm/khugepaged: reduce process visible downtime by
 pre-zeroing hugepage

On 08.03.24 08:49, Lance Yang wrote:
> The patch reduces the process visible downtime during hugepage
> collapse. This is achieved by pre-zeroing the hugepage before
> acquiring mmap_lock(write mode) if nr_pte_none >= 256, without
> affecting the efficiency of khugepaged.
> 
> On an Intel Core i5 CPU, the process visible downtime during
> hugepage collapse is as follows:
> 
> | nr_ptes_none  | w/o __GFP_ZERO | w/ __GFP_ZERO  |  Change |
> --------------------------------------------------—----------
> |      511      |     233us      |      95us      |  -59.21%|
> |      384      |     376us      |     219us      |  -41.20%|
> |      256      |     421us      |     323us      |  -23.28%|
> |      128      |     523us      |     507us      |   -3.06%|
> 
> Of course, alloc_charge_hpage() will take longer to run with
> the __GFP_ZERO flag.
> 
> |       Func           | w/o __GFP_ZERO | w/ __GFP_ZERO |
> |----------------------|----------------|---------------|
> | alloc_charge_hpage   |      198us     |      295us    |
> 
> But it's not a big deal because it doesn't impact the total
> time spent by khugepaged in collapsing a hugepage. In fact,
> it would decrease.

It does look sane to me and not overly complicated.

But, it's an optimization really only when we have quite a bunch of 
pte_none(), possibly repeatedly so that it really makes a difference.

Usually, when we repeatedly collapse that many pte_none() we're just 
wasting a lot of memory and should re-evaluate life choices :)

So my question is: do we really care about it that much that we care to 
optimize?

But again, LGTM.

-- 
Cheers,

David / dhildenb