linux-kernel - Re: [RFC PATCH v1 0/5] Alternative mTHP swap allocator improvements

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Date: Wed, 19 Jun 2024 15:19:46 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Ryan Roberts <ryan.roberts@....com>, Andrew Morton
 <akpm@...ux-foundation.org>
Cc: Chris Li <chrisl@...nel.org>,  Kairui Song <kasong@...cent.com>,  Kalesh
 Singh <kaleshsingh@...gle.com>,  Barry Song <baohua@...nel.org>,  "Hugh
 Dickins" <hughd@...gle.com>,  David Hildenbrand <david@...hat.com>,
  <linux-kernel@...r.kernel.org>,  <linux-mm@...ck.org>
Subject: Re: [RFC PATCH v1 0/5] Alternative mTHP swap allocator improvements

Hi, Ryan,

Ryan Roberts <ryan.roberts@....com> writes:

> Hi All,
>
> Chris has been doing great work at [1] to clean up my mess in the mTHP swap
> entry allocator.

I don't think the original behavior is something like mess.  It's just
the first step in the correct direction.  It's straightforward and
obviously correctly.  Then, we can optimize it step by step with data to
justify the increased complexity.

> But Barry posted a test program and results at [2] showing that
> even with Chris's changes, there are still some fallbacks (around 5% - 25% in
> some cases). I was interested in why that might be and ended up putting this PoC
> patch set together to try to get a better understanding. This series ends up
> achieving 0% fallback, even with small folios ("-s") enabled. I haven't done
> much testing beyond that (yet) but thought it was worth posting on the strength
> of that result alone.
>
> At a high level this works in a similar way to Chris's series; it marks a
> cluster as being for a particular order and if a new cluster cannot be allocated
> then it scans through the existing non-full clusters. But it does it by scanning
> through the clusters rather than assembling them into a list. Cluster flags are
> used to mark clusters that have been scanned and are known not to have enough
> contiguous space, so the efficiency should be similar in practice.
>
> Because its not based around a linked list, there is less churn and I'm
> wondering if this is perhaps easier to review and potentially even get into
> v6.10-rcX to fix up what's already there, rather than having to wait until v6.11
> for Chris's series? I know Chris has a larger roadmap of improvements, so at
> best I see this as a tactical fix that will ultimately be superseeded by Chris's
> work.

I don't think we need any mTHP swap entry allocation optimization to go
into v6.10-rcX.  There's no functionality or performance regression.
Per my understanding, we merge optimization when it's ready.

Hi, Andrew,

Please correct me if you don't agree.

[snip]

--
Best Regards,
Huang, Ying