linux-kernel - Re: [PATCH v5 4/4] mm: Introduce per-thpsize swapin control policy

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Message-ID: <CAGsJ_4zRLH_W=2fduujTt6Jh41+ZaLNkm7a_1kDorhCLjkbbAw@mail.gmail.com>
Date: Mon, 29 Jul 2024 16:49:32 +1200
From: Barry Song <21cnbao@...il.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, ying.huang@...el.com, 
	baolin.wang@...ux.alibaba.com, chrisl@...nel.org, david@...hat.com, 
	hannes@...xchg.org, hughd@...gle.com, kaleshsingh@...gle.com, 
	kasong@...cent.com, linux-kernel@...r.kernel.org, mhocko@...e.com, 
	minchan@...nel.org, nphamcs@...il.com, ryan.roberts@....com, 
	senozhatsky@...omium.org, shakeel.butt@...ux.dev, shy828301@...il.com, 
	surenb@...gle.com, v-songbaohua@...o.com, xiang@...nel.org, 
	yosryahmed@...gle.com
Subject: Re: [PATCH v5 4/4] mm: Introduce per-thpsize swapin control policy

On Mon, Jul 29, 2024 at 3:52 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Fri, Jul 26, 2024 at 09:46:18PM +1200, Barry Song wrote:
> > A user space interface can be implemented to select different swap-in
> > order policies, similar to the mTHP allocation order policy. We need
> > a distinct policy because the performance characteristics of memory
> > allocation differ significantly from those of swap-in. For example,
> > SSD read speeds can be much slower than memory allocation. With
> > policy selection, I believe we can implement mTHP swap-in for
> > non-SWAP_SYNCHRONOUS scenarios as well. However, users need to understand
> > the implications of their choices. I think that it's better to start
> > with at least always never. I believe that we will add auto in the
> > future to tune automatically, which can be used as default finally.
>
> I strongly disagree.  Use the same sysctl as the other anonymous memory
> allocations.

In versions v1-v4, we used the same controls as anonymous memory allocations.
Ying expressed concerns that this approach isn't always ideal, especially for
non-zRAM devices, as SSD read speeds can be much slower than memory
allocation. I think his concern is reasonable to some extent.

However, this patchset only addresses scenarios involving zRAM-like devices
and will not impact SSDs. I would like to get Ying's feedback on whether
it's acceptable to drop this one in v6.

Thanks
Barry