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

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Message-ID: <874j7zfqkk.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Mon, 05 Aug 2024 14:10:03 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Matthew Wilcox <willy@...radead.org>, Christoph Hellwig <hch@...radead.org>
Cc: Barry Song <21cnbao@...il.com>,  akpm@...ux-foundation.org,
  linux-mm@...ck.org,  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

Matthew Wilcox <willy@...radead.org> writes:

> 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.

I still believe we have some reasons for this tunable.

1. As Ryan pointed out in [1], swap-in with large mTHP orders may cause
   long latency, which some users might want to avoid.

[1] https://lore.kernel.org/lkml/f0c7f061-6284-4fe5-8cbf-93281070895b@arm.com/

2. We have readahead information available for swap-in, which is
   unavailable for anonymous page allocation.  This enables us to build
   an automatic swap-in order policy similar to that for page cache
   order based on readahead.

3. Swap-out/swap-in cycles present an opportunity to identify hot pages.
   In many use cases, we can utilize mTHP for hot pages and order-0 page
   for cold pages, especially under memory pressure.  When an mTHP has
   been swapped out, it indicates that it could be a cold page.
   Converting it to order-0 pages might be a beneficial policy.

--
Best Regards,
Huang, Ying