linux-kernel - Re: [PATCH v2 13/15] mm, swap: remove contention workaround for swap cache

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Message-ID: <2100bf81-e9c0-4440-8c19-b04acd3ced31@redhat.com>
Date: Mon, 8 Sep 2025 15:12:16 +0200
From: David Hildenbrand <david@...hat.com>
To: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 Chris Li <chrisl@...nel.org>, Barry Song <baohua@...nel.org>,
 Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>,
 Kemeng Shi <shikemeng@...weicloud.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Johannes Weiner <hannes@...xchg.org>, Yosry Ahmed <yosryahmed@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 linux-kernel@...r.kernel.org, kernel test robot <oliver.sang@...el.com>
Subject: Re: [PATCH v2 13/15] mm, swap: remove contention workaround for swap
 cache

On 05.09.25 21:13, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
> 
> Swap cluster setup will try to shuffle the clusters on initialization.
> It was helpful to avoid contention for the swap cache space. The cluster
> size (2M) was much smaller than each swap cache space (64M), so
> shuffling the cluster means the allocator will try to allocate swap
> slots that are in different swap cache spaces for each CPU, reducing the
> chance of two CPUs using the same swap cache space, and hence reducing
> the contention.
> 
> Now, swap cache is managed by swap clusters, this shuffle is pointless.
> Just remove it, and clean up related macros.
> 
> This also improves the HDD swap performance as shuffling IO is a bad
> idea for HDD, and now the shuffling is gone. Test have shown a ~40%
> performance gain for HDD [1]:
> 
> Doing sequential swap in of 8G data using 8 processes with usemem,
> average of 3 test runs:
> 
> Before: 1270.91 KB/s per process
> After:  1849.54 KB/s per process
> 
> Link: https://lore.kernel.org/linux-mm/CAMgjq7AdauQ8=X0zeih2r21QoV=-WWj1hyBxLWRzq74n-C=-Ng@mail.gmail.com/ [1]
> Reported-by: kernel test robot <oliver.sang@...el.com>
> Closes: https://lore.kernel.org/oe-lkp/202504241621.f27743ec-lkp@intel.com
> Signed-off-by: Kairui Song <kasong@...cent.com>
> Acked-by: Chris Li <chrisl@...nel.org>
> Reviewed-by: Barry Song <baohua@...nel.org>

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb