linux-kernel - Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole

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Message-ID: <emvsj7wfy24dzr6uxyac2qotp7nsdi7hnesihaldkvgo3mfzrf@u7fafr7mc3e7>
Date: Mon, 10 Jun 2024 13:43:10 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Chuanhua Han <chuanhuahan@...il.com>
Cc: Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>, 
	akpm@...ux-foundation.org, linux-mm@...ck.org, chengming.zhou@...ux.dev, chrisl@...nel.org, 
	david@...hat.com, hannes@...xchg.org, kasong@...cent.com, 
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org, mhocko@...e.com, nphamcs@...il.com, 
	shy828301@...il.com, steven.price@....com, surenb@...gle.com, 
	wangkefeng.wang@...wei.com, willy@...radead.org, xiang@...nel.org, ying.huang@...el.com, 
	yosryahmed@...gle.com, yuzhao@...gle.com, Chuanhua Han <hanchuanhua@...o.com>, 
	Barry Song <v-songbaohua@...o.com>
Subject: Re: [RFC PATCH v3 5/5] mm: support large folios swapin as a whole

On Thu, Mar 14, 2024 at 08:56:17PM GMT, Chuanhua Han wrote:
[...]
> >
> > So in the common case, swap-in will pull in the same size of folio as was
> > swapped-out. Is that definitely the right policy for all folio sizes? Certainly
> > it makes sense for "small" large folios (e.g. up to 64K IMHO). But I'm not sure
> > it makes sense for 2M THP; As the size increases the chances of actually needing
> > all of the folio reduces so chances are we are wasting IO. There are similar
> > arguments for CoW, where we currently copy 1 page per fault - it probably makes
> > sense to copy the whole folio up to a certain size.
> For 2M THP, IO overhead may not necessarily be large? :)
> 1.If 2M THP are continuously stored in the swap device, the IO
> overhead may not be very large (such as submitting bio with one
> bio_vec at a time).
> 2.If the process really needs this 2M data, one page-fault may perform
> much better than multiple.
> 3.For swap devices like zram,using 2M THP might also improve
> decompression efficiency.
>

Sorry for late response, do we have any performance data backing the
above claims particularly for zswap/swap-on-zram cases?