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

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Date: Tue, 11 Jun 2024 10:24:17 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Barry Song <21cnbao@...il.com>
Cc: Chuanhua Han <chuanhuahan@...il.com>, 
	Ryan Roberts <ryan.roberts@....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 Tue, Jun 11, 2024 at 12:23:41PM GMT, Barry Song wrote:
> On Tue, Jun 11, 2024 at 8:43 AM Shakeel Butt <shakeel.butt@...ux.dev> wrote:
> >
> > 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?
> 
> no need to say sorry. You are always welcome to give comments.
> 
> this, combining with zram modification, not only improves compression
> ratio but also reduces CPU time significantly. you may find some data
> here[1].
> 
> granularity   orig_data_size   compr_data_size   time(us)
> 4KiB-zstd      1048576000       246876055        50259962
> 64KiB-zstd     1048576000       199763892        18330605
> 
> On mobile devices, We tested the performance of swapin by running
> 100 iterations of swapping in 100MB of data ,and the results were
> as follows.the swapin speed increased by about 45%.
> 
>                 time consumption of swapin(ms)
> lz4 4k                  45274
> lz4 64k                 22942
> 
> zstdn 4k                85035
> zstdn 64k               46558

Thanks for the response. Above numbers are actually very fascinating and
counter intuitive (at least to me). Do you also have numbers for 2MiB
THP? I am assuming 64k is the right balance between too small or too
large. Did you experiment on server machines as well?

> 
> [1] https://lore.kernel.org/linux-mm/20240327214816.31191-1-21cnbao@gmail.com/
> 
> Thanks
> Barry