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

[Shakeel Butt]

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 at linux.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