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

[Barry Song <21cnbao@gmail.com>]

On Wed, Jun 12, 2024 at 5:24 AM Shakeel Butt <shakeel.butt@linux.dev> wrote:
>
> 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@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?

I don’t possess data on 2MiB, and regrettably, I lack a server machine
for testing. However, I believe that this type of higher compression ratio
and lower CPU consumption generally holds true for generic anonymous
memory.

64KB is a right balance. But nothing can stop THP from using 64KB to
swapin, compression and decompression. as you can see from the
zram/zsmalloc series,  we actually have a configuration
CONFIG_ZSMALLOC_MULTI_PAGES_ORDER

The default value is 4.

That means a 2MB THP can be compressed/decompressed as 32 * 64KB.
If we use 64KB as the swapin granularity, we still have the balance and
all the benefits if 2MB is a too large swap-in granularity which might cause
memory waste.

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

Thanks
Barry