Re: [PATCHv3 15/41] filemap: handle huge pages in do_generic_file_read()

Jan Kara
2016-11-03T20:40:12Z

On Wed 02-11-16 11:32:04, Kirill A. Shutemov wrote:
> On Tue, Nov 01, 2016 at 05:39:40PM +0100, Jan Kara wrote:
> > On Mon 31-10-16 21:10:35, Kirill A. Shutemov wrote:
> > > > If I understand the motivation right, it is mostly about being able to mmap
> > > > PMD-sized chunks to userspace. So my naive idea would be that we could just
> > > > implement it by allocating PMD sized chunks of pages when adding pages to
> > > > page cache, we don't even have to read them all unless we come from PMD
> > > > fault path.
> > >
> > > Well, no. We have one PG_{uptodate,dirty,writeback,mappedtodisk,etc}
> > > per-hugepage, one common list of buffer heads...
> > >
> > > PG_dirty and PG_uptodate behaviour inhered from anon-THP (where handling
> > > it otherwise doesn't make sense) and handling it differently for file-THP
> > > is nightmare from maintenance POV.
> >
> > But the complexity of two different page sizes for page cache and *each*
> > filesystem that wants to support it does not make the maintenance easy
> > either.
>
> I think with time we can make small pages just a subcase of huge pages.
> And some generalization can be made once more than one filesystem with
> backing storage will adopt huge pages.

My objection is that IMHO currently the code is too ugly to go in. Too many
places need to know about THP and I'm not even sure you have patched all
the places or whether some corner cases remained unfixed and how should I
find that out.

> > So I'm not convinced that using the same rules for anon-THP and
> > file-THP is a clear win.
>
> We already have file-THP with the same rules: tmpfs. Backing storage is
> what changes the picture.

Right, the ugliness comes from access to backing storage having to deal
with huge pages.

> > I'd also note that having PMD-sized pages has some obvious disadvantages as
> > well:
> >
> > 1) I'm not sure buffer head handling code will quite scale to 512 or even
> > 2048 buffer_heads on a linked list referenced from a page. It may work but
> > I suspect the performance will suck.
>
> Yes, buffer_head list doesn't scale. That's the main reason (along with 4)
> why syscall-based IO sucks. We spend a lot of time looking for desired
> block.
>
> We need to switch to some other data structure for storing buffer_heads.
> Is there a reason why we have list there in first place?
> Why not just array?
>
> I will look into it, but this sounds like a separate infrastructure change
> project.

As Christoph said iomap code should help you with that and make things
simpler. If things go as we imagine, we should be able to pretty much avoid
buffer heads. But it will take some time to get there.

> > 2) PMD-sized pages result in increased space & memory usage.
>
> Space? Do you mean disk space? Not really: we still don't write beyond
> i_size or into holes.
>
> Behaviour wrt to holes may change with mmap()-IO as we have less
> granularity, but the same can be seen just between different
> architectures: 4k vs. 64k base page size.

Yes, I meant different granularity of mmap based IO. And I agree it isn't a
new problem but the scale of the problem is much larger with 2MB pages than
with say 64K pages. And actually the overhead of higher IO granularity of
64K pages has been one of the reasons we have switched SLES PPC kernels
from 64K pages to 4K pages (we've got complaints from customers).

> > 3) In ext4 we have to estimate how much metadata we may need to modify when
> > allocating blocks underlying a page in the worst case (you don't seem to
> > update this estimate in your patch set). With 2048 blocks underlying a page,
> > each possibly in a different block group, it is a lot of metadata forcing
> > us to reserve a large transaction (not sure if you'll be able to even
> > reserve such large transaction with the default journal size), which again
> > makes things slower.
>
> I didn't saw this on profiles. And xfstests looks fine. I probably need to
> run them with 1k blocks once again.

You wouldn't see this in profiles - it is a correctness thing. And it won't
be triggered unless the file is heavily fragmented which likely does not
happen with any test in xfstests. If it happens you'll notice though - the
filesystem will just report error and shut itself down.

> The numbers below generated with fio. The working set is relatively small,
> so it fits into page cache and writing set doesn't hit dirty_ratio.
>
> I think the mmap performance should be enough to justify initial inclusion
> of an experimental feature: it useful for workloads that targets mmap()-IO.
> It will take time to get feature mature anyway.

I agree it will take time for feature to mature so I'me fine with
suboptimal performance in some cases. But I'm not fine with some of the
hacks you do currently because code maintenability is an issue even if
people don't actually use the feature...

> Configuration:
> - 2x E5-2697v2, 64G RAM;
> - INTEL SSDSC2CW24;
> - IO request size is 4k;
> - 8 processes, 512MB data set each;

The numbers indeed look interesting for mmaped case. Can you post the fio
cmdline? I'd like to compare profiles...

Honza
>
> Workload
> read/write baseline stddev huge=always stddev change
> --------------------------------------------------------------------------------
> sync-read
> read 21439.00 348.14 20297.33 259.62 -5.33%
> sync-write
> write 6833.20 147.08 3630.13 52.86 -46.88%
> sync-readwrite
> read 4377.17 17.53 2366.33 19.52 -45.94%
> write 4378.50 17.83 2365.80 19.94 -45.97%
> sync-randread
> read 5491.20 66.66 14664.00 288.29 167.05%
> sync-randwrite
> write 6396.13 98.79 2035.80 8.17 -68.17%
> sync-randrw
> read 2927.30 115.81 1036.08 34.67 -64.61%
> write 2926.47 116.45 1036.11 34.90 -64.60%
> libaio-read
> read 254.36 12.49 258.63 11.29 1.68%
> libaio-write
> write 4979.20 122.75 2904.77 17.93 -41.66%
> libaio-readwrite
> read 2738.57 142.72 2045.80 4.12 -25.30%
> write 2729.93 141.80 2039.77 3.79 -25.28%
> libaio-randread
> read 113.63 2.98 210.63 5.07 85.37%
> libaio-randwrite
> write 4456.10 76.21 1649.63 7.00 -62.98%
> libaio-randrw
> read 97.85 8.03 877.49 28.27 796.80%
> write 97.55 7.99 874.83 28.19 796.77%
> mmap-read
> read 20654.67 304.48 24696.33 1064.07 19.57%
> mmap-write
> write 8652.33 272.44 13187.33 499.10 52.41%
> mmap-readwrite
> read 6620.57 16.05 9221.60 399.56 39.29%
> write 6623.63 16.34 9222.13 399.31 39.23%
> mmap-randread
> read 6717.23 1360.55 21939.33 326.38 226.61%
> mmap-randwrite
> write 3204.63 253.66 12371.00 61.49 286.03%
> mmap-randrw
> read 2150.50 78.00 7682.67 188.59 257.25%
> write 2149.50 78.00 7685.40 188.35 257.54%
>
> --
> Kirill A. Shutemov
--
Jan Kara <jack@suse.com>
SUSE Labs, CR