Re: fiemap is slow on btrfs on files with multiple extents

[Pavel Tikhomirov]

On 01.09.2022 16:25, Filipe Manana wrote:
> On Fri, Aug 05, 2022 at 10:54:07AM +0100, Filipe Manana wrote:
> > On Fri, Aug 05, 2022 at 04:38:21PM +0900, Dominique MARTINET wrote:
> > > Pavel Tikhomirov wrote on Thu, Aug 04, 2022 at 07:30:52PM +0300:
> > > > I see a similar problem here
> > > > https://lore.kernel.org/linux-btrfs/Yr4nEoNLkXPKcOBi@xxxxxxxxxxxxxxxxx/#r ,
> > > > but in my case I have "5.18.6-200.fc36.x86_64" fedora kernel which does not
> > > > have 5ccc944dce3d ("filemap: Correct the conditions for marking a folio as
> > > > accessed") commit, so it should be something else.
> > >
> > > The root cause might be different but I guess they're related enough: if
> > > fiemap gets faster enough even when the whole file is in cache I guess
> > > that works for me :)
> > >
> > > Josef Bacik wrote on Thu, Aug 04, 2022 at 02:49:39PM -0400:
> > > > On Thu, Aug 04, 2022 at 07:30:52PM +0300, Pavel Tikhomirov wrote:
> > > > > I ran the below test on Fedora 36 (the test basically creates "very" sparse
> > > > > file, with 4k data followed by 4k hole again and again for the specified
> > > > > length and uses fiemap to count extents in this file) and face the problem
> > > > > that fiemap hangs for too long (for instance comparing to ext4 version).
> > > > > Fiemap with 32768 extents takes ~37264 us and with 65536 extents it takes
> > > > > ~34123954 us, which is x1000 times more when file only increased twice the
> > > > > size:
> > > >
> > > > Ah that was helpful, thank you.  I think I've spotted the problem, please give
> > > > this a whirl to make sure we're seeing the same thing.  Thanks,
> > >
> > > FWIW this patch does help a tiny bit, but I'm still seeing a huge
> > > slowdown: with patch cp goes from ~600MB/s (55s) to 136MB/s (3m55s) on
> > > the second run; and without the patch I'm getting 47s and 5m35
> > > respectively so this has gotten a bit better but these must still be
> > > cases running through the whole list (e.g. when not hitting a hole?)
> > >
> > >
> > > My reproducer is just running 'cp file /dev/null' twice on a file with
> > > 194955 extents (same file with mixed compressed & non-compressed extents
> > > as last time), so should be close enough to what Pavel was describing in
> > > just much worse.
> >
> > I remember your original report Dominique, it came along with the short
> > reads issue when using using io_uring with qemu.
> >
> > I had a quick look before going on vacations. In your post at:
> >
> > https://lore.kernel.org/linux-btrfs/Ysace25wh5BbLd5f@xxxxxxxxxxxxxxxxx/
> >
> > you mentioned a lot of time spent on count_range_bits(), and I quickly
> > came with a testing patch for that specific area:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/fdmanana/linux.git/commit/?h=fiemap_speedup&id=6bdc02edbb52786df2d8c2405d790390d9a9443c
> >
> > Basically whenever we call that, we start searching from the root of the
> > extent states rbtree - if the rbtree is large, that takes a lot of time.
> > The idea is to start the search from the last record instead.
> >
> > I haven't actually made any performance tests, as vacations came in and
> > I noticed that such change will very likely make little or no difference
> > because algorithmically btrfs' fiemap implementation is very ineficient
> > for several reasons. It basically works like this:
> >
> > 1) We start the search for the first extent. First we go search the inode's
> >     extent map rbtree - if we can't find it, then we will search in the
> >     fs b+tree - after this we create an extent map based on the file extent
> >     item we found in the b+tree and add it to the extent map rbtree.
> >
> >     We then pass to fiemap extent information based on the extent map
> >     (there's a few extra minor details, like merging, etc);
> >
> > 2) Then we search for the next extent, with a start offset based on the
> >     end offset of the previous one +1.
> >
> >     Again, if we can't find it in the extent map rbtree, we go search the
> >     fs b+tree, then create an extent map based on the file extent item we
> >     found there and add it to extent map rbtree.
> >
> >     This is silly. On each iteration the extent maps rbtree gets bigger and
> >     bigger, and we always search from the root node. We are spending time
> >     searching there and then allocating memory for the extent map and adding
> >     it to the rbtree, which is yet more cpu time spent.
> >
> >     We should only create extent maps when we are doing IO against, for a
> >     data write or read operation, we are just spending a lot of time on
> >     this and consuming memory too.
> >
> >     Then it's silly again because we will search the fs b+tree again, starting
> >     from the root. So we end up visting the same leaves over and over;
> >
> > 3) Whenever we find a hole, or a prealloc/unwritten extent, we have to check
> >     if there's pending dealloc for that region. That's where count_range_bits()
> >     is used - everytime it's called it starts from the root node of the extent
> >     states rbtree.
> >
> > My idea to address this is to basically rewrite fiemap so that it works like
> > this:
> >
> > 1) Go over each leaf in the fs b+tree and for each file extent item emit the
> >     extent information for fiemap - like this we don't do many repeated b+tree
> >     searches to end up in the same leaf;
> >
> > 2) Never create extent maps, so that we don't grow the extent maps rbtree
> >     unnecessarily, saving cpu time and avoiding memory allocations;
> >
> > 3) Whenever we find a hole or prealloc/unwritten extent, then check if there's
> >     pending delalloc in the range by using count_range_bits() like we currently
> >     do (and maybe add that patch to avoid always starting the search from the
> >     root).
> >
> >     If there's delalloc, then lookup for the correspond extent maps and use
> >     their info to emit extent information for fiemap. And keep using rb_next()
> >     while an extent map ends before the hole/unwritten range;
> >
> > 4) Because emitting all the extent information for fiemap and doing other things
> >     like checking if an extent is shared, calling count_range_bits(), etc can
> >     take some time, to avoid holding a read lock for too long on the fs b+tree
> >     leaf and block other tasks, clone the leaf, release the lock on the leaf and
> >     use the private clone. This is fine since we start fiemap we lock the file
> >     range, so no one else can go and create or drop extents in the range before
> >     fiemap finishes.
> >
> > That's the high level idea.
> >
> > There's another factor that can slowdown fiemap a lot, which is figuring out if
> > an extent is shared or not (reflinks, snapshots), but in your case you don't
> > have shared extents IIRC. I would have to look at that separetely, we probably
> > have some room for improvement there as well.
> >
> > I haven't had the time to work on that, as I've been working on other stuff
> > unrelated to fiemap, but maybe in a week or two I may start it.
>
> It took me a bit more than I expected, but here is the patchset to make fiemap
> (and lseek) much more efficient on btrfs:
>
> https://lore.kernel.org/linux-btrfs/cover.1662022922.git.fdmanana@xxxxxxxx/
>
> And also available in this git branch:
>
> https://git.kernel.org/pub/scm/linux/kernel/git/fdmanana/linux.git/log/?h=lseek_fiemap_scalability
>
> Running Pavel's test before applying the patchset:
>
>      *********** 256M ***********
>
>      size: 268435456
>      actual size: 134217728
>      fiemap: fm_mapped_extents = 32768
>      time = 4003133 us
>
>      size: 268435456
>      actual size: 134217728
>      fiemap: fm_mapped_extents = 32768
>      time = 4895330 us
>
>      *********** 512M ***********
>
>      size: 536870912
>      actual size: 268435456
>      fiemap: fm_mapped_extents = 65536
>      time = 30123675 us
>
>      size: 536870912
>      actual size: 268435456
>      fiemap: fm_mapped_extents = 65536
>      time = 33450934 us
>
>      *********** 1G ***********
>
>      size: 1073741824
>      actual size: 536870912
>      fiemap: fm_mapped_extents = 131072
>      time = 224924074 us
>
>      size: 1073741824
>      actual size: 536870912
>      fiemap: fm_mapped_extents = 131072
>      time = 217239242 us
>
> And running it after applying the patchset:
>
>      *********** 256M ***********
>
>      size: 268435456
>      actual size: 134217728
>      fiemap: fm_mapped_extents = 32768
>      time = 29475 us
>
>      size: 268435456
>      actual size: 134217728
>      fiemap: fm_mapped_extents = 32768
>      time = 29307 us
>
>      *********** 512M ***********
>
>      size: 536870912
>      actual size: 268435456
>      fiemap: fm_mapped_extents = 65536
>      time = 58996 us
>
>      size: 536870912
>      actual size: 268435456
>      fiemap: fm_mapped_extents = 65536
>      time = 59115 us
>
>      *********** 1G ***********
>
>      size: 1073741824
>      actual size: 536870912
>      fiemap: fm_mapped_extents = 116251
>      time = 124141 us
>
>      size: 1073741824
>      actual size: 536870912
>      fiemap: fm_mapped_extents = 131072
>      time = 119387 us
>
> There's a huge difference, so after it fiemap is a lot more usable on
> btrfs.
>
> It's still not as fast as ext4, but it's getting close to. On ext4 I
> get:
>
>      *********** 256M ***********
>
>      size: 268435456
>      actual size: 134217728
>      fiemap: fm_mapped_extents = 32768
>      time = 16877 us
>
>      size: 268435456
>      actual size: 134217728
>      fiemap: fm_mapped_extents = 32768
>      time = 17014 us
>
>      *********** 512M ***********
>
>      size: 536870912
>      actual size: 268435456
>      fiemap: fm_mapped_extents = 65536
>      time = 33764 us
>
>      size: 536870912
>      actual size: 268435456
>      fiemap: fm_mapped_extents = 65536
>      time = 33849 us
>
>      *********** 1G ***********
>
>      size: 1073741824
>      actual size: 536870912
>      fiemap: fm_mapped_extents = 131072
>      time = 69085 us
>
>      size: 1073741824
>      actual size: 536870912
>      fiemap: fm_mapped_extents = 131072
>      time = 68101 us
>
> However we do have extra work to do on btrfs because we have reflinks
> and snapshots, so it needs to check if extents are shared, while ext4
> does not have those features, thus less work to do for fiemap.
>
> Thanks for the report.

The results are amassing, would try it on my system. Thanks a lot for 
the fixes!

> > > --
> > > Dominique

--
Best regards, Tikhomirov Pavel
Software Developer, Virtuozzo.