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

[Pavel Tikhomirov <ptikhomirov@virtuozzo.com>]

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@atmark-techno.com/#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@atmark-techno.com/
>>
>> 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@suse.com/
> 
> 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.