Messages in this thread | | | Date | Fri, 5 Aug 2022 16:38:21 +0900 | From | Dominique MARTINET <> | Subject | Re: fiemap is slow on btrfs on files with multiple extents |
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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.
-- Dominique
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