On 12/23/20 3:31 PM, Holger Hoffstätte wrote:
On 2020-12-23 20:41, Josef Bacik wrote:
Could you give this a try? I'm not able to reproduce the problem, but
I'm
Since I wanted to rule out NVME/block layer/scheduler etc. I tried and
could reproduce it immediately, see below. Didn't notice it earlier since
most of btrfs is read-mostly.. :(
testing inside of a VM. I'm in the middle of Christmas stuff, but
I'll get
ahold of a giant machine at work tomorrow and see if I can reproduce
there.
Meanwhile can you give this a shot? I have a sneaking suspicion why
it happens
on your baremetal and not in VM's, and this will be a partial enough
of a revert
of the patch you bisected to validate what I'm thinking. THanks,
The patch doesn't apply to 5.10.x since btrfs_start_delalloc_roots()
does not
have the trailing true/false argument yet. I removed it, which seemed to
have
worked. :}
Results using -dsingle/-msingle/space tree, all on tmpfs:
Unpatched:
kernel tree, ~1.1G:
$time (cp -a /tmp/linux-5.10.3 /tmp/butter && sync -f /tmp/butter)
( cp -a /tmp/linux-5.10.3 /tmp/butter && sync -f /tmp/butter; ) 0.37s
user 3.26s system 6% cpu 52.144 total
-> slow as hell since it's thousands of small files. Writeback runs at
~5-10 MB/s.
large file:
$fallocate -l 2G /tmp/largefile
$time (cp -a /tmp/largefile /tmp/butter && sync -f /tmp/butter)
( cp -a /tmp/largefile /tmp/butter && sync -f /tmp/butter; ) 0.00s user
0.91s system 75% cpu 1.215 total
-> OK-ish since it's just one big file.
With your patch & the 'true' arg to btrfs_start_delalloc_roots() removed:
kernel tree:
$time (cp -a /tmp/linux-5.10.3 /tmp/butter && sync -f /tmp/butter)
( cp -a /tmp/linux-5.10.3 /tmp/butter && sync -f /tmp/butter; ) 0.28s
user 2.44s system 60% cpu 4.475 total
rewrite:
$time (cp -a /tmp/linux-5.10.3 /tmp/butter && sync -f /tmp/butter)
( cp -a /tmp/linux-5.10.3 /tmp/butter && sync -f /tmp/butter; ) 0.28s
user 2.87s system 93% cpu 3.357 total
Clearly better.
Hope this helps :)
Oops I built on top of our devel branch and not 5.10, my bad. This does
help and validates my theory. I'll work out a proper fix soon for
everybody to test, probably tomorrow so I don't risk being murdered by
my wife. Thanks,
Josef
