Most of io workers in btrfs don't take into account amount of disks they deal with:
fs/btrfs/disk-io.c: fs_info->thread_pool_size = min_t(unsigned long,
num_online_cpus() + 2, 8);
It might not be a problem for 'write-only' workloads,
but it's a serious problem for read/write ones.
Let's consider simple setup:
- dualcore (core2) laptop with sole HDD (5200 rpm)
- slightly aged btrfs: ~50% of 200GB is filled
and used as / (nothing special: stock mkfs.btrfs /dev/root)
- kernel does not matter much. any 3.3.0+ would fit.
3.5.0-rc3 runs here
- tried both anticipatory and deadline schedulers. does not
seem to matter that much
My "benchmark"[1] is gcc unpacking/perm-sanitizing. It
emulates typical workload of source based package manager.
When [1] is ran as 'sh torture.sh':
- first we see at least 4 'btrfs-endio-wri' hammering disk
with random reads and writes in the 'untar' phase
- then we see 4 'btrfs-delayed-m' with similar effect
in the 'chmod' phase
If it would be a quad-cpu laptop (typical i5) we would get 6(!)
threads chewing disk, so you get worse behaviour. System
becomes completely unusable. Right now I try to mitigate it
with 'thread_pool' mount option, but it looks like a crude hack.
Would be nice to have amount of parallel readers comparable to
number underlying spinning devices.
Thanks for your patience!
[1] torture.sh:
#!/bin/sh
gcc_url=http://distfiles.gentoo.org/distfiles/gcc-4.7.0.tar.bz2
gcc_tarball=gcc-4.7.0.tar.bz2
gcc_dir=gcc-4.7.0
[ -f "$gcc_tarball" ] || wget "$gcc_url"
chatty() {
echo "RUN: $@"
/usr/bin/time "$@"
}
torture() {
chatty rm -rf "${gcc_dir}"
### iotop pattern: scattered seeks/reads to death
#25499 be/4 root 60.34 K/s 79.20 K/s 0.00 % 99.99 %
[btrfs-endio-wri]
#28151 be/4 root 56.57 K/s 60.34 K/s 0.00 % 99.73 %
[btrfs-endio-wri]
# 6181 be/4 root 56.57 K/s 71.66 K/s 0.00 % 96.96 %
[flush-btrfs-1]
#23881 be/4 root 52.80 K/s 52.80 K/s 0.00 % 93.70 %
[btrfs-endio-wri]
chatty tar -xjf "${gcc_tarball}"
### iotop pattern: scattered seeks/reads to death
#29109 be/4 slyfox 870.05 K/s 0.00 B/s 0.00 % 97.05 % chmod -R
700 gcc-4.7.0/
#28067 be/4 root 162.66 K/s 949.49 K/s 0.00 % 77.72 %
[btrfs-delayed-m]
#28164 be/4 root 22.70 K/s 215.62 K/s 0.00 % 14.23 %
[btrfs-delayed-m]
# 4690 be/4 root 0.00 B/s 15.13 K/s 0.00 % 0.00 %
[btrfs-delayed-m]
echo "now look at iotop"
chatty chmod -R 700 "${gcc_dir}"/
chatty sync
}
torture
torture
torture
torture
--
Sergei
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