RHEL9: performance regression on ext4 writes over md/RAID5
Issue
- High 'await' is observed at the md, volume group, and ext4 layers.
- The perf top indicates that the kernel is spending a significant amount of time in the
ext4_mb_good_group
function.
# perf top
Samples: 87K of event 'cycles:P', 4000 Hz, Event count (approx.): 45245285879 lost: 0/0 drop: 0/0
Overhead Shared Object Symbol
20.85% [kernel] [k] ext4_mb_good_group
16.72% [kernel] [k] ext4_get_group_info
9.98% [kernel] [k] ext4_mb_choose_next_group_cr1.constprop.0
6.16% [kernel] [k] rwsem_spin_on_owner
4.43% [kernel] [k] __rcu_read_unlock
Environment
- Red Hat Enterprise Linux 9
kernel-5.14.0-570.18.1.el9_6
/kernel-5.14.0-503.40.1.el9_5
- md/raid5
- [ext4]
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