MDEV-34994: sql/mysqld: stop accept() loop after the first EAGAIN #3546
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vaintroub
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Sep 24, 2024
MaxKellermann
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Each time a listener socket becomes ready, MariaDB calls accept() ten
times (MAX_ACCEPT_RETRY), even if all but the first one return EAGAIN
because there are no more connections. This causes unnecessary CPU
usage - on our server, the CPU load of that thread, which does nothing
but accept(), saturates one CPU core by ~45%. The loop should stop
after the first EAGAIN.
Perf report:
11.01% mariadbd libc.so.6 [.] accept4
6.42% mariadbd [kernel.kallsyms] [k] finish_task_switch.isra.0
5.50% mariadbd [kernel.kallsyms] [k] _raw_spin_unlock_irqrestore
5.50% mariadbd [kernel.kallsyms] [k] syscall_enter_from_user_mode
4.59% mariadbd [kernel.kallsyms] [k] __fget_light
3.67% mariadbd [kernel.kallsyms] [k] kmem_cache_alloc
2.75% mariadbd [kernel.kallsyms] [k] fput
2.75% mariadbd [kernel.kallsyms] [k] mod_objcg_state
1.83% mariadbd [kernel.kallsyms] [k] __inode_wait_for_writeback
1.83% mariadbd [kernel.kallsyms] [k] __sys_accept4
1.83% mariadbd [kernel.kallsyms] [k] _raw_spin_unlock_irq
1.83% mariadbd [kernel.kallsyms] [k] alloc_inode
1.83% mariadbd [kernel.kallsyms] [k] call_rcu
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Force-push: rebase and added |
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Description
Each time a listener socket becomes ready, MariaDB calls accept() ten
times (MAX_ACCEPT_RETRY), even if all but the first one return EAGAIN
because there are no more connections. This causes unnecessary CPU
usage - on our server, the CPU load of that thread, which does nothing
but accept(), saturates one CPU core by ~45%. The loop should stop
after the first EAGAIN.
Perf report:
Release Notes
Reduce number of accept() system calls.
How can this PR be tested?
TODO: modify the automated test suite to verify that the PR causes MariaDB to behave as intended.
Consult the documentation on "Writing good test cases".
If the changes are not amenable to automated testing, please explain why not and carefully describe how to test manually.
Basing the PR against the correct MariaDB version
mainbranch.PR quality check