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SubjectRe: kswapd continuously active
Hi,


the perf top -a dumps:


Light fs activity (/bin/rm old build roots)
------------------------------------------------------------------------------
PerfTop: 4800 irqs/sec kernel:94.1% [100000 cycles], (all, 24 CPUs)
------------------------------------------------------------------------------
12352.00 - 17.7% : __lock_acquire
4975.00 - 7.1% : lock_release_holdtime
4793.00 - 6.9% : lock_release
3586.00 - 5.1% : lock_acquired
3544.00 - 5.1% : stick_get_tick
2245.00 - 3.2% : sched_clock
1938.00 - 2.8% : sched_clock_cpu
1851.00 - 2.7% : lock_acquire
1800.00 - 2.6% : tl0_linux64
1475.00 - 2.1% : ext4_mark_iloc_dirty
1041.00 - 1.5% : fget_light
1035.00 - 1.5% : find_busiest_group
977.00 - 1.4% : getnstimeofday
939.00 - 1.3% : do_sys_poll
926.00 - 1.3% : perf_poll
(Starting real work - file creation etc - now)

Calling sync a minute later when Dirty > 200MB in /proc/meminfo:
------------------------------------------------------------------------------
PerfTop: 28822 irqs/sec kernel:22.2% [100000 cycles], (all, 24 CPUs)
------------------------------------------------------------------------------
samples pcnt kernel function
_______ _____ _______________
22952.00 - 14.9% : __lock_acquire
9142.00 - 5.9% : lock_release_holdtime
8881.00 - 5.8% : stick_get_tick
8375.00 - 5.4% : lock_release
7401.00 - 4.8% : lock_acquired
5522.00 - 3.6% : tl0_linux64
4325.00 - 2.8% : sched_clock
3634.00 - 2.4% : ext4_rec_len_from_disk
3600.00 - 2.3% : sched_clock_cpu
3261.00 - 2.1% : lock_acquire
3012.00 - 2.0% : NGcopy_to_user
2988.00 - 1.9% : ext4_check_dir_entry
2909.00 - 1.9% : getnstimeofday
2010.00 - 1.3% : __delay
1915.00 - 1.2% : cpu_clock

Stopping all write processes so that sync can complete:

------------------------------------------------------------------------------
PerfTop: 2679 irqs/sec kernel:92.6% [100000 cycles], (all, 24 CPUs)
------------------------------------------------------------------------------
samples pcnt kernel function
_______ _____ _______________
6102.00 - 19.7% : __lock_acquire
2504.00 - 8.1% : lock_release_holdtime
2346.00 - 7.6% : lock_release
1685.00 - 5.5% : stick_get_tick
1532.00 - 5.0% : lock_acquired
1135.00 - 3.7% : sched_clock
1068.00 - 3.5% : find_busiest_group
1034.00 - 3.3% : sched_clock_cpu
1023.00 - 3.3% : tl0_linux64
832.00 - 2.7% : lock_acquire
658.00 - 2.1% : fget_light
656.00 - 2.1% : perf_poll
629.00 - 2.0% : journal_clean_one_cp_list
572.00 - 1.9% : find_next_bit
525.00 - 1.7% : getnstimeofday
------------------------------------------------------------------------------
PerfTop: 3182 irqs/sec kernel:90.5% [100000 cycles], (all, 24 CPUs)
------------------------------------------------------------------------------
samples pcnt kernel function
_______ _____ _______________
9076.00 - 19.9% : __lock_acquire
3702.00 - 8.1% : lock_release_holdtime
3420.00 - 7.5% : lock_release
2522.00 - 5.5% : stick_get_tick
2198.00 - 4.8% : lock_acquired
1700.00 - 3.7% : sched_clock
1539.00 - 3.4% : find_busiest_group
1507.00 - 3.3% : sched_clock_cpu
1402.00 - 3.1% : tl0_linux64
1277.00 - 2.8% : lock_acquire
1089.00 - 2.4% : journal_clean_one_cp_list
1085.00 - 2.4% : fget_light
1004.00 - 2.2% : perf_poll
860.00 - 1.9% : do_sys_poll
782.00 - 1.7% : find_next_bit

There's that journal_clean_one_cp_list thing ..


I have a gut feeling that sys_sync just waits until all data has been
flushed. That however won't fly, because there's always new dirty data
arriving when the box is not idle.

Of course I am not calling /bin/sync on purpose, but some software
packages do; rrdtool for example has an autoconf runtime
check for fsync(2), so it's inevitable that sync is called at some
point by something.


I also did

# perf top -a -k vmlinux-... -p $(that pid of jbd2-sda6)
------------------------------------------------------------------------------
PerfTop: 105 irqs/sec kernel:100.0% [100000 cycles], (target_pid: 574)
------------------------------------------------------------------------------
samples pcnt kernel function
_______ _____ _______________
42.00 - 12.8% : journal_clean_one_cp_list
34.00 - 10.4% : __lock_acquire
33.00 - 10.1% : __jbd2_journal_clean_checkpoint_list
28.00 - 8.5% : find_busiest_group
20.00 - 6.1% : clear_bit
16.00 - 4.9% : lock_release
16.00 - 4.9% : stick_get_tick
14.00 - 4.3% : test_and_set_bit
13.00 - 4.0% : lock_release_holdtime
12.00 - 3.7% : lock_acquired
9.00 - 2.7% : find_next_bit
7.00 - 2.1% : lock_acquire
7.00 - 2.1% : jbd2_journal_write_revoke_records
6.00 - 1.8% : jbd2_journal_switch_revoke_table
6.00 - 1.8% : cpu_clock


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