Messages in this thread | | | Date | Sun, 2 May 2004 19:17:09 -0700 | From | William Lee Irwin III <> | Subject | [0/2] filtered wakeups |
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The thundering herd issue in waitqueue hashing has been seen in practice. In order to preserve the space footprint reduction while improving performance, I wrote "filtered wakeups", which discriminate between waiters based on a key.
The following patch series, vs. 2.6.6-rc3-mm1, drastically reduces the kernel cpu consumption of tiobench --threads 512 --size 16384 (fed to tiotest by hand since apparently the perl script is buggy) on a 6x336MHz UltraSPARC III Sun Enterprise 3000 with 3.5GB RAM, ESP-366HME HBA, 10x10Krpm 18GB U160 SCSI disks configured for dm thusly: 0 355655680 striped 10 64 /dev/sda 0 /dev/sdb 0 /dev/sdc 0 /dev/sdd 0 \ /dev/sde 0 /dev/sdf 0 /dev/sdg 0 /dev/sdh 0 /dev/sdi 0 /dev/sdj 0 This was mkfs'd freshly to a single 171GB ext2 fs.
1/2, filtered page waitqueues, resolves the thundering herd issue with hashed page waitqueues. 2/2, filtered buffer_head waitqueues, resolves the thundering herd issue with hashed buffer_head waitqueues. Futexes appear to have their own solution to this issue, which is necessarily different from this as it needs to discriminate based on a longer key. They could in principle be consolidated by passing a comparator instead of comparing a key field or some similar strategy at the cost of indirect function calls.
I furthermore instrumented the calls to schedule(), possibly done indirectly, in patch 0.5/2 of the series, which isn't necessarily meant to be applied to anything, but merely shows how I collected some of the information in the runtime logs, which for space reasons I've posted as URL's instead of including them inline. ftp://ftp.kernel.org/pub/linux/kernel/people/wli/vm/filtered_wakeup/virgin_mm.log.tar.bz2 ftp://ftp.kernel.org/pub/linux/kernel/people/wli/vm/filtered_wakeup/filtered_wakeup.log.tar.bz2
Here "cpusec" represents 1 second of actual cpu consumed, counting cpu consumption of both user and kernel. Apart from regular sampling of profile data, no other load was running on the machine.
before: Tiotest results for 512 concurrent io threads: ,----------------------------------------------------------------------. | Item | Time | Rate | Usr CPU | Sys CPU | +-----------------------+----------+--------------+----------+---------+ | Write 16384 MBs | 1118.1 s | 14.654 MB/s | 1.6 % | 280.9 % | | Random Write 2000 MBs | 336.2 s | 5.950 MB/s | 0.8 % | 20.4 % | | Read 16384 MBs | 1717.1 s | 9.542 MB/s | 1.4 % | 31.8 % | | Random Read 2000 MBs | 465.2 s | 4.300 MB/s | 1.1 % | 36.1 % | `----------------------------------------------------------------------'
Throughput scaled by %cpu: Write: 5.1873MB/cpusec Random Write: 28.0660MB/cpusec Read: 28.7410MB/cpusec Random Read: 11.5591MB/cpusec
top 10 kernel cpu consumers: 21733 finish_task_switch 113.1927 11976 __wake_up 187.1250 11433 generic_file_aio_write_nolock 5.0321 9730 read_sched_profile 43.4375 9606 file_read_actor 42.8839 9116 __do_softirq 31.6528 8682 do_anonymous_page 19.3795 3635 prepare_to_wait 28.3984 2159 kmem_cache_free 16.8672 1944 buffered_rmqueue 3.3750
top 10 callers of scheduling functions: 9391185 wait_on_page_bit 32608.2812 7280055 cpu_idle 37916.9531 1458446 __lock_page 5064.0486 258142 __handle_preemption 16133.8750 134815 worker_thread 247.8217 45989 __wait_on_buffer 205.3080 22294 do_exit 21.7715 22187 generic_file_aio_write_nolock 9.7654 14932 sys_wait4 25.9236 14652 shrink_list 7.8944
after: Tiotest results for 512 concurrent io threads: ,----------------------------------------------------------------------. | Item | Time | Rate | Usr CPU | Sys CPU | +-----------------------+----------+--------------+----------+---------+ | Write 16384 MBs | 1099.5 s | 14.901 MB/s | 2.2 % | 279.3 % | | Random Write 2000 MBs | 333.8 s | 5.991 MB/s | 1.0 % | 14.9 % | | Read 16384 MBs | 1706.3 s | 9.602 MB/s | 1.4 % | 19.1 % | | Random Read 2000 MBs | 460.3 s | 4.345 MB/s | 1.1 % | 14.8 % | `----------------------------------------------------------------------'
Throughput scaled by %cpu: Write: 5.2934MB/cpusec Random Write: 37.6792MB/cpusec Read: 46.8390MB/cpusec Random Read: 27.3270MB/cpusec
top 10 kernel cpu consumers: 11873 generic_file_aio_write_nolock 5.2258 10245 file_read_actor 45.7366 10212 read_sched_profile 45.5893 10135 finish_task_switch 52.7865 9171 do_anonymous_page 20.4710 8619 __do_softirq 29.9271 2905 wake_up_filtered 18.1562 2325 __get_page_state 10.3795 2278 del_timer_sync 5.0848 2033 buffered_rmqueue 3.5295
top 10 callers of scheduling functions: 3985424 cpu_idle 20757.4167 2396754 wait_on_page_bit 7489.8562 209453 __handle_preemption 13090.8125 164071 worker_thread 301.6011 24321 do_exit 23.7510 21272 generic_file_aio_write_nolock 9.3627 16271 sys_wait4 28.2483 11080 pipe_wait 86.5625 9634 compat_sys_nanosleep 25.0885 7742 shrink_list 4.1713
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