Re: 2.6.37: Multi-second I/O latency while untarring

Andrew Lutomirski
2011-02-11T19:56:23Z

On Fri, Feb 11, 2011 at 10:44 AM, Chris Mason <chris.mason@oracle.com> wrote:
> Excerpts from Andrew Lutomirski's message of 2011-02-11 10:08:52 -0500:
>> As I type this, I have an ssh process running that's dumping data into
>> a fifo at high speed (maybe 500Mbps) and a tar process that's
>> untarring from the same fifo onto btrfs.  The btrfs fs is mounted -o
>> space_cache,compress.  This machine has 8GB ram, 8 logical cores, and
>> a fast (i7-2600) CPU, so it's not an issue with the machine struggling
>> under load.
>>
>> Every few tens of seconds, my system stalls for several seconds.
>> These stalls cause keyboard input to be lost, firefox to hang, etc.
>>
>> Setting tar's ionice priority to best effort / 7 or to idle makes no difference.
>>
>> ionice idle and queue_depth = 1 on the disk (a slow 2TB WD) also makes
>> no difference.
>>
>> max_sectors_kb = 64 in addition to the above doesn't help either.
>>
>> latencytop shows regular instances of 2-7 *second* latency, variously
>> in sync_page, start_transaction, btrfs_start_ordered_extent, and
>> do_get_write_access (from jbd2 on my ext4 root partition).
>>
>> echo 3 >drop_caches gave me 7 GB free RAM.  I still had stalls when
>> 4-5 GB were still free (so it shouldn't be a problem with important
>> pages being evicted).
>>
>> In case it matters, all of my partitions are on LVM on dm-crypt, but
>> this machine has AES-NI so the overhead from that should be minimal.
>> In fact, overall CPU usage is only about 10%.
>>
>> What gives?  I thought this stuff was supposed to be better on modern kernels.
>
> We can tell more if you post the full traces from latencytop.  I have a
> patch here for latencytop that adds a -c mode, which dumps the traces
> out to a text files.
>
> http://oss.oracle.com/~mason/latencytop.patch

Big dump at end of email from latencytop git + your patch.

>
> Based on what you have here, I think it's probably a latency problem
> between btrfs and the dm-crypt stuff.  How easily can setup a test
> partition without dm-crypt?

Not so easily on that disk. I left some space inside the LVM to play
with but none outside.

I'll try hooking up another disk over eSATA l (on a Cougar Point 3Gbps
controller, so it might blow up).

And here's the dump:

=============== Fri Feb 11 14:44:07 2011
Globals: Cause Maximum Percentage
synchronous write 4249.1 msec 35.5 %
Writing to a pipe 4248.5 msec 35.5 %
Writing a page to disk 105.9 msec 2.1 %
Page fault 23.7 msec 0.2 %
Reading from a pipe 4.7 msec 19.8 %
Waiting for event (select) 4.6 msec 6.4 %
Waiting for event (poll) 1.3 msec 0.0 %
Executing raw SCSI command 1.3 msec 0.2 %
opening cdrom device 1.3 msec 0.3 %
Process details:
Process ksoftirqd/1 (10) Total: 50.0 msec
[run_ksoftirqd] 4.8 msec 100.0 %
run_ksoftirqd kthread kernel_thread_helper
Process ksoftirqd/2 (15) Total: 8.7 msec
[run_ksoftirqd] 4.9 msec 100.0 %
run_ksoftirqd kthread kernel_thread_helper
Process ksoftirqd/3 (19) Total: 2.9 msec
[run_ksoftirqd] 2.9 msec 100.0 %
run_ksoftirqd kthread kernel_thread_helper
Process ksoftirqd/5 (27) Total: 80.6 msec
[run_ksoftirqd] 5.0 msec 100.0 %
run_ksoftirqd kthread kernel_thread_helper
Process scsi_eh_1 (62) Total: 45.0 msec
Executing internal ATA command 0.7 msec 62.3 %
ata_exec_internal_sg ata_exec_internal atapi_eh_request_sense
ata_eh_link_autopsy ata_eh_autopsy sata_pmp_error_handler
ahci_error_handler ata_scsi_error scsi_error_handler kthread
kernel_thread_helper
SCSI error handler 0.6 msec 37.7 %
scsi_error_handler kthread kernel_thread_helper
Process kworker/7:1 (76) Total: 8.7 msec
. 3.9 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process kworker/4:1 (139) Total: 124.0 msec
. 4.9 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process kworker/6:1 (140) Total: 11.7 msec
. 3.8 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process kworker/5:1 (141) Total: 12.5 msec
. 4.9 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process kworker/2:1 (142) Total: 26.1 msec
. 4.9 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process kworker/1:1 (143) Total: 47.1 msec
. 4.9 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process kworker/3:1 (150) Total: 4.6 msec
. 3.1 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process jbd2/dm-1-8 (376) Total: 66.7 msec
Writing buffer to disk (synchronous) 66.7 msec 100.0 %
sync_buffer __wait_on_buffer wait_on_buffer
jbd2_journal_commit_transaction kjournald2 kthread
kernel_thread_helper
Process btrfs-submit-0 (829) Total: 1719.4 msec
[worker_loop] 2.2 msec 100.0 %
worker_loop kthread kernel_thread_helper
Process btrfs-endio-met (833) Total: 540.5 msec
[worker_loop] 4.8 msec 100.0 %
worker_loop kthread kernel_thread_helper
Process btrfs-freespace (836) Total: 31.7 msec
[worker_loop] 1.8 msec 100.0 %
worker_loop kthread kernel_thread_helper
Process btrfs-transacti (838) Total: 5567.3 msec
Writing buffer to disk (synchronous) 352.5 msec 6.8 %
sync_buffer __wait_on_buffer write_dev_supers write_all_supers
write_ctree_super btrfs_commit_transaction transaction_kthread
kthread kernel_thread_helper
Writing a page to disk 91.4 msec 93.2 %
sync_page wait_on_page_bit read_extent_buffer_pages
btree_read_extent_buffer_pages.clone.56 read_tree_block
read_block_for_search.clone.38 btrfs_search_slot
lookup_inline_extent_backref __btrfs_free_extent
run_clustered_refs btrfs_run_delayed_refs
btrfs_commit_transaction
Process hald-addon-stor (1349) Total: 39.5 msec
Executing raw SCSI command 1.3 msec 49.1 %
blk_execute_rq scsi_execute scsi_execute_req
sr_test_unit_ready sr_drive_status cdrom_ioctl sr_block_ioctl
__blkdev_driver_ioctl blkdev_ioctl block_ioctl do_vfs_ioctl
sys_ioctl
opening cdrom device 1.3 msec 43.4 %
blk_execute_rq scsi_execute scsi_execute_req
sr_test_unit_ready sr_media_change media_changed
cdrom_media_changed sr_block_media_changed check_disk_change
cdrom_open sr_block_open __blkdev_get
SCSI cdrom ioctl 0.6 msec 7.5 %
blk_execute_rq scsi_execute sr_do_ioctl sr_packet
cdrom_get_media_event sr_drive_status cdrom_ioctl
sr_block_ioctl __blkdev_driver_ioctl blkdev_ioctl block_ioctl
do_vfs_ioctl
Process gdm-binary (1486) Total: 0.3 msec
Waiting for event (poll) 0.3 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process Xorg (1520) Total: 0.9 msec
[i915_do_wait_request] 0.3 msec 70.7 %
i915_do_wait_request i915_gem_object_wait_rendering
i915_gem_object_set_to_gtt_domain i915_gem_set_domain_ioctl
drm_ioctl do_vfs_ioctl sys_ioctl system_call_fastpath
Process upowerd (1645) Total: 0.2 msec
Waiting for event (poll) 0.2 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process gnome-session (1752) Total: 0.1 msec
Waiting for event (poll) 0.1 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process udisks-daemon (1880) Total: 0.1 msec
Waiting for event (poll) 0.1 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process udisks-daemon (1881) Total: 19.2 msec
opening cdrom device 1.2 msec 88.1 %
blk_execute_rq scsi_execute scsi_execute_req
sr_test_unit_ready sr_media_change media_changed
cdrom_media_changed sr_block_media_changed check_disk_change
cdrom_open sr_block_open __blkdev_get
Executing raw SCSI command 0.5 msec 11.9 %
blk_execute_rq scsi_execute scsi_execute_req
ioctl_internal_command.clone.3 scsi_set_medium_removal
sr_lock_door cdrom_release sr_block_release __blkdev_put
blkdev_put blkdev_close fput
Process bluetooth-apple (1915) Total: 0.2 msec
Waiting for event (poll) 0.2 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process gpk-update-icon (1942) Total: 0.2 msec
Waiting for event (poll) 0.2 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process gnome-screensav (2040) Total: 0.3 msec
Waiting for event (poll) 0.2 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process flush-btrfs-1 (2120) Total: 173.3 msec
Writing a page to disk 161.4 msec 100.0 %
sync_page __lock_page lock_page
extent_write_cache_pages.clone.11.clone.18 extent_writepages
btrfs_writepages do_writepages writeback_single_inode
writeback_sb_inodes writeback_inodes_wb wb_writeback
wb_do_writeback
Process btrfs-delalloc- (2402) Total: 0.2 msec
[worker_loop] 0.2 msec 100.0 %
worker_loop kthread kernel_thread_helper
Process btrfs-endio-met (2408) Total: 52.1 msec
[worker_loop] 2.5 msec 100.0 %
worker_loop kthread kernel_thread_helper
Process btrfs-endio-wri (2768) Total: 1561.5 msec
[worker_loop] 4.1 msec 100.0 %
worker_loop kthread kernel_thread_helper
Process compiz (6845) Total: 0.8 msec
Waiting for event (poll) 0.2 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process clock-applet (6928) Total: 1.4 msec
Waiting for event (poll) 1.3 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process kworker/0:0 (20743) Total: 1867.9 msec
. 5.0 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process tar (22372) Total: 6614.1 msec
synchronous write 4249.1 msec 64.2 %
btrfs_throttle btrfs_file_aio_write do_sync_write vfs_write
sys_write system_call_fastpath
Reading from a pipe 4.7 msec 35.8 %
pipe_wait pipe_read do_sync_read vfs_read sys_read
system_call_fastpath
Process ssh (22533) Total: 4974.4 msec
Writing to a pipe 4248.5 msec 85.4 %
start_transaction btrfs_join_transaction btrfs_dirty_inode
__mark_inode_dirty file_update_time pipe_write do_sync_write
vfs_write sys_write system_call_fastpath
Waiting for event (select) 4.6 msec 14.6 %
poll_schedule_timeout do_select core_sys_select sys_select
system_call_fastpath
Process kworker/u:3 (23092) Total: 1901.4 msec
Creating block layer request 14.7 msec 84.7 %
get_request_wait __make_request generic_make_request
kcryptd_crypt_write_io_submit kcryptd_crypt process_one_work
worker_thread kthread kernel_thread_helper
. 4.7 msec 15.3 %
worker_thread kthread kernel_thread_helper
Process kworker/u:0 (23763) Total: 2108.1 msec
. 5.0 msec 100.0 %
worker_thread kthread kernel_thread_helper
Process packagekitd (24691) Total: 0.2 msec
Waiting for event (poll) 0.2 msec 100.0 %
poll_schedule_timeout do_sys_poll sys_poll
system_call_fastpath
Process sshd (24718) Total: 35.8 msec
Waiting for event (select) 3.1 msec 99.9 %
poll_schedule_timeout do_select core_sys_select sys_select
system_call_fastpath
Process bash (24719) Total: 5.1 msec
Reading from a pipe 1.0 msec 82.5 %
pipe_wait pipe_read do_sync_read vfs_read sys_read
system_call_fastpath
Waiting for a process to die 0.9 msec 17.5 %
do_wait sys_wait4 system_call_fastpath

It seems a little sad that ssh took a big latency hit just marking the
fifo dirty.

--Andy