On 01/05/15 22:35, Mike Snitzer wrote: > On Fri, Jan 02 2015 at 12:53pm -0500, > Bart Van Assche <bvanassche@xxxxxxx> wrote: >> Thanks, my tests confirm that this patch indeed fixes the issue I had >> reported. Unfortunately this doesn't mean that the blk-mq multipath code >> is already working perfectly. Most of the time I/O requests are >> processed within the expected time but sometimes I/O processing takes >> much more time than what I expected: >> >> # /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null >> 0.02 >> # /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null >> 0.02 >> # /usr/bin/time -f %e mkfs.xfs -f /dev/dm-0 >/dev/null >> 8.68 >> >> However, if I run the same command on the underlying device it always >> completes within the expected time. > > I don't have very large blk-mq devices, but I can work on that. > How large is the blk-mq device in question? > > Also, how much memory does the system have? Is memory fragmented at > all? With this change the requests are cloned using memory allocated > from block core's blk_get_request (rather than a dedicated mempool in DM > core). > > Any chance you could use 'perf record' to try to analyze where the > kernel is spending its time? Hello Mike, The device used in this test was a tmpfs file with a size of 16 MB. That file had been created as follows: dd if=/dev/zero of=/dev/vdisk bs=1M count=16. The initiator and target systems did have enough memory to keep this tmpfs file in RAM all the time (32 GB and 4 GB respectively). For the runs that took much longer than expected the CPU load was low. This probably means that the system was waiting for one or another I/O timer to expire. The output triggered by "echo w > /proc/sysrq-trigger" during a run that took longer than expected was as follows: SysRq : Show Blocked State task PC stack pid father kdmwork-253:0 D ffff8807c1fd3b78 0 10396 2 0x00000000 ffff8807c1fd3b78 ffff88083b6b6cc0 0000000000012ec0 ffff8807c1fd3fd8 0000000000012ec0 ffff880824225aa0 ffff88083b6b6cc0 ffff88081b0cb2c0 ffff88085fc537c8 ffff8807c1fd3c98 ffff8807f7a99d70 ffffe8ffffc43bc0 Call Trace: [<ffffffff814d5230>] io_schedule+0xa0/0x130 [<ffffffff8125a3f7>] bt_get+0x117/0x1b0 [<ffffffff81256580>] ? blk_mq_queue_enter+0x30/0x2a0 [<ffffffff81094cf0>] ? prepare_to_wait_event+0x110/0x110 [<ffffffff8125a76f>] blk_mq_get_tag+0x9f/0xd0 [<ffffffff8125591b>] __blk_mq_alloc_request+0x1b/0x210 [<ffffffff812571c9>] blk_mq_alloc_request+0x139/0x150 [<ffffffff8124c16e>] blk_get_request+0x2e/0xe0 [<ffffffff8109a60d>] ? trace_hardirqs_on+0xd/0x10 [<ffffffffa07f7d0f>] __multipath_map.isra.15+0x1cf/0x210 [dm_multipath] [<ffffffffa07f7d6a>] multipath_clone_and_map+0x1a/0x20 [dm_multipath] [<ffffffffa039dbb5>] map_tio_request+0x1d5/0x3a0 [dm_mod] [<ffffffff8109a53d>] ? trace_hardirqs_on_caller+0xfd/0x1c0 [<ffffffff81075cbe>] kthread_worker_fn+0x7e/0x1b0 [<ffffffff81075c40>] ? __init_kthread_worker+0x60/0x60 [<ffffffff81075bc8>] kthread+0xf8/0x110 [<ffffffff81075ad0>] ? kthread_create_on_node+0x210/0x210 [<ffffffff814dacac>] ret_from_fork+0x7c/0xb0 [<ffffffff81075ad0>] ? kthread_create_on_node+0x210/0x210 dmraid D ffff8807f4cafc88 0 25099 25064 0x00000000 ffff8807f4cafc88 ffff8807c0b52440 0000000000012ec0 ffff8807f4caffd8 0000000000012ec0 ffffffff81a194e0 ffff8807c0b52440 ffff8807c09ec1c0 ffff88085fc137c8 ffff88085ff8ce38 ffff8807f4cafd30 0000000000000082 Call Trace: [<ffffffff814d5990>] ? bit_wait+0x50/0x50 [<ffffffff814d5230>] io_schedule+0xa0/0x130 [<ffffffff814d59bc>] bit_wait_io+0x2c/0x50 [<ffffffff814d578b>] __wait_on_bit_lock+0x4b/0xb0 [<ffffffff8113b45a>] __lock_page_killable+0x9a/0xa0 [<ffffffff81094d30>] ? autoremove_wake_function+0x40/0x40 [<ffffffff8113da78>] generic_file_read_iter+0x408/0x640 [<ffffffff8109a60d>] ? trace_hardirqs_on+0xd/0x10 [<ffffffff811d5f57>] blkdev_read_iter+0x37/0x40 [<ffffffff8119866e>] new_sync_read+0x7e/0xb0 [<ffffffff81199858>] __vfs_read+0x18/0x50 [<ffffffff81199916>] vfs_read+0x86/0x140 [<ffffffff81199a19>] SyS_read+0x49/0xb0 [<ffffffff814dad52>] system_call_fastpath+0x12/0x17 Bart. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
