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    SubjectRe: kswapd continuously active
    On Mon, Feb 08 2010, Jan Engelhardt wrote:
    >
    > On Sunday 2010-02-07 11:50, Jan Engelhardt wrote:
    > >On Friday 2010-02-05 14:24, Jan Engelhardt wrote:
    > >>>> The ext4 filesystem is already mounted with barrier=0. If there
    > >>>> is any block-level barriers I also can turn off, what would be
    > >>>> the command?
    > >>>
    > >>>barrier=0 is enough. I do wonder why your writeback rate is that slow,
    > >>>then. The disk has write back caching enabled?
    > >>
    > >>Yes, that seems to be the case at least. In fact the box is all fluffy
    > >>when nobody runs sync(1), which is what makes it so strange.
    > >>
    > >>sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't
    > >>support DPO or FUA
    > >
    > >Here is an alternate trace that just happened.
    > >
    > >INFO: task flush-8:0:343 blocked for more than 120 seconds.
    > >"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
    > >flush-8:0 D 0000000000555930 6664 343 2 0x18000000000
    > >Call Trace:
    > > [00000000005555f4] start_this_handle+0x324/0x4b0
    > > [0000000000555930] jbd2_journal_start+0x94/0xc0
    > > [000000000052ddb8] ext4_da_writepages+0x1e0/0x460
    > > [000000000049bf30] do_writepages+0x28/0x48
    > > [00000000004e6d58] writeback_single_inode+0xf0/0x330
    > > [00000000004e7b24] writeback_inodes_wb+0x4c8/0x5d8
    > > [00000000004e7da4] wb_writeback+0x170/0x1ec
    > > [00000000004e8074] wb_do_writeback+0x188/0x1a4
    > > [00000000004e80b8] bdi_writeback_task+0x28/0xa0
    > > [00000000004a76c8] bdi_start_fn+0x64/0xc4
    > > [0000000000475f84] kthread+0x58/0x6c
    > > [000000000042ade0] kernel_thread+0x30/0x48
    > > [0000000000475ee0] kthreadd+0xb8/0x104
    >
    > Could it be that there is something synchronize_rcu()-like in the
    > game that ??? as a result of how RCU works ??? just takes ages with 24
    > VCPUs?

    The only synchronize_rcu() involved in the writeback code happens when a
    bdi exits, so you should not hit that. It'll do call_rcu() for work
    completions, but 1) you should not see a lot of work entries, and 2)
    lots of other kernel code will do that, too.

    Are you seeing a lot of CPU usage? What does eg perf top -a say?

    And what setup is this, I didn't realize you were running a virtualized
    setup?

    --
    Jens Axboe

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