linux-kernel - Re: kswapd continuously active

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Date:	Mon, 8 Feb 2010 01:07:40 +0100 (CET)
From:	Jan Engelhardt <jengelh@...ozas.de>
To:	Jens Axboe <jens.axboe@...cle.com>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kswapd continuously active


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?
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