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Date: Mon, 8 Feb 2010 15:08:37 +0100 From: Jens Axboe <jens.axboe@...cle.com> To: Jan Engelhardt <jengelh@...ozas.de> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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