[Xen-users] Very slow VBD writes in domU under Linux 3.3, ok under 3.0.x

[Jacek Konieczny <jajcus@xxxxxxxxxx>]

Hello,

After upgrading my DomUs to Linux kernel 3.3.0 I have noticed some
applications are _inceribly_ slow. Eventually I fount out that are some
filesystem writes blocking for long time.

The problem occurs no matter if I use file-backed VBD or
LVM-device-backed VBD, it doesn't matter if I run the domU under Xen
4.1.2 or under Xen 3.4.3. It does not matter if I write through 
a file system in domU or if I write directly to the xvd device.
But the same operations work well under kernels 3.0.x, but fail in
3.3.x.

Any ideas what is causing that? How can I prevent this in any other way
than downgrading the domU kernel?

[root@vpbx15 ~]# time dd if=/dev/zero bs=1024 count=10000 of=/dev/xvdc1 
10000+0 records in
10000+0 records out
10240000 bytes (10 MB) copied, 52.9366 s, 193 kB/s
   88.23s real     0.00s user     0.36s system

while blocked:

[root@vpbx15 ~]# cat /proc/`pidof dd`/stack    
[<c016891a>] check_preempt_curr+0x6a/0x80
[<c0109f3c>] xen_clocksource_read+0x2c/0x60
[<c0179f10>] ktime_get_ts+0xf0/0x120
[<c01bd285>] sleep_on_page+0x5/0x10
[<c01bd280>] sleep_on_page+0x0/0x10
[<c01bd4cd>] wait_on_page_bit+0x8d/0xa0
[<c015c8a0>] wake_bit_function+0x0/0x60
[<c01bd6e1>] grab_cache_page_write_begin+0x91/0xd0
[<c0220d1d>] block_write_begin+0x2d/0x90
[<c01bcab7>] generic_file_buffered_write+0xe7/0x250
[<c0225e10>] blkdev_get_block+0x0/0x70
[<c01696d8>] get_parent_ip+0x8/0x20
[<c01be43e>] __generic_file_aio_write+0x27e/0x500
[<c0109880>] xen_read_cr2_direct+0x0/0x10
[<c0430a81>] do_page_fault+0x161/0x4d0
[<c0224feb>] blkdev_aio_write+0x3b/0xa0
[<c01f5a95>] do_sync_write+0xc5/0x110
[<c01f625a>] rw_verify_area+0x6a/0x130
[<c01f59d0>] do_sync_write+0x0/0x110
[<c01f6620>] vfs_write+0xa0/0x160
[<c01f6901>] sys_write+0x41/0x80
[<c0433f5c>] sysenter_do_call+0x12/0x2c
[<ffffffff>] 0xffffffff

Greets,
        Jacek

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