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

[Jacek Konieczny <jajcus@xxxxxxxxxx>]

On Thu, Jun 14, 2012 at 11:41:38AM +0200, Jacek Konieczny wrote:
> 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.
[...]
> [root@vpbx15 ~]# time dd if=/dev/zero bs=1024 count=10000 of=/dev/xvdc1 
> 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

I have managed to upgrade my image to kernel 3.4.4 â that still didn't
help, but when I switched to a 64-bit kernel, the problem disappeared.

I have been using 64-bit hypervisor for years now, but the domU kernels
were always 32-bit. Upgrading dom0 kernel to 64-bit did not change
anything.

I am not 100% sure if it is the kernel architecture changes or some
difference in kernel configurations (some options which where enabled
in 32-bit kernel where automatically disabled when compiling for
64-bit), but I cannot find any such obvious setting change.

Anyway, using 64-bit kernel fixes the problem for me and that is good
enough. Since I am able to use a 64-bit kernel I have no reason to use
32 bit Linux kernels under Xen any more.

Greets,
        Jacek

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