Re: [Xen-devel] Very slow disk I/O performance in PV & PVHVM domU (Xen 4.1.2)

[Oscar <invictusjs@xxxxxxxxx>]

Thank you for your reply.

On Sun, Jan 5, 2014 at 11:38 AM, Anthony Liguori <anthony@xxxxxxxxxxxxx> wrote:
> On Sat, Jan 4, 2014 at 4:12 AM, Jeongseok Son <invictusjs@xxxxxxxxx> wrote:
>> Hello, I've used Xen 4.1.2 for 2 years to experiment some ideas.
>>
>> HVM DomU works well. However, with PV domU, or with PVHVM domU, the
>> disk I/O performance is severely slow. The following are the results
>> of dd executions in each domUs.
>>
>> - HVM
>> $ dd if=/dev/zero of=disk.img count=512k bs=1k
>> 524288+0 records in
>> 524288+0 records out
>> 536870912 bytes (537 MB) copied, 5.42523 s, 99.0 MB/s
>
> This is not doing what you think it is.  You are writing to the page
> cache so chances very little has hit disk by the time your benchmark
> finishes.  Instead, you want:
>
> $ dd if=/dev/zero of=disk.img oflag=direct,sync count=128k bs=4k
>
> Expect the results to be far worse than your above test.

Yes you are right. The write speed is less than a 1MB/s in HVM guest,
and so does in PV guest.

Probably, disk I/O performance is not a real cause of performance
degradation of PV.

>
>> - PV
>> $ if=/dev/zero of=disk.img count=512k bs=1k
>> 524288+0 records in
>> 524288+0 records out
>> 536870912 bytes (537 MB) copied, 45.898 s, 11.7 MB/s
>
> Since you are writing to the page cache, you are primarily bound by
> memcpy speed and syscall overhead.  I assume you are testing a 64-bit
> guest.  If so, syscalls are much more expensive on PV than HVM.

Then, is it a normal performance of 64 bit PV guest? I also copied a
large file, compiled linux kernel source code, and did some other
tests on PV guest and usually all of them took 10 times longer than
did on HVM guest.

And as I have said, in Xen 4.3, PV domU shows good performance like
HVM domU. Were there any patches related to this issue since Xen
4.1.2?

Best Regards,

Jeongseok

>
> Regards,
>
> Anthony Liguori
>
>> And after copying, the responsiveness of PV domU becomes too slow to
>> use and so does PVHVM domU. But in Xen 4.3, PV domU shows right
>> performance.
>>
>> I configured my PV domU to use disk file image like this.
>>
>> root = '/dev/xvda2 ro'
>> disk = [
>>     'file:/path/to/pv/images/disk.img,xvda2,w',
>>     'file:/path/to/pv/images/swap.img,xvda1,w',
>> ]
>>
>> My dom0 runs Ubuntu 12.04.2 LTS with linux kernel 3.9.4, and PV domU
>> runs Ubuntu 10.04 LTS / Ubuntu 12.04 LTS with the same kernel. (I
>> tried with an older linux kernel (2.6.39) but the result was same.)
>>
>> In my own guess, there's some issue when using disk image with PV
>> block device driver in Xen 4.1.2. But I have to use disk file image
>> because of some other experimental issues.
>>
>> Why my PV domU is so slow in Xen 4.1.2 but not in Xen 4.3? Which part
>> of code should I modify to fix this problem in Xen 4.1.2?
>>
>> Thank you for any help you can provide.
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxx
>> http://lists.xen.org/xen-devel

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