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Re: [Qemu-devel] QEMU throughput is down with SMP


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] QEMU throughput is down with SMP
Date: Fri, 1 Oct 2010 09:41:05 +0100

On Thu, Sep 30, 2010 at 8:19 PM, Venkateswararao Jujjuri (JV)
<address@hidden> wrote:
> On 9/30/2010 2:13 AM, Stefan Hajnoczi wrote:
>>
>> On Thu, Sep 30, 2010 at 1:50 AM, Venkateswararao Jujjuri (JV)
>> <address@hidden>  wrote:
>>>
>>> Code: Mainline QEMU (git://git.qemu.org/qemu.git)
>>> Machine: LS21 blade.
>>> Disk: Local disk through VirtIO.
>>> Did not select any cache option. Defaulting to writethrough.
>>>
>>> Command tested:
>>> 3 parallel instances of : dd if=/dev/zero of=/pmnt/my_pw bs=4k
>>> count=100000
>>>
>>> QEMU with  smp=1
>>> 19.3 MB/s + 19.2 MB/s + 18.6 MB/s = 57.1 MB/s
>>>
>>> QEMU with smp=4
>>> 15.3 MB/s + 14.1 MB/s + 13.6 MB/s = 43.0 MB/s
>>>
>>> Is this expected?
>>
>> Did you configure with --enable-io-thread?
>
> Yes I did.
>>
>> Also, try using dd oflag=direct to eliminate effects introduced by the
>> guest page cache and really hit the disk.
>
> With oflag=direct , I see no difference and the throughput is so slow and I
> would not
> expect to see any difference.
> It is 225 kb/s  for each thread either with smp=1 or with smp=4.

If I understand correctly you are getting:

QEMU oflag=direct with smp=1
225 KB/s + 225 KB/s + 225 KB/s = 675 KB/s

QEMU oflag=direct with smp=4
225 KB/s + 225 KB/s + 225 KB/s = 675 KB/s

This suggests the degradation for smp=4 is guest kernel page cache or
buffered I/O related.  Perhaps lockholder preemption?

Stefan



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