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

[Ryan Harper]

* Stefan Hajnoczi <address@hidden> [2010-10-01 03:48]:
> 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?

or just a single spindle maxed out because the blade hard drive doesn't
have writecache enabled (it's disabled by default).  

-- 
Ryan Harper
Software Engineer; Linux Technology Center
IBM Corp., Austin, Tx
address@hidden