Re: [Qemu-devel] [PATCH RFC v2 0/4] block/qapi: refactor and optimize the qmp_query_blockstats()

[Dou Liyang]

Hi Stefan,

At 12/20/2016 05:39 PM, Stefan Hajnoczi wrote:
> On Tue, Dec 20, 2016 at 12:32:40AM +0800, Fam Zheng wrote:
> > On Mon, 12/19 15:02, Stefan Hajnoczi wrote:
> > > On Mon, Dec 19, 2016 at 04:51:22PM +0800, Dou Liyang wrote:
> > > > These patches aim to refactor the qmp_query_blockstats() and
> > > > improve the performance by reducing the running time of it.
> > > >
> > > > qmp_query_blockstats() is used to monitor the blockstats, it
> > > > querys all the graph_bdrv_states or monitor_block_backends.
> > > >
> > > > There are the two jobs:
> > > >
> > > > 1 For the performance:
> > > >
> > > > 1.1 the time it takes(ns) in each time:
> > > > the disk numbers     | 10    | 500
> > > > -------------------------------------
> > > > before these patches | 19429 | 667722
> > > > after these patches  | 17516 | 557044
> > > >
> > > > 1.2 the I/O performance is degraded(%) during the monitor:
> > > >
> > > > the disk numbers     | 10    | 500
> > > > -------------------------------------
> > > > before these patches | 1.3   | 14.2
> > > > after these patches  | 0.8   | 9.1
> > >
> > > Do you know what is consuming the remaining 9.1%?
> > >
> > > I'm surprised to see such a high performance impact caused by a QMP
> > > command.
> >
> > If it's "performance is 9.1% worse only during the 557044 ns when the QMP
> > command is being processed", it's probably becaues the main loop is stalled a
> > bit, and it's not a big problem. I'd be very surprised if the degradation is
> > more longer than that.
>
> It would be interesting to compare against virtio-blk dataplane.  That
> way the QMP command can execute without interfering with disk I/O activity.

Yes, I will try to do it for you.

I have compared against the disk of IDE/SATA, I think the I/O requests
is handled by the vcpu threads. it isn't be disturbed like virtio-blk
dataplane. but its performance also degraded.


Thanks,

        Liyang.


> qemu-system-x86_64 ... \
>     -object iothread,id=iothread0 \
>     -drive if=none,id=drive0,file=vm.img,format=raw,aio=native,cache=none \
>     -device virtio-blk-pci,drive=drive0,iothread=iothread0
>
> Stefan