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

[Stefan Hajnoczi]

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.

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

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