Re: [Qemu-devel] [PATCH v13 19/25] replay: add BH oneshot event for block layer

[Kevin Wolf]

Am 06.03.2019 um 09:33 hat Pavel Dovgalyuk geschrieben:
> > From: Kevin Wolf [mailto:address@hidden
> > Am 05.03.2019 um 12:04 hat Pavel Dovgalyuk geschrieben:
> > > > > > > @@ -1349,8 +1351,8 @@ static BlockAIOCB 
> > > > > > > *blk_aio_prwv(BlockBackend *blk, int64_t
> > offset,
> > > > int
> > > > > > bytes,
> > > > > > >
> > > > > > >      acb->has_returned = true;
> > > > > > >      if (acb->rwco.ret != NOT_DONE) {
> > > > > > > -        aio_bh_schedule_oneshot(blk_get_aio_context(blk),
> > > > > > > -                                blk_aio_complete_bh, acb);
> > > > > > > +        
> > > > > > > replay_bh_schedule_oneshot_event(blk_get_aio_context(blk),
> > > > > > > +                                         blk_aio_complete_bh, 
> > > > > > > acb);
> > > > > > >      }
> > > > > >
> > > > > > This, and a few other places that you convert, are in fast paths 
> > > > > > and add
> > > > > > some calls that are unnecessary for non-replay cases.
> > > > >
> > > > > I don't think that this can make a noticeable slowdown, but we can run
> > > > > the tests if you want.
> > > > > We have the test suite which performs disk-intensive computation.
> > > > > It was created to measure the effect of running BH callbacks through
> > > > > the virtual timer infrastructure.
> > > >
> > > > I think this requires quite fast storage to possibly make a difference.
> > >
> > > True.
> > >
> > > > Or if you don't have that, maybe a ramdisk or even a null-co:// backend
> > > > could do the trick. Maybe null-co:// is actually the best option.
> > >
> > > We've got tests with file copying and compression on qcow2 disks.
> > > How the null backend can be applied there?
> > 
> > With qcow2, it can't really. null-co:// would work for running something
> > like fio directly against a virtual disk, without any image format
> > involved. Getting the image format out of the way makes things even a
> > little bit faster.
> > 
> > Maybe we should run a micro-benchmark fio with null-co just in addition
> > to your higher level tests?
> 
> We run something like that:
>   -drive file={},if=none,id=drv,snapshot -device ide-hd,drive=drv -drive
> file={},if=none,id=tst,snapshot -device ide-hd,drive=tst -net none -monitor 
> stdio --enable-kvm -m 2G
> 
> I don't really get your idea. What should be added to the command line
> to run null-co benchmark?

Something like:

-drive file=null-co://,if=none,id=null -device virtio-blk,drive=null

(Testing with IDE is useless, IDE is slow, you'll never get very high
IOPS or bandwidth with it, neither before nor after your series.)

Kevin