Re: [Qemu-devel] Re: [PATCH] Add cache=volatile parameter to -drive

[Anthony Liguori]

On 05/26/2010 10:40 AM, Aurelien Jarno wrote:
> > I highly doubt that this is even visible on benchmarks without using
> > KVM.  The improvement on a microbenchmark was relatively small and the
> > cost from TCG would almost certainly dwarf it.
> >      
> It is something clearly visible. Before fsync() was not used, and it
> happens this syscall can be very expensive (ie a few seconds, especially
> with some other i/o load on the system) on ext3 with not so old kernels.
> A google search for "firefox fsync" will give you a few pointers.
>    

I'm well aware.  I've asked multiple times for benchmark data with ext3 
because of this issue.  So far, it hasn't been noticable.

> > Also, remember before KVM, we had single threaded IO and posix-aio
> > (which is still single threaded).  If KVM never happened, block
> > performance would be far, far worse than it is today with cache=writeback.
> >
> >      
> io thread is not enable by default in QEMU.
>    

But the thread pool based aio implementation is always enabled 
regardless of io thread.  I was going to build and run 0.9.1 to do a 
benchmark of TCG comparing that to the latest git, but I don't have an 
easy way to get a version of gcc-3.  I've forgotten how annoying that 
used to be :-)

I would be very surprised if TCG + IDE had any observable performance 
difference pre/post the ide flush implementation.  I'm almost inclined 
to suggestion that flush controls should be a qdev property verses a 
drive property since that's really where the change happened.

Regards,

Anthony Liguori