Re: [9fans] I/O load crashes Qemu

[Bakul Shah <bakul+plan9@bitblocks.com>]

On Fri, 13 Jun 2008 20:39:48 EDT erik quanstrom <quanstro@quanstro.net>  wrote:
> > On a T42 running FreeBSD,  a stock FreeBSD-4.11/qemu gets
> > 18MB/s & plan9/qemu gets 3MB/s.  Both tested by writing 100MB
> > from /dev/zero to a file.  Neither needs any special drivers.
> >
> > I think part of the performance problem is qemu emulates an
> > early Intel ATA controller chip (PIIX3) and perhaps plan9
> > does not do certain optimizations.  It would not be too hard
> > to emulate a more modern controller.
>
> try turning dma on.  it is very unlikely that plan 9 is missing some
> important ata optimization.

Already tried.  echo 'dma on' > /dev/sdC0/ctl doesn't make
any difference in performance.

> > IMHO a virtualizable processor is the necessary first step as
> > it clears one's mind about what not to do in an efficient
> > virtualizable IO architecture!
>
> unless you are contemplating a processor with i/o instructions,
> what does the processor have to do with i/o architecture?

Just that if you have no incentive to virtualize IO, you are
unlikely to think about making it efficient.

> i find there's a certain simplicty in dealing directly
> with hardware, provided one has documentation.

Provided it is complete and the h/w well designed and
interface regular.  Unfortunately not all that common.

> but just wait, there will come a day when people complain
> about the nasty registers in vm and how it would be good to
> abstract that stuff out.

Ha!  First we have to get there.