Re: Calculating kernel logical address ..

Richard B. Johnson
2002-09-09T18:50:54Z

On Mon, 9 Sep 2002, Daniel Phillips wrote:

> On Monday 09 September 2002 20:43, Richard B. Johnson wrote:
> > On Mon, 9 Sep 2002, Daniel Phillips wrote:
> >
> > > On Monday 09 September 2002 20:00, Richard B. Johnson wrote:
> > > > For some reason, (claimed performance reasons) user-mode code
> > > > has to be able to get data directly from hardware with no
> > > > intervening copy operation. I think any claimed advantage goes
> > > > away when you look at the overhead necessary for user-mode
> > > > code to sleep before, and awaken after, the DMA operation but
> > > > often marketing departments make those decisions.
> > >
> > > Pfft. Try turning off ide dma and see what happens.
> >
> > I know that DMA works, I'm talking about DMA direct-to-user
> > which is not what the file-systems that use DMA do.
>
> The next generation of fast, parallel filesystems relies on dma
> to/from user space. Besides, what do you think happens when you
> read/write a mmap?

You write to some memory that may (perhaps never) be written to
the underlying device, using whatever I/O method that underlying
device uses, including network.

And, if you are going to DMA direct to/from user-space, you have
a real big performance problem when the user changes a single byte
or a small number of bytes in a file. So your (theoretical) next
generation, as you say, "fast" parallel filesystems won't be doing
this.

Cheers,
Dick Johnson
Penguin : Linux version 2.4.18 on an i686 machine (797.90 BogoMips).
The US military has given us many words, FUBAR, SNAFU, now ENRON.
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