Re: OS I/O operations concepts

From: Martin Waitz
Date: Thu Dec 16 2004 - 03:29:57 EST

hoi :)

On Wed, Dec 15, 2004 at 09:10:03PM +0000, tony osborne wrote:
> What about the disk bitmap and the one loaded into the memory. Will this be
> updated at each Byte write operation? This will slow down extremely the
> system speed.

no, all data is first written into the buffer cache and will be written
to disk later.

> Should the programmer force the second option (by using BufferOutputStream
> as in java) or is it done automatically by the JVM or OS?

Writing single bytes can be slow because of another reason:
system calls are expensive and it makes sense to buffer data in the
application and send it to the operating system in one big system call.

> Does the I/O controller (once the device driver installed) full privileges
> as the main CPU when on kernel mode?

I'm not sure if i understood your question correctly but yes,
The IO Controller has full access to physical memory through DMA.

> is Java (system.out.println) synchronous or asynchronous I/O
> Op

They are synchronous. Otherwise you couldn't access your data at the
moment read() returns.

Martin Waitz

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