Re: Threads reading a file at the same time

From:

Daniel Pitts <newsgroup.spamfilter@virtualinfinity.net>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 30 Jul 2010 09:54:29 -0700

Message-ID:

<8tD4o.46821$3%3.543@newsfe23.iad>

On 7/30/2010 9:21 AM, Knute Johnson wrote:

On 7/29/2010 9:35 PM, Boris Punk wrote:

"Arne Vajh?j"<arne@vajhoej.dk> wrote in message
news:4c52403f$0$283$14726298@news.sunsite.dk...

On 29-07-2010 22:58, Boris Punk wrote:

I'm not sure about this one. The basic IDE hard drive hasn't got the
capability to read from two disk locations at the same time has it?
Modern
SSD drives may have.

This lock is stating that multiple reads are ok, but just one write
at a
time is ok:
http://download.oracle.com/javase/6/docs/api/java/util/concurrent/locks/ReadWriteLock.html

How can that be? Imagine Thread A calling seek on the disk, then
Thread B
calling seek. Thread A then reads from B's location surely?

The ReadWriteLock is not related to the actual disk IO - it only
coordinates between two or more threads.

Arne

Fair enough - I was just read somewhere someone tried using it for a
disk IO

That is a common usage for RWL. The reading threads can be blocked when
a write is about to occur. The actual read/write to the disk is another
matter altogether. Caching and other things can affect that as well.

Perhaps its a common usage for the general concept of read/write lock,
but in Java, the ReadWriteLock wouldn't be useful, since it doesn't
prevent external processes from accessing the file.

One place that the Java ReadWriteLock is useful for a Many-Access,
Few-Write data-structure (eg, configuration object which changes rarely,
but is used in many places).

--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>