parallel I/O on shared-memory multi-CPU machines

[David Schwartz]

> I write scientific number-crunching codes which deal with large
> input and output files (Gb, tens of Gb, as much as hundreds of
> Gygabytes on occasions). Now there are these multi-core setups
> like Intel Core 2 Duo becoming available at a low cost. Disk I/O
> is one of the biggest bottlenecks. I would very much like to put
> to use the multiple processors to read or write in parallel
> (probably using OMP directives in a C program).

How do you think more processors is going to help? Disk I/O doesn't take
much processor. It's hard to imagine a realistic disk I/O application that
was somehow limited by available CPU.

> My question is -- can the filesystem actually read/write to files
> truly in parallel

Yes.

> (more processors -- faster read/write),

No. Parallel I/O does not require more processors.

> or if I
> write such a code the I/O commands from the CPUs will just queue
> after each other and it would be the same thing as using a single
> CPU?

Why do you think the I/O commands juse queue after each other on a single
CPU? That is a very mistaken view. As further progress is capable to make on
each operaiton, a single CPU will make the progress. But that doesn't mean
it works one operation from start to finish before it gets to the next one.

> If parallel I/O is possible, can it be accomplished entirely
> transparently, or using special libraries, or only in special
> circumstances, like reading in parallel with N CPUs from N
> different physical disks? Or only on some types of hardware? Is
> there a max nr of threads/processes that can write to disk in
> parallel? If ext3 does not do this, which (stable) Linux
> filesystem does it?

You're asking the wrong questions because you asking about CPUs. One CPU can
issue multiple I/O requests without waiting for each one to finish before
issuing the next one. It is a legitimate question how you perform I/O
asynchronously. There are a few ways such as threads and POSIX asynchronous
I/O.

DS