[Beowulf] scratch File system for small cluster

Greg Lindahl lindahl at pbm.com
Thu Sep 25 16:08:23 PDT 2008

On Thu, Sep 25, 2008 at 02:08:23PM -0400, Joe Landman wrote:

>   It looks like people use mmap files to explicitly avoid seeks,  
> replacing semantics of file IO with memory access semantics.

Well, it explicitly avoids having to call I/O functions all the time
as you skip around a file. It also is better at sharing, if several
processes have the same file open, or if the file was just written.
Remember that read() usually has to copy the data. mmap() sometimes
reduces the number of copies.

> Sadly, mmap  
> on linux uses the paging mechanism which is pretty much stuck at 4kB  
> pages for most distributions.  I think the SiCortex folks and a few  
> others are working with 64 kB page kernels.

That's a function of architecture. The Linux mips and powerpc ports
have supported larger-than-4k pages for a long time -- it's easy to do
if you pick a single page-size at boot. x86 only has Overly Small and
Really Too Huge pages.

-- greg

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