OT - Journaling File Systems?

[Tom Mitchell]

On Wed, Apr 28, 2004 at 12:17:25PM -0500, Edwards, Scott (MED, Kelly IT Resouces) wrote:
....
> >XFS requires effective atomic and strictly ordered writes for meta
> >data consistency.  In multiple processor environments strong mutual
> >exclusion locks are a requirement.  I suspect that this is true for 
> >all file systems.  Is this a multiprocessor box?
> 
> It only has one CPU.  I'm confused about the whole Hyper-Threading
> thing, Linux seems to treat it as multiple processors.

Do you have a Hyper-Threading capable box?
If so how is the BIOS set etc.

===
The Hyper-Threading is multi processing on a single package.

Start with a processor that has a rich set of internal registers and
execution units.  Add a second execution block (instruction decode)
units that run independent of the first, add a second register file
and exclusion logic to shares execution blocks and it looks MP.

So for example one program with an integer rich loop could leave the
floating point hardware near idle.  Another process could have a
floating point rich loop and leave the integer hardware idle.

It is not that simple in hardware but the result acts like multiple
processors.  Interrupts, cache state, interlocks, support chips, all
need to work or be used multi processor safe.

The internal parallel hardware tricks to make a single processor
faster (SIMD/MMX instructions, multiple floating point blocks,
multiple integer units, register renaming) set the stage for multiple
instruction decode and execution blocks.

With high clock rate long pipeline designs it makes hardware sense.

http://www.intel.com/products/ht/hyperthreading.htm

  "² Hyper-Threading Technology requires a computer system with an
  Intel® Pentium®4 processor supporting Hyper-Threading Technology and
  an HT Technology enabled chipset, BIOS and operating
  system. Performance will vary depending on the specific hardware and
  software you use. See http://www.intel.com/info/hyperthreading/ for
  more information including details on which processors support HT
  Technology"

A personal opinion is that enabling Hyper-Threading might be a net
loss for some folks depending on the application mix.  

A compiler that understands the P4 and some of the more exotic
instructions can keep enough hardware busy that the parallel thread
would be starved for registers or functional blocks.  Since this is
uncommon outside of hand coded library blocks.  Hyper-Threading is often
a win.  However since hand coded library blocks exist in Linux testing
will tell you if you really want to turn it on.

Device drivers must be MP safe.



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