[Openmcl-devel] Quick HW question...

[Gary Byers]

Someone asked about the i7, and I remember professing ignorance (several
paragraphs of it.)

The Mac Pro (at least some models) use/have used Intel XEON processors
which in turn use HTT; it's reasonable to assume that the OSX scheduler's
been HTT-aware for some time.  (I don't know if it's true, but it's a
reasonable assumption.)

CCL uses spinlocks on most platforms; acquiring a spinlock involves a
loop like:

(block got-lock
   (loop
     (dotimes (i *spin-lock-tries*)
       (if (%store-conditional something somewhere)
         (return-from got-lock t)))
     (give-up-timeslice)))

where *spin-lock-tries* is 1 on a uniprocessor system and maybe a few
thousand on a multiprocessor system.  On a system that uses
hyperthreading, that sort of loop interferes with the hardware's 
ability to schedule another hardware thread, and it's necessary to
use a special "pause" instruction (a kind of NOP) in the loop to
advise the hardware that the current thread wasn't really making
progress.

While profiling a customer's application a few years ago, we found
that - when run on XEONs - a lot of time was reported as being spent
looping around waiting for spinlocks.  I said "D'oh!" and added a
"pause" instruction on the execution path, but that didn't make things
better; on Linux, we replaced the use of spinlocks with a slightly
more expensive mechanism, and (paradoxically) things improved, at least
on the XEONs.

That never made sense to me, and I always suspected that something was
just wrong in the implementation (the "pause" instruction happened
out-of-line) or that we were seeing profiling artifacts.  At the time,
there wasn't time to explore this issue more fully, but those suspicions
have lingered.

CCL generally does a lot less locking (of hash-tables, streams, ...) 
than it did a few years ago.  We still use spinlocks (some) on
non-Linux platforms, so if there was a bad interaction between spinlock
loops and hyperthreading it's likely still there but may not show up
as often.

Other than that issue, I'm not aware of any way in which HTT is directly
visible to non-OS code.



On Mon, 15 Nov 2010, Jon Anthony wrote:

> Hi,
>
> I know the Wiki page for SysReq states no known issues with X86_64, but
> I seem to recall something passing through here about some "issue" with
> Intel Core i7 on Mac, or maybe generally, or am I just plain
> misremembering?  On a related note, does MacOS understand HTT (does it
> know the diff between physical and logical cores)?  Thanks for any info!
>
> /Jon
>
>
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