On Thu, Nov 01, 2007 at 04:01:45PM -0400, Chuck Ebbert wrote:
> On 11/01/2007 10:03 AM, Nick Piggin wrote:
>
> [edited to show the resulting code]
>
> > + __asm__ __volatile__ (
> > + LOCK_PREFIX "xaddw %w0, %1\n"
> > + "1:\t"
> > + "cmpb %h0, %b0\n\t"
> > + "je 2f\n\t"
> > + "rep ; nop\n\t"
> > + "movb %1, %b0\n\t"
> > + /* don't need lfence here, because loads are in-order */
> > "jmp 1b\n"
> > + "2:"
> > + :"+Q" (inc), "+m" (lock->slock)
> > + :
> > + :"memory", "cc");
> > }
>
> If you really thought you might get long queues, you could figure out
> how far back you are and use that to determine how long to wait before
> testing the lock again. That cmpb could become a subb without adding
> overhead to the fast path -- that would give you the queue length (or
> its complement anyway.)
Indeed. You can use this as a really nice input into a backoff
algorithm (eg. if you're next in line, don't back off, or at least
don't go into exponential backoff; if you've got people in front
of you, start throttling harder).
I think I'll leave that to SGI if they come up with a big x86 SSI ;)
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