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 ;) - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
