don't use ld.bias

Patch to fix spin_lock to not use ld.bias since it causes a bunch of bus activity that hurts scalability when a lock is contended.

Thanks,
Jesse

=== include/asm-ia64/spinlock.h 1.13 vs edited ==--- 1.13/include/asm-ia64/spinlock.h Fri May 30 19:59:40 2003
+++ edited/include/asm-ia64/spinlock.h Mon Jul 21 16:54:15 2003
@@ -93,13 +93,16 @@
* Streamlined test_and_set_bit(0, (x)). We use test-and-test-and-set
* rather than a simple xchg to avoid writing the cache-line when
* there is contention.
+ *
+ * Don't use ld.bias since it generates unnecessary bus activity on
+ * multiprocessor systems when the lock is contended.
*/
#define _raw_spin_lock(x) __asm__ __volatile__ ( \
"mov ar.ccv = r0\n" \
"mov r29 = 1\n" \
";;\n" \
"1:\n" \
- "ld4.bias r2 = [%0]\n" \
+ "ld4 r2 = [%0]\n" \
";;\n" \
"cmp4.eq p0,p7 = r0,r2\n" \
"(p7) br.cond.spnt.few 1b \n" \

bus

It's dangerous to say something without knowing the whole context, but I can't resist asking why not fix the contended lock area instead? Sounds like you have some serious contention.

Maybe I shouldn't have said 'contended'. This causes problems even when a lock is nearly uncontended. I agree that fixing lock contention is a good idea, but as I read this code, it'll only make the very best case (totally uncontended) only a little better than if the ld.bias were just an ld and it makes the average and contended cases _much_ worse.

I can get some data if you're not convinced by reading the code :)

Thanks,
Jesse

It does make sense to not use the ld.bias in the spinlock, it will make more cacheline bouncing in a larger SMP system.
Thanks,
Asit

it certainly makes a mockery of the comment:

* Streamlined test_and_set_bit(0, (x)). We use test-and-test-and-set
* rather than a simple xchg to avoid writing the cache-line when
* there is contention.

since there's no real advantage (AIUI) to have the cacheline clean-exclusive vs dirty-exclusive.

but why not delete this code? it's under #else /* !NEW_LOCK */ and NEW_LOCK looks to be the better code as well as the default in 2.5.

"It's not Hollywood.  War is real, war is primarily not about defeat or
victory, it is about death.  I've seen thousands and thousands of dead bodies.
Do you think I want to have an academic debate on this subject?" -- Robert Fisk

Jesse> Patch to fix spin_lock to not use ld.bias since it causes a Jesse> bunch of bus activity that hurts scalability when a lock is Jesse> contended.

Yes, using ld.bias really was wrong there. However, instead of applying your patch, I dropped the old lock code (which was effectively dead code) and instead removed the .bias from the ia64_spinlock_contention() routine in head.S.

Thanks,

	--david

Even better. Thanks.

Jesse