Re: spinlocks on powerpc

On 2012-01-03 04:44, Robert Haas wrote:
> On read-only workloads, you get spinlock contention, because everyone
> who wants a snapshot has to take the LWLock mutex to increment the
> shared lock count and again (just a moment later) to decrement it.

Does the LWLock protect anything but the shared lock count? If not
then the usually quickest manipulation is along the lines of:

loop: lwarx r5,0,r3 #load and reserve
add r0,r4,r5 #increment word
stwcx. r0,0,r3 #store new value if still reserved
bne- loop #loop if lost reservation

(per IBM's software ref manual,
https://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/852569B20050FF778525699600719DF2
)

The same sort of thing generally holds on other instruction-sets also.

Also, heavy-contention locks should be placed in cache lines away from other
data (to avoid thrashing the data cache lines when processors are fighting
over the lock cache lines).
--
Jeremy