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

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