In a computer system having a multithreaded application and a generational garbage collector that dynamically pre-tenures objects from a predetermined number of candidate allocation sites, allocated byte accounting is performed by each application thread using an array that contains a number of entries equal to the total number of candidate sites at any given time. Each array is indexed by a site number assigned to that site and contains a bytes allocated count for that site. At compilation time, object allocation code that is generated by the compiler is modified to update an array entry associated with a site number that is assigned when the site is selected as a candidate site. Since each array is local to the thread that contains it, each thread can write into its array without using atomic operations or locks.