This was not the only source of performance problems. Another centered on the problems of trying to handle memory properly when physical memory ran low and paging had to occur. In the traditional monolithic operating systems the authors had direct experience with which parts of the kernel called which others, allowing them to fine-tune their pager to avoid paging out code that was about to be used. Under Mach this wasn't possible because the kernel had no real idea what the operating system consisted of. Instead they had to use a single one-size-fits-all solution that added to the performance problems. Mach 3 attempted to address this problem by providing a simple pager, relying on user-space pagers for better specialization. But this turned out to have little effect. In practice, any benefits it had were wiped out by the expensive IPC needed to call it in.