Lazy Allocation Policy

In lazy allocation, swap space is reserved dynamically as the system needs to reclaim physical memory instead of having to allocate it in advance for every page of anonymous memory (that is, memory devoted to the stack and heap of a process, and to data that is not file-backed).

However, when the list of active pages falls below the preconfigured limit and the OSF memory manager attempts to reclaim pages for the free list by paging out virtual pages, if the available swap space has already been exhausted, the OSF memory manager does not back off of the page-out and instead simply discards the page. Thus, when the process whose page has been discarded takes a page fault and attempts to reactivate the missing page, unpredictable behavior results, including system hangs, panics, and at times data corruption.

To correct this problem, Digital reworked the page-out algorithm to ensure that pages are not lost if the memory manager is unable to allocate swap space for a virtual page.

As swap space decreases, the Digital UNIX Version 4.0 memory manager logs warning messages at the console until finally, if the memory manager is unable to allocate swap space for a page-out, it selects the oldest idle process and kills it, thereby freeing up swap space and returning virtual pages to the free list.