On Thu, 23 Feb 2012 15:00:34 -0500 Rik van Riel <riel@xxxxxxxxxx> wrote: > Some programs have a large number of VMAs, and make frequent calls > to mmap and munmap. Having munmap constantly cause the search > pointer for get_unmapped_area to get reset can cause a significant > slowdown for such programs. > > Likewise, starting all the way from the top any time we mmap a small > VMA can greatly increase the amount of time spent in > arch_get_unmapped_area_topdown. > > For programs with many VMAs, a next-fit algorithm would be fastest, > however that could waste a lot of virtual address space, and potentially > page table memory. > > A compromise is to reset the search pointer for get_unmapped_area > after we have unmapped 1/8th of the normal memory in a process. ick! > For > a process with 1000 similar sized VMAs, that means the search pointer > will only be reset once every 125 or so munmaps. The cost is that > the program may use about 1/8th more virtual space for these VMAs, > and up to 1/8th more page tables. > > We do not count special mappings, since there are programs that > use a large fraction of their address space mapping device memory, > etc. > > The benefit is that things scale a lot better, and we remove about > 200 lines of code. We've been playing whack-a-mole with this search for many years. What about developing a proper data structure with which to locate a suitable-sized hole in O(log(N)) time? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>