Messages in this thread |  | | From | Daniel Phillips <> | Subject | Re: Note describing poor dcache utilization under high memory pressure | Date | Tue, 29 Jan 2002 23:53:21 +0100 |
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On January 29, 2002 10:00 pm, Oliver Xymoron wrote: > On Tue, 29 Jan 2002, Daniel Phillips wrote: > > > On January 29, 2002 06:25 pm, Rik van Riel wrote: > > > On Tue, 29 Jan 2002, Oliver Xymoron wrote: > > > > > > > Daniel's approach seems to be workable (once he's spelled out all the > > > > details) but it misses the big performance win for fork/exec, which is > > > > surely the common case. Given that exec will be throwing away all these > > > > mappings, we can safely assume that we will not be inheriting many shared > > > > mappings from parents of parents so Daniel's approach also still ends up > > > > marking most of the pages RO still. > > > > > > It gets worse. His approach also needs to adjust the reference > > > counts on all pages (and swap pages). > > > > Well, Rik, time to present your algorithm. I assume it won't reference > > counts on pages, and will do some kind of traversal of the mm tree. Note > > however, that I did investigate the class of algorithm you are interested in, > > and found only nasty, complex solutions there, with challenging locking > > problems. (I also looked at a number of possible improvements to virtual > > scanning, as you know, and likewise only found ugly or inadequate solutions.) > > I think it goes something like this: > > fork: > detach page tables from parent > retain pointer to "backing page tables" in parent and child > update use count in page tables > "prefault" tables for current stack and instruction pages in both parent > and child > > page fault: > if faulted on page table: > look up backing page tables > if use count > 1: copy, dec use count > else: take ownership > > > Before you sink a lot of time into it though, you might add up the actual > > overhead you're worried about above, and see if it moves the needle in a real > > system. > > I'm pretty sure something like the above does signficantly less work in > the fork/exec case, which is the important one.
With fork/exec, for each page table there are two cases:
- The parent instantiated the page table. In this case the extra work to set the ptes RO (only for CoW pages) is insignificant.
- The parent is still sharing the page table with its parent and so the ptes are still set RO.
I don't see how there is a whole lot of fat to cut here.
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