Messages in this thread | | | From | Daniel Phillips <> | Subject | Re: Note describing poor dcache utilization under high memory pressure | Date | Tue, 29 Jan 2002 10:55:24 +0100 |
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On January 29, 2002 10:20 am, William Lee Irwin III wrote: > On Tue, Jan 29, 2002 at 09:55:02AM +0100, Daniel Phillips wrote: > > It's only touching the ptes on tables that are actually used, so if a parent > > with a massive amount of mapped memory forks a child that only instantiates > > a small portion of it (common situation) then the saving is pretty big. > > Please correct my attempt at clarifying this:
Sorry, it's my fault, my explanation above is ambiguous, or even incorrect.
> The COW markings are done at the next higher level of hierarchy above > the pte's themselves, and so experience the radix tree branch factor > reduction in the amount of work done at fork-time in comparison to a > full pagetable copy on fork.
The CoW markings are done at the same level they always have been - directly in the ptes (the 4 byte thingies, please don't get confused by the unfortunate overloading of 'pte' to mean 'page table' in some contexts, e.g., zap_pte_range). But the CoW marking only has to be done for page tables that have use count == 1 at the time of fork. So if the parent inherited some page tables then these already have use count > 1, i.e., are shared, and they don't have to be set up for CoW again when the child forks. Only page tables that the parent instantiated for itself have to be re-marked for CoW.
Wow, this is getting subtle, isn't it?
> On Tue, Jan 29, 2002 at 09:55:02AM +0100, Daniel Phillips wrote: > > Note that I'm not counting on this to be a huge performance win, except in > > the specific case that that is bothering rmap. This is already worth the > > price of admission. > > It is an overall throughput loss in the cases where the majority of the > page table entries are in fact referenced by the child, and this is > more than acceptable because it is more incremental, reference-all is > an uncommon case, and once all the page table entries are referenced, > there are no longer any penalties. Defeating this scheme would truly > require a contrived application, and penalizes only that application.
True. Also, since the child is doing all that work anyway, the cost of instantiating one page table (and extending the pte_chains) per 1K referenced pages will get lost in the noise.
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