Messages in this thread | | | Date | Tue, 03 Aug 2010 09:51:25 +0300 | From | Avi Kivity <> | Subject | Re: [PATCH] kvm cleanup: Introduce sibling_pte and do cleanup for reverse map and parent_pte |
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On 08/03/2010 05:30 AM, Lai Jiangshan wrote: > This patch is just a big cleanup. it reduces 220 lines of code. > > It introduces sibling_pte array for tracking identical sptes, so the > identical sptes can be linked as a single linked list by their > corresponding sibling_pte. A reverse map or a parent_pte points at > the head of this single linked list. So we can do cleanup for > reverse map and parent_pte VERY LARGELY. > > BAD: > If most rmap have only one entry or most sp have only one parent, > this patch may use more memory than before.
That is the case with NPT and EPT. Each page has exactly one spte (except a few vga pages), and each sp has exactly one parent_pte (except the root pages).
> GOOD: > 1) Reduce a lot of code, The functions which are in hot path becomes > very very simple and terrifically fast. > 2) rmap_next(): O(N) -> O(1). traveling a ramp: O(N*N) -> O(N)
The existing rmap_next() is not O(N), it's O(RMAP_EXT), which is 4. The data structure was chosen over a simple linked list to avoid extra cache misses.
> 3) Remove the ugly interlayer: struct kvm_rmap_desc, struct kvm_pte_chain
kvm_rmap_desc and kvm_pte_chain are indeed ugly, but they do save a lot of memory and cache misses.
> 4) We don't need to allocate any thing when we change the mappings. > So we can avoid allocation when we have held kvm mmu spin lock. > (this feature is very helpful in future). > 5) better readability.
I agree the new code is more readable. Unfortunately it uses more memory and is likely to be slower. You add a cache miss for every spte, while kvm_rmap_desc amortizes the cache miss among 4 sptes, and special cases 1 spte to have no cache misses (or extra memory requirements).
-- I have a truly marvellous patch that fixes the bug which this signature is too narrow to contain.
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