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SubjectRe: [benchmark] 1% performance overhead of paravirt_ops on native kernels
Ingo Molnar wrote:
> Fork is pretty much the worst-case measurement for larger pte
> overhead, as it has to copy around a lot of pagetables.
>

We could eliminate that if we use the R/W bit on pgd entries. fork()
would be 256 clear_bit()s (1536 and 768 on i386 pae and nonpae).

copy_one_pte() disagrees though:

if (unlikely(!pte_present(pte))) {
if (!pte_file(pte)) {
swp_entry_t entry = pte_to_swp_entry(pte);

swap_duplicate(entry);
/* make sure dst_mm is on swapoff's mmlist. */
if (unlikely(list_empty(&dst_mm->mmlist))) {
spin_lock(&mmlist_lock);
if (list_empty(&dst_mm->mmlist))
list_add(&dst_mm->mmlist,
&src_mm->mmlist);
spin_unlock(&mmlist_lock);
}
if (is_write_migration_entry(entry) &&
is_cow_mapping(vm_flags)) {
/*
* COW mappings require pages in both parent
* and child to be set to read.
*/
make_migration_entry_read(&entry);
pte = swp_entry_to_pte(entry);
set_pte_at(src_mm, addr, src_pte, pte);
}
}
goto out_set_pte;
}


Not sure how we can enlaze this thing.

--
error compiling committee.c: too many arguments to function



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