On Fri, Jun 25, 2021, Peter Xu wrote:
> Currently rmap array element only contains 3 entries. However for EPT=N there
> could have a lot of guest pages that got tens of even hundreds of rmap entry.
>
> A normal distribution of a 6G guest (even if idle) shows this with rmap count
> statistics:
>
> Rmap_Count: 0 1 2-3 4-7 8-15 16-31 32-63 64-127 128-255 256-511 512-1023
> Level=4K: 3089171 49005 14016 1363 235 212 15 7 0 0 0
> Level=2M: 5951 227 0 0 0 0 0 0 0 0 0
> Level=1G: 32 0 0 0 0 0 0 0 0 0 0
>
> If we do some more fork some pages will grow even larger rmap counts.
>
> This patch makes PTE_LIST_EXT bigger so it'll be more efficient for the general
> use case of EPT=N as we do list reference less and the loops over PTE_LIST_EXT
> will be slightly more efficient; but still not too large so less waste when
> array not full.
>
> It should not affecting EPT=Y since EPT normally only has zero or one rmap
> entry for each page, so no array is even allocated.
>
> With a test case to fork 500 child and recycle them ("./rmap_fork 500" [1]),
> this patch speeds up fork time of about 22%.
>
> Before: 367.20 (+-4.58%)
> After: 302.00 (+-5.30%)
>
> [1] https://github.com/xzpeter/clibs/commit/825436f825453de2ea5aaee4bdb1c92281efe5b3
>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
> arch/x86/kvm/mmu/mmu.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index b3f738a7c05e..9b093985a2ef 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -137,8 +137,8 @@ module_param(dbg, bool, 0644);
>
> #include <trace/events/kvm.h>
>
> -/* make pte_list_desc fit well in cache line */
> -#define PTE_LIST_EXT 3
> +/* make pte_list_desc fit well in cache lines */
> +#define PTE_LIST_EXT 15
Ha, I was going to say that this should be '14' to fit pte_list_desc within two
cache lines, but looks like Paolo fixed it up on commit.
Also, if the whole cache line thing actually matters, sptes[] and spte_count
should be swapped since spte_count is always read, whereas spte_count[7:14] will
be read iff there are 8+ SPTEs.
> struct pte_list_desc {
> u64 *sptes[PTE_LIST_EXT];
> --
> 2.31.1
>
