Re: [Qemu-devel] [RESEND][PATCH] migration: drop MADVISE_DONT_NEED for incoming zero pages

[Zhanghaoyu (A)]

The comments of ram_handle_compressed needs to be changed accordingly,

"Do not memset pages to zero if they already read as zero to avoid allocating 
zero pages and consuming memory unnecessarily."

Thanks,
Zhang Haoyu

> The madvise for zeroed out pages was introduced when every transferred
> zero page was memset to zero and thus allocated. Since commit
> 211ea740 we check for zeroness of a target page before we memset
> it to zero. Additionally we memmap target memory so it is essentially
> zero initialized (except for e.g. option roms and bios which are loaded
> into target memory although they shouldn't).
> 
> It was reported recently that this madvise causes a performance
> degradation
> in some situations. As the madvise should only be called rarely and if
> it's called
> it is likely on a busy page (it was non-zero and changed to zero during
> migration)
> drop it completely.
> 
> Reported-By: Zhang Haoyu <address@hidden>
> Acked-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Peter Lieven <address@hidden>
> ---
>  arch_init.c |    8 --------
>  1 file changed, 8 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 7545d96..e0acbc5 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -850,14 +850,6 @@ void ram_handle_compressed(void *host, uint8_t ch,
> uint64_t size)
>  {
>      if (ch != 0 || !is_zero_range(host, size)) {
>          memset(host, ch, size);
> -#ifndef _WIN32
> -        if (ch == 0 && (!kvm_enabled() || kvm_has_sync_mmu())) {
> -            size = size & ~(getpagesize() - 1);
> -            if (size > 0) {
> -                qemu_madvise(host, size, QEMU_MADV_DONTNEED);
> -            }
> -        }
> -#endif
>      }
>  }
> 
> --
> 1.7.9.5
>