Re: [RFC PATCH] mm: add a vma to vmacache when addr overlaps the vma range

[Matthew Wilcox <willy@infradead.org>]

On Tue, Oct 16, 2018 at 09:47:12PM +0800, Wei Yang wrote:
> Based on my understanding, this change would put more accurate vma entry in the
> cache, which means reduce unnecessary vmacache update and vmacache find.
> 
> But the test result is not as expected. From the original changelog, I don't
> see the explanation to add this non-overlap entry into the vmacache, so
> curious about why this performs a little better than putting an overlapped
> entry.

What makes you think this performs any differently for this test-case?
The numbers all seem to fall within a "reasonable variation" range to me.
You're going to need to do some statistics (with a much larger sample
size) to know whether there's any difference at all.

> Below is the test result for building kernel in two cases:
> 
>          make -j4                   make -j8
> base-line:
> 
> real    6m15.947s          real    5m11.684s
> user    21m14.481s         user    27m23.471s
> sys     2m34.407s          sys     3m13.233s
> 
> real    6m16.089s          real    5m11.445s
> user    21m18.295s         user    27m24.045s
> sys     2m35.551s          sys     3m13.443s
> 
> real    6m16.239s          real    5m11.218s
> user    21m17.590s         user    27m19.133s
> sys     2m35.252s          sys     3m12.684s
> 
> patched:
> 
> real    6m15.416s          real    5m10.810s
> user    21m21.800s         user    27m25.223s
> sys     2m33.398s          sys     3m14.784s
> 
> real    6m15.114s          real    5m12.285s
> user    21m19.986s         user    27m32.055s
> sys     2m34.718s          sys     3m13.107s
> 
> 
> real    6m16.206s          real    5m11.509s
> user    21m22.557s         user    27m28.265s
> sys     2m35.637s          sys     3m12.747s
> 
> 
> ---
>  mm/mmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/mmap.c b/mm/mmap.c
> index 2e0daf666f42..dda495d84862 100644
> --- a/mm/mmap.c
> +++ b/mm/mmap.c
> @@ -2214,7 +2214,7 @@ struct vm_area_struct *find_vma(struct mm_struct *mm, unsigned long addr)
>  			rb_node = rb_node->rb_right;
>  	}
>  
> -	if (vma)
> +	if (vma && vma->vm_start <= addr)
>  		vmacache_update(addr, vma);
>  	return vma;
>  }
> -- 
> 2.15.1
>