Re: [PATCH -mm -v2] mm: Clear to access sub-page last when clearing huge page

[Michal Hocko]

On Tue 15-08-17 09:46:18, Huang, Ying wrote:
> From: Huang Ying <ying.huang@xxxxxxxxx>
> 
> Huge page helps to reduce TLB miss rate, but it has higher cache
> footprint, sometimes this may cause some issue.  For example, when
> clearing huge page on x86_64 platform, the cache footprint is 2M.  But
> on a Xeon E5 v3 2699 CPU, there are 18 cores, 36 threads, and only 45M
> LLC (last level cache).  That is, in average, there are 2.5M LLC for
> each core and 1.25M LLC for each thread.  If the cache pressure is
> heavy when clearing the huge page, and we clear the huge page from the
> begin to the end, it is possible that the begin of huge page is
> evicted from the cache after we finishing clearing the end of the huge
> page.  And it is possible for the application to access the begin of
> the huge page after clearing the huge page.
> 
> To help the above situation, in this patch, when we clear a huge page,
> the order to clear sub-pages is changed.  In quite some situation, we
> can get the address that the application will access after we clear
> the huge page, for example, in a page fault handler.  Instead of
> clearing the huge page from begin to end, we will clear the sub-pages
> farthest from the the sub-page to access firstly, and clear the
> sub-page to access last.  This will make the sub-page to access most
> cache-hot and sub-pages around it more cache-hot too.  If we cannot
> know the address the application will access, the begin of the huge
> page is assumed to be the the address the application will access.
> 
> With this patch, the throughput increases ~28.3% in vm-scalability
> anon-w-seq test case with 72 processes on a 2 socket Xeon E5 v3 2699
> system (36 cores, 72 threads).  The test case creates 72 processes,
> each process mmap a big anonymous memory area and writes to it from
> the begin to the end.  For each process, other processes could be seen
> as other workload which generates heavy cache pressure.  At the same
> time, the cache miss rate reduced from ~33.4% to ~31.7%, the
> IPC (instruction per cycle) increased from 0.56 to 0.74, and the time
> spent in user space is reduced ~7.9%

The patch looks good to me alebit little bit tricky to read.

But I am still wondering. Have you considered non-temporal stores for
clearing?

> Christopher Lameter suggests to clear bytes inside a sub-page from end
> to begin too.  But tests show no visible performance difference in the
> tests.  May because the size of page is small compared with the cache
> size.
> 
> Thanks Andi Kleen to propose to use address to access to determine the
> order of sub-pages to clear.
> 
> The hugetlbfs access address could be improved, will do that in
> another patch.
> 
> [Use address to access information]
> Suggested-by: Andi Kleen <andi.kleen@xxxxxxxxx>
> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> Acked-by: Jan Kara <jack@xxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Nadia Yvette Chambers <nyc@xxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Matthew Wilcox <mawilcox@xxxxxxxxxxxxx>
> Cc: Hugh Dickins <hughd@xxxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Shaohua Li <shli@xxxxxx>
> Cc: Christopher Lameter <cl@xxxxxxxxx>
> Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>

Reviewed-by: Michal Hocko <mhocko@xxxxxxxx>

> +	for (i = 0; i < l; i++) {

I would find it a bit easier to read if this was
		int left_idx = base + i;
		int right_idx = base + 2*l - 1 - i

> +		cond_resched();
> +		clear_user_highpage(page + base + i,
> +				    addr + (base + i) * PAGE_SIZE);
		clear_user_highpage(page + left_idx, addr + left_idx * PAGE_SIZE);

>  		cond_resched();
> -		clear_user_highpage(page + i, addr + i * PAGE_SIZE);
> +		clear_user_highpage(page + base + 2 * l - 1 - i,
> +				    addr + (base + 2 * l - 1 - i) * PAGE_SIZE);
		clear_user_highpage(page + right_idx, addr + right_idx * PAGE_SIZE);
>  	}
>  }
-- 
Michal Hocko
SUSE Labs