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

[Huang\, Ying]

Mike Kravetz <mike.kravetz@xxxxxxxxxx> writes:

> On 08/07/2017 12:21 AM, 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%
>> 
>> 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.
>
> hugetlb_fault masks off the actual faulting address with,
>         address &= huge_page_mask(h);
> before calling hugetlb_no_page.
>
> But, we could pass down the actual (unmasked) address to take advantage
> of this optimization for hugetlb faults as well.  hugetlb_fault is the
> only caller of hugetlb_no_page, so this should be pretty straight forward.
>
> Were you thinking of additional improvements?

No.  I am thinking of something like this.  If the basic idea is
accepted, I plan to add better support like this for hugetlbfs in
another patch.

Best Regards,
Huang, Ying