Re: [PATCH] mm: Compute mTHP order efficiently

[David Hildenbrand <david@redhat.com>]

On 17.09.24 05:55, Dev Jain wrote:
> 
> On 9/16/24 18:54, Matthew Wilcox wrote:
>> On Fri, Sep 13, 2024 at 02:49:02PM +0530, Dev Jain wrote:
>>> We use pte_range_none() to determine whether contiguous PTEs are empty
>>> for an mTHP allocation. Instead of iterating the while loop for every
>>> order, use some information, which is the first set PTE found, from the
>>> previous iteration, to eliminate some cases. The key to understanding
>>> the correctness of the patch is that the ranges we want to examine
>>> form a strictly decreasing sequence of nested intervals.
>> This is a lot more complicated.  Do you have any numbers that indicate
>> that it's faster?  Yes, it's fewer memory references, but you've gone
>> from a simple linear scan that's easy to prefetch to an exponential scan
>> that might confuse the prefetchers.
> 
> I do have some numbers, I tested with a simple program, and also used
> ktime API, with the latter, enclosing from "order = highest_order(orders)"
> till "pte_unmap(pte)" (enclosing the entire while loop), a rough average
> estimate is that without the patch, it takes 1700 ns to execute, with the
> patch, on an average it takes 80 - 100ns less. I cannot think of a good
> testing program...

And that is likely what Willy is actually wondering about: does it have 
any real world impact or is the benefit just noise. :)

Change does not look too wild to me, but yes, it increases complexity.

-- 
Cheers,

David / dhildenb