Re: [PATCH] mm: Compute mTHP order efficiently

[Dev Jain <dev.jain@arm.com>]

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...

For the prefetching thingy, I am still doing a linear scan, and in each
iteration, with the patch, the range I am scanning is going to strictly
lie inside the range I would have scanned without the patch. Won't the
compiler and the CPU still do prefetching, but on a smaller range; where
does the prefetcher get confused? I confess, I do not understand this
very well.