linux-kernel - Re: [PATCH] mm: Compute mTHP order efficiently

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Message-ID: <42d6f3db-33db-4475-97e3-fbd28ea131ea@redhat.com>
Date: Tue, 17 Sep 2024 12:12:05 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, Matthew Wilcox <willy@...radead.org>
Cc: akpm@...ux-foundation.org, ryan.roberts@....com,
 anshuman.khandual@....com, baohua@...nel.org, hughd@...gle.com,
 ioworker0@...il.com, wangkefeng.wang@...wei.com,
 baolin.wang@...ux.alibaba.com, gshan@...hat.com,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: Compute mTHP order efficiently

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