linux-kernel - Re: Potential heapify performance issue in dm-vdo

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Message-ID: <9501613e-e8e5-4734-aa2f-ca3a3e4ca990@redhat.com>
Date: Wed, 21 May 2025 14:57:26 -0400
From: Matthew Sakai <msakai@...hat.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: dm-devel@...ts.linux.dev, linux-kernel@...r.kernel.org,
 jserv@...s.ncku.edu.tw
Subject: Re: Potential heapify performance issue in dm-vdo



On 5/21/25 11:04 AM, Kuan-Wei Chiu wrote:
> Hi Matthew,
> 
> Recently noticed that the current heapify method in min_heap.h may
> degrade performance when the heap contains many compare-equal elements,
> compared to the previous version.
> 
> In detail, the new heapify reduces the number of comparisons by about
> 50% when all elements are distinct. However, when all elements are
> equal, the comparison count can degrade from O(1) to O(log n).
> 
> I don't have enough domain knowledge of dm-vdo, so I'd like to ask
> whether it uses heaps with many compare-equal elements. If so, I'll
> work on fixing the issue.
> 
> Regards,
> Kuan-Wei

Hi Kuan-Wei,

dm-vdo uses heapify for two different operations, but in both cases we 
define heap elements that can never be equal to each other. So I think 
this is not an issue for dm-vdo. (We have not noticed any issues with 
this in our regular testing, either.)

Matt