linux-kernel - Re: [PATCH 0/5] Optimize number of comparisons for heap/heapsort implementaion

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Date: Mon, 22 Jan 2024 11:06:39 -0500
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: colyli@...e.de, bfoster@...hat.com, jserv@...s.ncku.edu.tw, 
	linux-bcache@...r.kernel.org, linux-kernel@...r.kernel.org, linux-bcachefs@...r.kernel.org
Subject: Re: [PATCH 0/5] Optimize number of comparisons for heap/heapsort
 implementaion

On Mon, Jan 22, 2024 at 11:06:54PM +0800, Kuan-Wei Chiu wrote:
> On Sun, Jan 21, 2024 at 12:41:55PM -0500, Kent Overstreet wrote:
> > On Mon, Jan 22, 2024 at 12:55:51AM +0800, Kuan-Wei Chiu wrote:
> > > On Sun, Jan 21, 2024 at 11:21:06AM -0500, Kent Overstreet wrote:
> > > > On Sun, Jan 21, 2024 at 11:36:44PM +0800, Kuan-Wei Chiu wrote:
> > > > > Hello,
> > > > > 
> > > > > The existing implementations of heap/heapsort follow the conventional
> > > > > textbook approach, where each heapify operation requires approximately
> > > > > 2*log2(n) comparisons. In this series, I introduce a bottom-up variant
> > > > > that reduces the number of comparisons during heapify operations to
> > > > > approximately log2(n), while maintaining the same number of swap
> > > > > operations.
> > > > > 
> > > > > Thanks,
> > > > > Kuan-Wei
> > > > > 
> > > > > Kuan-Wei Chiu (5):
> > > > >   bcachefs: Optimize eytzinger0_sort() using bottom-up heapsort
> > > > >   bcachefs: Introduce parent function for sort_cmp_size()
> > > > >   bcachefs: Optimize sort_cmp_size() using bottom-up heapsort
> > > > >   bcachefs: Optimize number of comparisons in heap_sift_down
> > > > >   bcache: Optimize number of comparisons in heap_sift
> > > > > 
> > > > >  drivers/md/bcache/util.h |  23 +++++----
> > > > >  fs/bcachefs/util.c       | 109 ++++++++++++++++++++++++++-------------
> > > > >  fs/bcachefs/util.h       |  23 +++++----
> > > > >  3 files changed, 98 insertions(+), 57 deletions(-)
> > > > 
> > > > Good stuff
> > > > 
> > > > While we're looking at this code, we should be doing some cleanup too -
> > > > there's no reason for the heap code to be duplicated in bcache and
> > > > bcachefs anymore, and it'd also be nice to get fs/bcachefs/eytzinger.h
> > > > moved to include/linux and bcache converted to use it.
> > > > 
> > > > I also would not be surprised if there's another heap implementation in
> > > > include/linux; we'll want to check for that and if there is decide which
> > > > is worth keeping.
> > > >
> > > Yes, we have 'min_heap.h' in include/linux.
> > 
> > So that has the advantage of more readable code - functions instead of
> > macros - whereas my version has the type safe interface.
> > 
> > We could combine the two approaches, and put a type-safe interface on
> > top of the min_heap.h code with some small macro wrappers - see
> > generic-radix-tree.h for an example of how that's done.
> 
> Without modifying the interface provided by min_heap.h, it seems
> challenging to implement the functionality of heap_add due to the
> relationship with heap_setbackpointer.

min_heap.h has the same functionality, different interface - updating
the callers for an interface change is fine.

> 
> Additionally, when looking into the code in generic-radix-tree.h,
> should we replace type[0] with type[]? This is because zero-length
> arrays are deprecated language features mentioned in document [1].

Zero length arrays are deprecated as VLAs, but this isn't a VLA - we're
not storing anything there, the variable is just so that macros have
access to the type.

> Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#zero-length-and-one-element-arrays [1]
> > 
> > min_heap.h has only one user though? I don't think I can quite believe
> > that's the only other code in the kernel using a heap, there must be
> > more open coded out there...
> 
> I'm not sure why, but it seems that in the kernel, other places using
> the heap implement their own subsystem-specific solutions rather than
> utilizing a generic heap interface. For instance,
> kernel/sched/cpudeadline.c and net/sched/sch_cake.c both have their own
> implementations.

Sounds like a fun cleanup project :)