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Message-ID: <CAJhEC05pmnTd9mROTazKMFzSO+CcpY8au57oypCiXGaqhpA_2Q@mail.gmail.com>
Date: Thu, 12 Jun 2025 22:00:14 +0900
From: Robert Pang <robertpang@...gle.com>
To: Kuan-Wei Chiu <visitorckw@...il.com>
Cc: corbet@....net, colyli@...nel.org, kent.overstreet@...ux.dev,
akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
linux-doc@...r.kernel.org, linux-bcache@...r.kernel.org,
jserv@...s.ncku.edu.tw, stable@...r.kernel.org
Subject: Re: [PATCH 1/8] lib min_heap: Add equal-elements-aware sift_down variant
Hi Kuan-Wei
Thanks for this patch series to address the bcache latency regression.
I tested it but results show regression still remains. Upon review of
the patch changes, I notice that the min_heap_sift_down_eqaware_inline
#define macro in this patch may have been mapped incorrectly:
+#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args) \
+ __min_heap_sift_down_inline(container_of(&(_heap)->nr,
min_heap_char, nr), _pos, \
+ __minheap_obj_size(_heap), _func, _args)
I changed it to map to its "eqaware" counterpart like this and the
regression does not happen again.
+#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args) \
+ __min_heap_sift_down_eqaware_inline(container_of(&(_heap)->nr,
min_heap_char, nr), _pos, \
+ __minheap_obj_size(_heap), _func, _args)
Do you think this correction is appropriate?
Best regards
Robert Pang
On Wed, Jun 11, 2025 at 6:55 AM Kuan-Wei Chiu <visitorckw@...il.com> wrote:
>
> The existing min_heap_sift_down() uses the bottom-up heapify variant,
> which reduces the number of comparisons from ~2 * log2(n) to
> ~1 * log2(n) when all elements are distinct. However, in workloads
> where the heap contains many equal elements, this bottom-up variant
> can degenerate and perform up to 2 * log2(n) comparisons, while the
> traditional top-down variant needs only O(1) comparisons in such cases.
>
> To address this, introduce min_heap_sift_down_eqaware(), a top-down
> heapify variant optimized for scenarios with many equal elements. This
> variant avoids unnecessary comparisons and swaps when elements are
> already equal or in the correct position.
>
> Cc: stable@...r.kernel.org # 6.11+
> Signed-off-by: Kuan-Wei Chiu <visitorckw@...il.com>
> ---
> include/linux/min_heap.h | 51 ++++++++++++++++++++++++++++++++++++++++
> lib/min_heap.c | 7 ++++++
> 2 files changed, 58 insertions(+)
>
> diff --git a/include/linux/min_heap.h b/include/linux/min_heap.h
> index 79ddc0adbf2b..b0d603fe5379 100644
> --- a/include/linux/min_heap.h
> +++ b/include/linux/min_heap.h
> @@ -292,6 +292,52 @@ void __min_heap_sift_down_inline(min_heap_char *heap, size_t pos, size_t elem_si
> __min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos, \
> __minheap_obj_size(_heap), _func, _args)
>
> +/*
> + * Sift the element at pos down the heap.
> + *
> + * Variants of heap functions using an equal-elements-aware sift_down.
> + * These may perform better when the heap contains many equal elements.
> + */
> +static __always_inline
> +void __min_heap_sift_down_eqaware_inline(min_heap_char * heap, size_t pos, size_t elem_size,
> + const struct min_heap_callbacks *func, void *args)
> +{
> + void *data = heap->data;
> + void (*swp)(void *lhs, void *rhs, void *args) = func->swp;
> + /* pre-scale counters for performance */
> + size_t a = pos * elem_size;
> + size_t b, c, smallest;
> + size_t n = heap->nr * elem_size;
> +
> + if (!swp)
> + swp = select_swap_func(data, elem_size);
> +
> + for (;;) {
> + b = 2 * a + elem_size;
> + c = b + elem_size;
> + smallest = a;
> +
> + if (b >= n)
> + break;
> +
> + if (func->less(data + b, data + smallest, args))
> + smallest = b;
> +
> + if (c < n && func->less(data + c, data + smallest, args))
> + smallest = c;
> +
> + if (smallest == a)
> + break;
> +
> + do_swap(data + a, data + smallest, elem_size, swp, args);
> + a = smallest;
> + }
> +}
> +
> +#define min_heap_sift_down_eqaware_inline(_heap, _pos, _func, _args) \
> + __min_heap_sift_down_inline(container_of(&(_heap)->nr, min_heap_char, nr), _pos, \
> + __minheap_obj_size(_heap), _func, _args)
> +
> /* Sift up ith element from the heap, O(log2(nr)). */
> static __always_inline
> void __min_heap_sift_up_inline(min_heap_char *heap, size_t elem_size, size_t idx,
> @@ -433,6 +479,8 @@ void *__min_heap_peek(struct min_heap_char *heap);
> bool __min_heap_full(min_heap_char *heap);
> void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
> const struct min_heap_callbacks *func, void *args);
> +void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
> + const struct min_heap_callbacks *func, void *args);
> void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
> const struct min_heap_callbacks *func, void *args);
> void __min_heapify_all(min_heap_char *heap, size_t elem_size,
> @@ -455,6 +503,9 @@ bool __min_heap_del(min_heap_char *heap, size_t elem_size, size_t idx,
> #define min_heap_sift_down(_heap, _pos, _func, _args) \
> __min_heap_sift_down(container_of(&(_heap)->nr, min_heap_char, nr), _pos, \
> __minheap_obj_size(_heap), _func, _args)
> +#define min_heap_sift_down_eqaware(_heap, _pos, _func, _args) \
> + __min_heap_sift_down_eqaware(container_of(&(_heap)->nr, min_heap_char, nr), _pos, \
> + __minheap_obj_size(_heap), _func, _args)
> #define min_heap_sift_up(_heap, _idx, _func, _args) \
> __min_heap_sift_up(container_of(&(_heap)->nr, min_heap_char, nr), \
> __minheap_obj_size(_heap), _idx, _func, _args)
> diff --git a/lib/min_heap.c b/lib/min_heap.c
> index 96f01a4c5fb6..2225f40d0d7a 100644
> --- a/lib/min_heap.c
> +++ b/lib/min_heap.c
> @@ -27,6 +27,13 @@ void __min_heap_sift_down(min_heap_char *heap, size_t pos, size_t elem_size,
> }
> EXPORT_SYMBOL(__min_heap_sift_down);
>
> +void __min_heap_sift_down_eqaware(min_heap_char *heap, size_t pos, size_t elem_size,
> + const struct min_heap_callbacks *func, void *args)
> +{
> + __min_heap_sift_down_eqaware_inline(heap, pos, elem_size, func, args);
> +}
> +EXPORT_SYMBOL(__min_heap_sift_down_eqaware);
> +
> void __min_heap_sift_up(min_heap_char *heap, size_t elem_size, size_t idx,
> const struct min_heap_callbacks *func, void *args)
> {
> --
> 2.34.1
>
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