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From: Junio C Hamano <gitster@pobox.com>
To: Taylor Blau <me@ttaylorr.com>
Cc: git@vger.kernel.org, dstolee@microsoft.com, peff@peff.net
Subject: Re: [PATCH 01/23] ewah/ewah_bitmap.c: grow buffer past 1
Date: Sun, 22 Nov 2020 11:36:17 -0800	[thread overview]
Message-ID: <xmqqblfpqj4e.fsf@gitster.c.googlers.com> (raw)
In-Reply-To: <36deaad366d66d10b96755dd6969bfe51123a2d4.1605123652.git.me@ttaylorr.com> (Taylor Blau's message of "Wed, 11 Nov 2020 14:41:44 -0500")

Taylor Blau <me@ttaylorr.com> writes:

> When the buffer size is exactly 1, we fail to grow it properly, since
> the integer truncation means that 1 * 3 / 2 = 1. This can cause a bad
> write on the line below.

When the buffer_size is exactly (alloc_size - 1), we can fit the new
element at the last word in the buffer array, but we still grow.  Is
this because we anticipate that we would need to add more soon?

> Bandaid this by first padding the buffer by 16, and then growing it.
> This still allows old blocks to fit into new ones, but fixes the case
> where the block size equals 1.

Adding 16 unconditionally is not "to pad".  If somebody really wants
"to pad", a likely implementation would be that the size resulting
from some computation (e.g. multiplying by 1.5) is round up to a
multiple of some number, than rounding up the original number before
multiplying it by 1.5, so the use of that verb in the explanation
did not help me understand what is going on.

Having said that, I see you used the word "bandaid" to signal that
we shouldn't worry about this being optimal or even correct and we
should be happy as long as it is not wrong ;-), but is there any
reason behind this 16 (as opposed to picking, say, 8 or 31), or is
that pulled out of thin air?

I think this probably mimics what alloc_nr() computes for ALLOC_GROW().
I wonder why buffer_grow() cannot be built around ALLOC_GROW() instead?

Nothing in the code is wrong per-se, but just what I noticed while
re-reading the patch.

Thanks.

> Co-authored-by: Jeff King <peff@peff.net>
> Signed-off-by: Jeff King <peff@peff.net>
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>  ewah/ewah_bitmap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/ewah/ewah_bitmap.c b/ewah/ewah_bitmap.c
> index d59b1afe3d..3fae04ad00 100644
> --- a/ewah/ewah_bitmap.c
> +++ b/ewah/ewah_bitmap.c
> @@ -45,7 +45,7 @@ static inline void buffer_grow(struct ewah_bitmap *self, size_t new_size)
>  static inline void buffer_push(struct ewah_bitmap *self, eword_t value)
>  {
>  	if (self->buffer_size + 1 >= self->alloc_size)
> -		buffer_grow(self, self->buffer_size * 3 / 2);
> +		buffer_grow(self, (self->buffer_size + 16) * 3 / 2);
>  
>  	self->buffer[self->buffer_size++] = value;
>  }

  reply	other threads:[~2020-11-22 19:40 UTC|newest]

Thread overview: 173+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-11 19:41 [PATCH 00/23] pack-bitmap: bitmap generation improvements Taylor Blau
2020-11-11 19:41 ` [PATCH 01/23] ewah/ewah_bitmap.c: grow buffer past 1 Taylor Blau
2020-11-22 19:36   ` Junio C Hamano [this message]
2020-11-23 16:22     ` Taylor Blau
2020-11-24  2:48       ` Jeff King
2020-11-24  2:51         ` Jeff King
2020-12-01 22:56           ` Taylor Blau
2020-11-11 19:41 ` [PATCH 02/23] pack-bitmap: fix header size check Taylor Blau
2020-11-12 17:39   ` Martin Ågren
2020-11-11 19:42 ` [PATCH 03/23] pack-bitmap: bounds-check size of cache extension Taylor Blau
2020-11-12 17:47   ` Martin Ågren
2020-11-13  4:57     ` Jeff King
2020-11-13  5:26       ` Martin Ågren
2020-11-13 21:29       ` Taylor Blau
2020-11-13 21:39         ` Jeff King
2020-11-13 21:49           ` Taylor Blau
2020-11-13 22:11             ` Jeff King
2020-11-11 19:42 ` [PATCH 04/23] t5310: drop size of truncated ewah bitmap Taylor Blau
2020-11-11 19:42 ` [PATCH 05/23] rev-list: die when --test-bitmap detects a mismatch Taylor Blau
2020-11-11 19:42 ` [PATCH 06/23] ewah: factor out bitmap growth Taylor Blau
2020-11-11 19:42 ` [PATCH 07/23] ewah: make bitmap growth less aggressive Taylor Blau
2020-11-22 20:32   ` Junio C Hamano
2020-11-23 16:49     ` Taylor Blau
2020-11-24  3:00       ` Jeff King
2020-11-24 20:11         ` Junio C Hamano
2020-11-11 19:43 ` [PATCH 08/23] ewah: implement bitmap_or() Taylor Blau
2020-11-22 20:34   ` Junio C Hamano
2020-11-23 16:52     ` Taylor Blau
2020-11-11 19:43 ` [PATCH 09/23] ewah: add bitmap_dup() function Taylor Blau
2020-11-11 19:43 ` [PATCH 10/23] pack-bitmap-write: reimplement bitmap writing Taylor Blau
2020-11-11 19:43 ` [PATCH 11/23] pack-bitmap-write: pass ownership of intermediate bitmaps Taylor Blau
2020-11-11 19:43 ` [PATCH 12/23] pack-bitmap-write: fill bitmap with commit history Taylor Blau
2020-11-11 19:43 ` [PATCH 13/23] bitmap: add bitmap_diff_nonzero() Taylor Blau
2020-11-11 19:43 ` [PATCH 14/23] commit: implement commit_list_contains() Taylor Blau
2020-11-11 19:43 ` [PATCH 15/23] t5310: add branch-based checks Taylor Blau
2020-11-11 20:58   ` Derrick Stolee
2020-11-11 21:04     ` Junio C Hamano
2020-11-15 23:26       ` Johannes Schindelin
2020-11-11 19:43 ` [PATCH 16/23] pack-bitmap-write: rename children to reverse_edges Taylor Blau
2020-11-11 19:43 ` [PATCH 17/23] pack-bitmap-write: build fewer intermediate bitmaps Taylor Blau
2020-11-13 22:23   ` SZEDER Gábor
2020-11-13 23:03     ` Jeff King
2020-11-14  6:23       ` Jeff King
2020-11-11 19:43 ` [PATCH 18/23] pack-bitmap-write: ignore BITMAP_FLAG_REUSE Taylor Blau
2020-11-11 19:44 ` [PATCH 19/23] pack-bitmap: factor out 'bitmap_for_commit()' Taylor Blau
2020-11-11 19:44 ` [PATCH 20/23] pack-bitmap: factor out 'add_commit_to_bitmap()' Taylor Blau
2020-11-11 19:44 ` [PATCH 21/23] pack-bitmap-write: use existing bitmaps Taylor Blau
2020-11-11 19:44 ` [PATCH 22/23] pack-bitmap-write: relax unique rewalk condition Taylor Blau
2020-11-11 19:44 ` [PATCH 23/23] pack-bitmap-write: better reuse bitmaps Taylor Blau
2020-11-17 21:46 ` [PATCH v2 00/24] pack-bitmap: bitmap generation improvements Taylor Blau
2020-11-17 21:46   ` [PATCH v2 01/24] ewah/ewah_bitmap.c: grow buffer past 1 Taylor Blau
2020-11-17 21:46   ` [PATCH v2 02/24] pack-bitmap: fix header size check Taylor Blau
2020-11-17 21:46   ` [PATCH v2 03/24] pack-bitmap: bounds-check size of cache extension Taylor Blau
2020-11-17 21:46   ` [PATCH v2 04/24] t5310: drop size of truncated ewah bitmap Taylor Blau
2020-11-17 21:46   ` [PATCH v2 05/24] rev-list: die when --test-bitmap detects a mismatch Taylor Blau
2020-11-17 21:46   ` [PATCH v2 06/24] ewah: factor out bitmap growth Taylor Blau
2020-11-17 21:47   ` [PATCH v2 07/24] ewah: make bitmap growth less aggressive Taylor Blau
2020-11-17 21:47   ` [PATCH v2 08/24] ewah: implement bitmap_or() Taylor Blau
2020-11-17 21:47   ` [PATCH v2 09/24] ewah: add bitmap_dup() function Taylor Blau
2020-11-17 21:47   ` [PATCH v2 10/24] pack-bitmap-write: reimplement bitmap writing Taylor Blau
2020-11-25  0:53     ` Jonathan Tan
2020-11-28 17:27       ` Taylor Blau
2020-11-17 21:47   ` [PATCH v2 11/24] pack-bitmap-write: pass ownership of intermediate bitmaps Taylor Blau
2020-11-25  1:00     ` Jonathan Tan
2020-11-17 21:47   ` [PATCH v2 12/24] pack-bitmap-write: fill bitmap with commit history Taylor Blau
2020-11-22 21:50     ` Junio C Hamano
2020-11-23 14:54       ` Derrick Stolee
2020-11-25  1:14     ` Jonathan Tan
2020-11-28 17:21       ` Taylor Blau
2020-11-30 18:33         ` Jonathan Tan
2020-11-17 21:47   ` [PATCH v2 13/24] bitmap: add bitmap_diff_nonzero() Taylor Blau
2020-11-22 22:01     ` Junio C Hamano
2020-11-23 20:19       ` Taylor Blau
2020-11-17 21:47   ` [PATCH v2 14/24] commit: implement commit_list_contains() Taylor Blau
2020-11-17 21:47   ` [PATCH v2 15/24] t5310: add branch-based checks Taylor Blau
2020-11-25  1:17     ` Jonathan Tan
2020-11-28 17:30       ` Taylor Blau
2020-11-17 21:47   ` [PATCH v2 16/24] pack-bitmap-write: rename children to reverse_edges Taylor Blau
2020-11-17 21:47   ` [PATCH v2 17/24] pack-bitmap.c: check reads more aggressively when loading Taylor Blau
2020-11-17 21:48   ` [PATCH v2 18/24] pack-bitmap-write: build fewer intermediate bitmaps Taylor Blau
2020-11-24  6:07     ` Jonathan Tan
2020-11-25  1:46     ` Jonathan Tan
2020-11-30 18:41       ` Derrick Stolee
2020-11-17 21:48   ` [PATCH v2 19/24] pack-bitmap-write: ignore BITMAP_FLAG_REUSE Taylor Blau
2020-12-02  7:13     ` Jonathan Tan
2020-11-17 21:48   ` [PATCH v2 20/24] pack-bitmap: factor out 'bitmap_for_commit()' Taylor Blau
2020-12-02  7:17     ` Jonathan Tan
2020-11-17 21:48   ` [PATCH v2 21/24] pack-bitmap: factor out 'add_commit_to_bitmap()' Taylor Blau
2020-12-02  7:20     ` Jonathan Tan
2020-11-17 21:48   ` [PATCH v2 22/24] pack-bitmap-write: use existing bitmaps Taylor Blau
2020-12-02  7:28     ` Jonathan Tan
2020-12-02 16:21       ` Taylor Blau
2020-11-17 21:48   ` [PATCH v2 23/24] pack-bitmap-write: relax unique rewalk condition Taylor Blau
2020-12-02  7:44     ` Jonathan Tan
2020-12-02 16:30       ` Taylor Blau
2020-12-07 18:19         ` Jonathan Tan
2020-12-07 18:43           ` Derrick Stolee
2020-12-07 18:45             ` Derrick Stolee
2020-12-07 18:48           ` Jeff King
2020-11-17 21:48   ` [PATCH v2 24/24] pack-bitmap-write: better reuse bitmaps Taylor Blau
2020-12-02  8:08     ` Jonathan Tan
2020-12-02 16:35       ` Taylor Blau
2020-12-02 18:22         ` Derrick Stolee
2020-12-02 18:25           ` Taylor Blau
2020-12-07 18:26             ` Jonathan Tan
2020-12-07 18:24           ` Jonathan Tan
2020-12-07 19:20             ` Derrick Stolee
2020-11-18 18:32   ` [PATCH v2 00/24] pack-bitmap: bitmap generation improvements SZEDER Gábor
2020-11-18 19:51     ` Taylor Blau
2020-11-22  2:17       ` Taylor Blau
2020-11-22  2:28         ` Taylor Blau
2020-11-20  6:34   ` Martin Ågren
2020-11-21 19:37     ` Junio C Hamano
2020-11-21 20:11       ` Martin Ågren
2020-11-22  2:31         ` Taylor Blau
2020-11-24  2:43           ` Jeff King
2020-12-01 23:04             ` Taylor Blau
2020-12-01 23:37               ` Jonathan Tan
2020-12-01 23:43                 ` Taylor Blau
2020-12-02  8:11                   ` Jonathan Tan
2020-12-08  0:04 ` [PATCH v3 " Taylor Blau
2020-12-08  0:04   ` [PATCH v3 01/24] ewah/ewah_bitmap.c: avoid open-coding ALLOC_GROW() Taylor Blau
2020-12-08  0:04   ` [PATCH v3 02/24] pack-bitmap: fix header size check Taylor Blau
2020-12-08  0:04   ` [PATCH v3 03/24] pack-bitmap: bounds-check size of cache extension Taylor Blau
2020-12-08  0:04   ` [PATCH v3 04/24] t5310: drop size of truncated ewah bitmap Taylor Blau
2020-12-08  0:04   ` [PATCH v3 05/24] rev-list: die when --test-bitmap detects a mismatch Taylor Blau
2020-12-08  0:04   ` [PATCH v3 06/24] ewah: factor out bitmap growth Taylor Blau
2020-12-08  0:04   ` [PATCH v3 07/24] ewah: make bitmap growth less aggressive Taylor Blau
2020-12-08  0:04   ` [PATCH v3 08/24] ewah: implement bitmap_or() Taylor Blau
2020-12-08  0:04   ` [PATCH v3 09/24] ewah: add bitmap_dup() function Taylor Blau
2020-12-08  0:04   ` [PATCH v3 10/24] pack-bitmap-write: reimplement bitmap writing Taylor Blau
2020-12-08  0:05   ` [PATCH v3 11/24] pack-bitmap-write: pass ownership of intermediate bitmaps Taylor Blau
2020-12-08  0:05   ` [PATCH v3 12/24] pack-bitmap-write: fill bitmap with commit history Taylor Blau
2020-12-08  0:05   ` [PATCH v3 13/24] bitmap: implement bitmap_is_subset() Taylor Blau
2020-12-08  0:05   ` [PATCH v3 14/24] commit: implement commit_list_contains() Taylor Blau
2020-12-08  0:05   ` [PATCH v3 15/24] t5310: add branch-based checks Taylor Blau
2020-12-08  0:05   ` [PATCH v3 16/24] pack-bitmap-write: rename children to reverse_edges Taylor Blau
2020-12-08  0:05   ` [PATCH v3 17/24] pack-bitmap.c: check reads more aggressively when loading Taylor Blau
2020-12-08  0:05   ` [PATCH v3 18/24] pack-bitmap-write: build fewer intermediate bitmaps Taylor Blau
2020-12-08  0:05   ` [PATCH v3 19/24] pack-bitmap-write: ignore BITMAP_FLAG_REUSE Taylor Blau
2020-12-08  0:05   ` [PATCH v3 20/24] pack-bitmap: factor out 'bitmap_for_commit()' Taylor Blau
2020-12-08  0:05   ` [PATCH v3 21/24] pack-bitmap: factor out 'add_commit_to_bitmap()' Taylor Blau
2020-12-08  0:05   ` [PATCH v3 22/24] pack-bitmap-write: use existing bitmaps Taylor Blau
2020-12-08  0:05   ` [PATCH v3 23/24] pack-bitmap-write: relax unique rewalk condition Taylor Blau
2020-12-08  0:05   ` [PATCH v3 24/24] pack-bitmap-write: better reuse bitmaps Taylor Blau
2020-12-08 20:56   ` [PATCH v3 00/24] pack-bitmap: bitmap generation improvements Junio C Hamano
2020-12-08 21:03     ` Taylor Blau
2020-12-08 22:03       ` Junio C Hamano
2020-12-08 22:03 ` [PATCH v4 " Taylor Blau
2020-12-08 22:03   ` [PATCH v4 01/24] ewah/ewah_bitmap.c: avoid open-coding ALLOC_GROW() Taylor Blau
2020-12-08 22:03   ` [PATCH v4 02/24] pack-bitmap: fix header size check Taylor Blau
2020-12-08 22:03   ` [PATCH v4 03/24] pack-bitmap: bounds-check size of cache extension Taylor Blau
2020-12-08 22:03   ` [PATCH v4 04/24] t5310: drop size of truncated ewah bitmap Taylor Blau
2020-12-08 22:03   ` [PATCH v4 05/24] rev-list: die when --test-bitmap detects a mismatch Taylor Blau
2020-12-08 22:03   ` [PATCH v4 06/24] ewah: factor out bitmap growth Taylor Blau
2020-12-08 22:03   ` [PATCH v4 07/24] ewah: make bitmap growth less aggressive Taylor Blau
2020-12-08 22:03   ` [PATCH v4 08/24] ewah: implement bitmap_or() Taylor Blau
2020-12-08 22:03   ` [PATCH v4 09/24] ewah: add bitmap_dup() function Taylor Blau
2020-12-08 22:03   ` [PATCH v4 10/24] pack-bitmap-write: reimplement bitmap writing Taylor Blau
2020-12-08 22:03   ` [PATCH v4 11/24] pack-bitmap-write: pass ownership of intermediate bitmaps Taylor Blau
2020-12-08 22:04   ` [PATCH v4 12/24] pack-bitmap-write: fill bitmap with commit history Taylor Blau
2020-12-08 22:04   ` [PATCH v4 13/24] bitmap: implement bitmap_is_subset() Taylor Blau
2020-12-08 22:04   ` [PATCH v4 14/24] commit: implement commit_list_contains() Taylor Blau
2020-12-08 22:04   ` [PATCH v4 15/24] t5310: add branch-based checks Taylor Blau
2020-12-08 22:04   ` [PATCH v4 16/24] pack-bitmap-write: rename children to reverse_edges Taylor Blau
2020-12-08 22:04   ` [PATCH v4 17/24] pack-bitmap.c: check reads more aggressively when loading Taylor Blau
2020-12-08 22:04   ` [PATCH v4 18/24] pack-bitmap-write: build fewer intermediate bitmaps Taylor Blau
2020-12-08 22:04   ` [PATCH v4 19/24] pack-bitmap-write: ignore BITMAP_FLAG_REUSE Taylor Blau
2020-12-08 22:04   ` [PATCH v4 20/24] pack-bitmap: factor out 'bitmap_for_commit()' Taylor Blau
2020-12-08 22:05   ` [PATCH v4 21/24] pack-bitmap: factor out 'add_commit_to_bitmap()' Taylor Blau
2020-12-08 22:05   ` [PATCH v4 22/24] pack-bitmap-write: use existing bitmaps Taylor Blau
2020-12-08 22:05   ` [PATCH v4 23/24] pack-bitmap-write: relax unique revwalk condition Taylor Blau
2020-12-08 22:05   ` [PATCH v4 24/24] pack-bitmap-write: better reuse bitmaps Taylor Blau

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