From: Eric Biggers <ebiggers@kernel.org>
To: Yangtao Li <frank.li@vivo.com>
Cc: tytso@mit.edu, linux-kernel@vger.kernel.org,
adilger.kernel@dilger.ca, viro@zeniv.linux.org.uk,
hsiangkao@linux.alibaba.com, linux-ext4@vger.kernel.org,
linux-erofs@lists.ozlabs.org
Subject: Re: [PATCH] ext4: convert to DIV_ROUND_UP() in mpage_process_page_bufs()
Date: Thu, 9 Mar 2023 22:41:01 -0800 [thread overview]
Message-ID: <ZArQ/WaBrw0LR56v@sol.localdomain> (raw)
In-Reply-To: <20230310062738.69663-1-frank.li@vivo.com>
On Fri, Mar 10, 2023 at 02:27:38PM +0800, Yangtao Li wrote:
> > Please don't do this. This makes the code compile down to a division, which is
> > far less efficient. I've verified this by checking the assembly generated.
>
> How much is the performance impact? So should the following be modified as shift operations as well?
>
> fs/erofs/namei.c:92: int head = 0, back = DIV_ROUND_UP(dir->i_size, EROFS_BLKSIZ) - 1;
> fs/erofs/zmap.c:252: const unsigned int totalidx = DIV_ROUND_UP(inode->i_size, EROFS_BLKSIZ);
> fs/erofs/decompressor.c:14:#define LZ4_MAX_DISTANCE_PAGES (DIV_ROUND_UP(LZ4_DISTANCE_MAX, PAGE_SIZE) + 1)
> fs/erofs/decompressor.c:56: DIV_ROUND_UP(distance, PAGE_SIZE) + 1 :
> fs/erofs/decompressor.c:70: unsigned long bounced[DIV_ROUND_UP(LZ4_MAX_DISTANCE_PAGES,
> fs/erofs/data.c:84: nblocks = DIV_ROUND_UP(inode->i_size, EROFS_BLKSIZ);
>
No, compilers can handle division by a power-of-2 constant just fine. It's just
division by a non-constant that is inefficient.
- Eric
WARNING: multiple messages have this Message-ID (diff)
From: Eric Biggers <ebiggers@kernel.org>
To: Yangtao Li <frank.li@vivo.com>
Cc: tytso@mit.edu, adilger.kernel@dilger.ca,
hsiangkao@linux.alibaba.com, linux-ext4@vger.kernel.org,
linux-erofs@lists.ozlabs.org, linux-kernel@vger.kernel.org,
viro@zeniv.linux.org.uk
Subject: Re: [PATCH] ext4: convert to DIV_ROUND_UP() in mpage_process_page_bufs()
Date: Thu, 9 Mar 2023 22:41:01 -0800 [thread overview]
Message-ID: <ZArQ/WaBrw0LR56v@sol.localdomain> (raw)
In-Reply-To: <20230310062738.69663-1-frank.li@vivo.com>
On Fri, Mar 10, 2023 at 02:27:38PM +0800, Yangtao Li wrote:
> > Please don't do this. This makes the code compile down to a division, which is
> > far less efficient. I've verified this by checking the assembly generated.
>
> How much is the performance impact? So should the following be modified as shift operations as well?
>
> fs/erofs/namei.c:92: int head = 0, back = DIV_ROUND_UP(dir->i_size, EROFS_BLKSIZ) - 1;
> fs/erofs/zmap.c:252: const unsigned int totalidx = DIV_ROUND_UP(inode->i_size, EROFS_BLKSIZ);
> fs/erofs/decompressor.c:14:#define LZ4_MAX_DISTANCE_PAGES (DIV_ROUND_UP(LZ4_DISTANCE_MAX, PAGE_SIZE) + 1)
> fs/erofs/decompressor.c:56: DIV_ROUND_UP(distance, PAGE_SIZE) + 1 :
> fs/erofs/decompressor.c:70: unsigned long bounced[DIV_ROUND_UP(LZ4_MAX_DISTANCE_PAGES,
> fs/erofs/data.c:84: nblocks = DIV_ROUND_UP(inode->i_size, EROFS_BLKSIZ);
>
No, compilers can handle division by a power-of-2 constant just fine. It's just
division by a non-constant that is inefficient.
- Eric
next prev parent reply other threads:[~2023-03-10 6:41 UTC|newest]
Thread overview: 22+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-10 6:07 [PATCH] ext4: convert to DIV_ROUND_UP() in mpage_process_page_bufs() Yangtao Li
2023-03-10 6:17 ` Eric Biggers
2023-03-10 6:27 ` Yangtao Li
2023-03-10 6:27 ` Yangtao Li
2023-03-10 6:35 ` Gao Xiang
2023-03-10 6:35 ` Gao Xiang
2023-03-10 6:37 ` Gao Xiang
2023-03-10 6:37 ` Gao Xiang
2023-03-10 6:41 ` Eric Biggers [this message]
2023-03-10 6:41 ` Eric Biggers
2023-03-10 6:37 ` Al Viro
2023-03-10 6:43 ` Al Viro
2023-03-10 6:43 ` Eric Biggers
2023-03-10 6:46 ` Al Viro
2023-03-10 6:54 ` Eric Biggers
2023-03-10 7:05 ` Al Viro
2023-03-10 7:12 ` Al Viro
2023-03-10 21:47 ` Eric Biggers
2023-03-10 19:07 ` kernel test robot
2023-03-10 19:38 ` kernel test robot
2023-03-10 23:21 ` Theodore Ts'o
2023-03-13 9:25 ` David Laight
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