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SubjectRe: [PATCH] ext4: convert to DIV_ROUND_UP() in mpage_process_page_bufs()
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

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