[PATCH]: block_read_full_page: micro optimization

[PATCH]: block_read_full_page: micro optimization

[Paul Drynoff]

I wonder, may be with such change kernel become little faster?

Signed-off-by: Paul Drynoff <pauldrynoff@gmail.com>

---

Index: linux-2.6.17-mm1/fs/buffer.c
===================================================================
--- linux-2.6.17-mm1.orig/fs/buffer.c
+++ linux-2.6.17-mm1/fs/buffer.c
@@ -2093,7 +2093,7 @@ int block_read_full_page(struct page *pa
 			}
 			if (!buffer_mapped(bh)) {
 				void *kaddr = kmap_atomic(page, KM_USER0);
-				memset(kaddr + i * blocksize, 0, blocksize);
+				memset(kaddr + (i << inode->i_blkbits), 0, blocksize);
 				flush_dcache_page(page);
 				kunmap_atomic(kaddr, KM_USER0);
 				if (!err)

Re: [PATCH]: block_read_full_page: micro optimization

[Ray Lee]

On 6/24/06, Paul Drynoff <pauldrynoff@gmail.com> wrote:
> I wonder, may be with such change kernel become little faster?
> -                               memset(kaddr + i * blocksize, 0, blocksize);
> +                               memset(kaddr + (i << inode->i_blkbits), 0, blocksize);

It's likely slower with this change, as you now require the CPU to
load inode->i_blkbits which, chances are, isn't hanging around handily
in a register like blocksize obviously is. So, you've increased
register pressure and added more fetches from memory instead of just
doing a simple (and cheap) multiplication. Not good.

Check the generated code for both cases. Smaller is usually better.
Benchmarking would be best, however, code changes that cause cache
misses or register pressure can be hard to measure well.

Ray