Re: [PATCH 10/11] readahead: dont do start-of-file readahead afterlseek()

[Linus Torvalds]

On Tue, 2 Feb 2010, Olivier Galibert wrote:
>
> On Tue, Feb 02, 2010 at 10:40:41AM -0800, Linus Torvalds wrote:
> > IOW, if you start off with a SEEK_END, I think it's reasonable to expect 
> > it to _not_ read the whole thing.
> 
> I've seen a lot of:
>   int fd = open(...);
>   size = lseek(fd, 0, SEEK_END);
>   lseek(fd, 0, SEEK_SET);
> 
>   data = malloc(size);
>   read(fd, data, size);
>   close(fd);
> 
> Why not fstat?  I don't know.

Well, the above will work perfectly with or without the patch, since it 
does the read of the full size. There is no read-ahead hint necessary for 
that kind of single read behavior.

Rememebr: read-ahead is about filling the empty IO spaces _between_ reads, 
and turning many smaller reads into one bigger one. If you only have a 
single big read, read-ahead cannot help.

Also, keep in mind that read-ahead is not always a win. It can be a huge 
loss too. Which is why we have _heuristics_. They fundamentally cannot 
catch every case, but what they aim for is to do a good job on average.

			Linus
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/