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SubjectRe: [PATCH 10/11] readahead: dont do start-of-file readahead after lseek()


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


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