Re: [RFC 0/3] Add madvise(..., MADV_WILLWRITE)

Andy Lutomirski
2013-08-09T00:11:51Z

On Thu, Aug 8, 2013 at 12:25 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> Whoops -- I read your email too quickly. I haven't tried
> MADV_WILLNEED, but I think I tried reading each page to fault them in.
> Is there any reason to expect MADV_WILLNEED to do any better? I'll
> try to do some new tests to see how well this all works.
>
> (I imagine that freshly fallocated files are somehow different when
> read, since there aren't zeros on the disk backing them until they get
> written.)

Well, this will teach me to write code based on an old benchmark from
memory. It seems that prefaulting for read is okay on Linux 3.9 --
latencytop isn't do_wp_page or ext4* at all, at least not for the last
couple minutes on my test box.

I wonder if ext4 changed its handling of fallocated extents somewhere
between 3.5 and 3.9. In any case, please consider these patches
withdrawn for the time being.

* With file_update_time stubbed out. I need to dust off my old
patches to fix that part.