Re: [ck] Re: 2.6.20-ck1

Radoslaw Szkodzinski
2007-02-18T00:41:10Z

On 2/18/07, Andrew Morton <akpm@linux-foundation.org> wrote:
> Generally, the penalties for getting this stuff wrong are very very high:
> orders of magnitude slowdowns in the right situations. Which I suspect
> will make any system-wide knob ultimately unsuccessful.
>

Yes, they were. Now, it's an extremely light and well-tuned patch.
kprefetchd should only run on a totally idle system now.

> The ideal way of getting this *right* is to change every application in the
> world to get smart about using sync_page_range() and/or posix_fadvise(),
> then to add a set of command-line options to each application in the world
> so the user can control its pagecache handling.

We don't live in a perfect world. :-)

> Obviously that isn't practical. But what _could_ be done is to put these
> pagecache smarts into glibc's read() and write() code. So the user can do:
>
> MAX_PAGECACHE=4M MAX_DIRTY_PAGECACHE=2M rsync foo bar
>
> This will provide pagecache control for pretty much every application. It
> has limitations (fork+exec behaviour??) but will be useful.

Not too useful for interactive applications with unpredictable memory
consumption behaviour, where swap-prefetch still helps.

> A kernel-based solution might use new rlimits, but would not be as flexible
> or successful as a libc-based one, I suspect.