Re: [PATCH prototype] [0/8] Predictive bitmaps for ELF executables

Andrew Morton
2008-03-18T07:37:12Z

On Tue, 18 Mar 2008 02:09:34 +0100 (CET) Andi Kleen <andi@firstfloor.org> wrote:

> This patchkit is an experimental optimization I played around with
> some time ago.
>
> This is more a prototype still, but I wanted to push it out
> so that other people can play with it.
>
> The basic idea is that most programs have the same working set
> over multiple runs. So instead of demand paging all the text pages
> in the order the program runs save the working set to disk and prefetch
> it at program start and then save it at program exit.
>
> This allows some optimizations:
> - it can avoid unnecessary disk seeks because the blocks will be fetched in
> sorted offset order instead of program execution order.
> - batch kernel entries (each demand page exception has some
> overhead just for entering the kernel). This keeps the caches hot too.
> - The prefetch could be in theory done in the background while the program
> runs (although that is not implemented currently)

Should be worthwhile for some things.

> Some details on the implementation:

Can't this all be done in userspace? Hook into exit() with an LD_PRELOAD,
use /proc/self/maps and the new pagemap code to work out which pages of
which files were faulted in, write that info into the elf file (or a
separate per-executable shadow file), then use that info the next time the
app is executed, either with an LD_PRELOAD or just a wrapper.

> Drawbacks:
> - No support for dynamic libraries right now (except very clumpsily
> through the mmap_slurp hack). This is the main reason it is not
> very useful for speed up desktops currently.
>
> - Executable files have to be writable by the user executing it
> currently to get bitmap updates. It would be possible to let the
> kernel bypass this, but I haven't thought too much about the security
> implications of it.
> However any user can use the bitmap data written by a user with
> write rights.

Those all get fixed with the userspace version?