Messages in this thread | | | Date | Tue, 18 Mar 2008 00:36:20 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH prototype] [0/8] Predictive bitmaps for ELF executables |
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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?
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