Messages in this thread | | | Date | Sat, 03 Mar 2007 20:23:07 -0500 | From | Rik van Riel <> | Subject | Re: userspace pagecache management tool |
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Andrew Morton wrote: > On Sat, 03 Mar 2007 19:01:01 -0500 Rik van Riel <riel@redhat.com> wrote:
>> The use-once policy we have in the kernel should work >> perfectly fine for backups. All we need to do is >> actually honor the accessed bit on active page cache >> pages, instead of flushing them onto the inactive >> list. >> >> What am I overlooking? > > That'll improve backups but will break other things. > > To do this effectively we'd need to change the policy so that new pagecache > allocations cause no scanning of used-twice pages at all. So that even > after many gigs of backing up, the working set is still there. > > Problem is, (for example) what about the person who has 80% of memory in > used-twice state and who then reads a file or files which are 20% or more of > the size of memory, two or more times. It'll be 100% cache misses, every time. > This will happen quite a lot. IOW, once those pages are in used-twice state, > how does further pagecache activity ever get them _out_ of that state? Only > by joining the used-twice page set, and that can't happen if the used-once-so-far > pages got reclaimed. > > Doing a refault thing would help a bit, but stops working at a certain point.
At what point does it stop working?
I am not asking this to be difficult, I just want to get Linux a VM that does not need to be kludged up every time a distro ships it to its customers.
I believe one starting point would be a concept that people cannot shoot holes in any more. That is no guarantee, but as long as the concept has known holes coding it up is likely to be a waste of time since the code will need kludges to deal with the problems later on and we'd be back to square one.
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