On Sat, 03 Mar 2007 20:23:07 -0500 Rik van Riel <[EMAIL PROTECTED]> wrote:
> Andrew Morton wrote: > > On Sat, 03 Mar 2007 19:01:01 -0500 Rik van Riel <[EMAIL PROTECTED]> 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? We need to store that this-page-got-reclaimed info somewhere. I don't know how space-efficient that is. Did anyone ever do an implementation? Of course, the pages need to be re-read again so there's a potential 100% hit there, which is in fact not a huge amount in this context. Depends how often it occurs (all the time when refault is being useful?) versus what we gain from it. > 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. We have a communication problem here. Please please please work harder to get these problems communicated to the MM developers. The only vendor MM kludge of which I'm aware is a thing which Andrea is working on to address a large-shm-segment versus bulk-IO problem (yup, database). If you have enough of an understanding of a problem to be able to develop and productise a fix then share that info madly, asap. otoh, rhel-on-the-desktop-or-smaller probably isn't a huge priority, which can be taken advantage of. > 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. You mean design it and review the design before coding it? You'll find few objections there. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
